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Design and Operation of Farm Irrigation Systems, 2nd Edition -17


CHAPTER 16

DESIGN AND OPERATION OF SPRINKLER SYSTEMS
Derrel L. Martin (University of Nebraska, Lincoln, Nebraska) Dennis C. Kincaid (USDA-ARS, Kimberly, Idaho) William M. Lyle

(Texas A&M University, Lubbock, Texas)
Abstract. The design and operation of sprinkler irrigation systems are presented and discussed in this chapter. Information is provided concerning the components of irrigation systems and the characteristics of the system that affect water application efficiency and uniformity. The interactions of system characteristics with soil and crop properties are evaluated relative to design and operation of irrigation systems to minimize runoff, deep percolation, and excessive evaporation losses. Keywords. Application efficiency, Center pivot, Lateral move, Runoff, Side roll, Solid set, Sprinkler irrigation, Towline, Uniformity.

16.1 INTRODUCTION
Sprinkler systems have revolutionized the development of irrigated agriculture. Efficient water application with sprinkler irrigation involves the design and operation of pumps, pipes, and sprinkler devices to match soil, crop, and resource conditions. Thus, sprinkler systems can be designed and operated for efficient irrigation for a wide array of conditions. Sprinkler irrigation also facilitated the expansion of irrigated agriculture onto lands classified as unsuitable for surface irrigation. Initially the labor required to transport the system across the field impeded the adoption of sprinkler irrigation. Through automation the labor required for sprinkler irrigation has been reduced significantly. Reduced labor requirements enabled producers to irrigate more frequently with smaller water applications which diminished unintentional leaching and increased the potential to store precipitation in the crop root zone while satisfying crop requirements. Morgan (1993) indicated that sprinkling devices were used as early as 1873. By 1898 seventeen patents had been issued for sprinkler devices. Since that early begin-

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Chapter 16 Design and Operation of Sprinkler Systems

ning many developments have occurred. The patent for impact sprinklers as we know them today was issued in 1934. Aluminum pipe with rubber gaskets were first produced in the late 1940s while an early version of the side roll machine was first produced in the 1950s. Self-propelled center pivot and lateral move systems were invented in the late 1940s. Producers quickly recognized that controlling an irrigation system was essential for proper performance. One of the first controllers for sprinkler irrigation was installed in 1924. These early developments laid the foundation for the growth of sprinkler irrigation. In the late 1940s and early 1950s development began in earnest and continued with large increases in the 1960s and 1970s when automated systems were commercialized. The amount of land irrigated with sprinkler systems continues to increase. Most of the development today is devoted to automated and semi-automated sprinkler systems. According to the Farm and Ranch Irrigation Survey (USDA-NASS, 2003), sprinkler irrigation was used on approximately 51% of the land irrigated in the United States (Table 16.1). Currently, center pivots are used on approximately 79% of the land that is irrigated by sprinkler. Slightly more low-pressure center pivots are used than medium-pressure pivots. Relatively few high-pressure pivots were used in 2002.
Table 16.1. Distribution of sprinkler irrigated land in the United States in 2002 (source: USDA-NASS, 2003). Land Area (hectares) by Pressure Range System Center pivot Lateral move Solid set Side roll Traveler or big gun < 207 kPa 210 to 414 kPa > 414 kPa 3,925,883 60,553 112,551 --3,909,860 78,784 364,353 ---784,943 Total Area % of Sprinklerhectares Irrigated Land 8,620,685 139,337 476,904 739,231 256,351 673,498 10,906,006 21,288,338 79.0 1.3 4.4 6.8 2.4 6.2

Hand move -Total sprinkler-irrigated land Total land irrigated with all types of systems

16.2 COMPONENTS OF SPRINKLER SYSTEMS
Sprinkler irrigation systems generally consist of a pump used to lift and pressurize water, a main line piping system to convey water from the pump to the lateral pipelines used to transport water across the irrigated field, and sprinkler devices to apply the water within the field (Figure 16.1). In some cases the main line is subdivided into submains to convey water to portions of the field. Multiple laterals may be used if the field is large or if the field needs to be irrigated frequently. Sprinkler devices are installed at intervals along the lateral. Often a pipe called a riser is used to adjust the height of the sprinkler device to the desired height to avoid crop interference with the jet of water discharged from the sprinkler. For some applications with center pivots or lateral move systems, the sprinkler device is suspended below the lateral using drop tubes to minimize evaporation and drift of sprinkler droplets.

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Figure 16.1. Components and general layout of sprinkler irrigation systems.

The proper spacing of sprinklers along the lateral is crucial to providing enough overlap of water from adjacent sprinklers to achieve uniform irrigation. Sprinkler laterals can be permanently installed as with solid set systems, or the laterals can be moved across the field. Laterals can be periodically moved from one location (often called a “set”) to the next location as with hand move systems or the laterals can move continuously across the field as with center pivot and lateral move systems. When the sprinkler lateral is periodically moved across the field the distance between subsequent sets of the lateral must be narrow enough to provide for adequate overlap. Many kinds of sprinkler devices are available; some are shown in Figure 16.2. All sprinkler devices use a nozzle to control the discharge from the sprinkler. The diameter and shape of the nozzle orifice and the water pressure at the nozzle control the rate of flow. The nozzle converts pressure within the piping system to velocity upon discharge from the sprinkler. The velocity propels droplets through the air to provide a wetted pattern about the sprinkler. The design of the nozzle and the sprinkler device determines the diameter of coverage and the distribution of water within the wetted region. As with any irrigation system, the sprinkler system must be laid out to conform to the boundaries of the field. The dimensions of the field physically or economically often dictate the type of sprinkler system to be used. Ultimately the components of the sprinkler system must be matched to irrigate efficiently and economically. Inappropriate specification of any component will negatively affect the entire system.

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Figure 16.2. Examples of sprinkler devices used on irrigation systems.

16.3 DESIGN FUNDAMENTALS
Sprinkler irrigation systems should be designed to satisfy crop water requirements while applying water at a rate that minimizes runoff and excess leaching. To accomplish these objectives, the pressure distribution in the main line and sprinkler lateral must be appropriate as discussed in Chapter 15. Efforts are underway to design systems that apply variable depths of water within a field for site-specific management. However, emphasis is currently on systems to apply water uniformly throughout the field. General concepts that apply to design and operation of sprinkler systems are developed in this section. Details for specific systems are presented later. 16.3.1 Application Efficiency The application efficiency is the fraction of the applied water that remains in the crop root zone following irrigation. Water that infiltrates and remains in the root zone is the net irrigation. The application efficiency is then the ratio of the net irrigation depth (dn) to the gross depth of water applied (dg). The possible losses of water in sprinkler irrigation are illustrated in Figure 16.3. Some water may evaporate before it reaches the crop canopy or the soil surface. A portion of the water is intercepted by the canopy while the remainder falls through the canopy to the soil surface. Water that is intercepted by the canopy can be retained on the canopy, may run down the stem of the plant to the soil surface, or may drip from the plant. Steiner et al. (1983) showed that for a mature corn crop up to 50% of the

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Figure 16.3. Diagram of water losses for sprinkler irrigation.

water applied with a center pivot flowed along the leaves and stems to the ground. Water that is intercepted and/or retained on the canopy can evaporate during and after irrigation. Water that reaches the soil surface can evaporate, infiltrate, run off, or be stored in depressions along the soil surface. When infiltration exceeds the available storage in the crop root zone, the excess application will slowly drain through the soil profile inducing deep percolation. Water stored on the soil surface has been called surface storage, depression storage, or retention storage. This water is available to infiltrate after the rate of application of water decreases below the infiltration rate of the soil. Water that runs off the point of application can infiltrate at a downstream location and may be used. If the runoff leaves the field, the water is lost to that field. If the runoff accumulates in low-lying areas deep percolation may occur. The type of system, its design and operation, and the soil and climatological conditions at the time of irrigation all affect the application efficiency. Design of sprinkler systems is based upon average application efficiencies. Typical efficiencies are summarized in Table 16.2. 16.3.2 System Discharge The discharge required for the irrigation system is one of the first considerations in the design process. The discharge must be sufficient to meet crop water requirements for the climatological conditions of the region. We are using the net system capacity as the indicator of the supply required to meet crop needs. The net system capacity (Cn) is the rate water must be continually supplied to satisfy plant water requirements. Heermann et al. (1974) used a daily soil water balance to simulate the effect of net system capacities on the soil water content. For a given capacity, they determined the maximum soil water depletion reached during a year. They simulated the soil water balance for a 59-year period to develop data for a probability distribution of the net

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Chapter 16 Design and Operation of Sprinkler Systems

Table 16.2. Characteristics of sprinkler irrigation systems (adapted from Keller et al., 1980). Installation Annual Cost of Labor per Irrigation, Application Coefficient Maintenance (hour ha-1) Type of Cost ($ Efficiency of -1 System hour ) (% of purchase) Preseason In Season (%) Uniformity Hand move Towline Side roll Side move Solid set Center pivot standard with corner Lateral move ditch fed hose fed pipe fed Travelers 300 500 675 2000 800 925 -1250 -900 2 3 2 4 2 5 6 0.25` 0.25 0.25 0.49 0.25 0.12 0.12 0.15 0.12 0.25 1.73 1.00 0.86 0.62 0.15 0.05 0.06 0.10 0.15 0.07 0.62 65-75 70-80 70-80 70-85 80-92 70-85 70-85 70-85 75-85 85-95

6 6

85-92 60-70

85-95 70-80

system capacity requirement. Results from von Bernuth et al. (1984) were used to develop design guidelines for center pivots in Nebraska (Figure 16.4). Howell et al. (1989), and Lundstrom and Stegman (1988) used similar procedures for other areas in the Great Plains. The inflow rate required for the system is the amount of water that must be supplied to avoid water stress. The system flow rate is given by
? d g Ai ? ? C T ?? A ? ? QS = 0.116? n i ?? i ? = 0.116? ? E ?? T ? ? T ? ? a ?? o ? ? o ?

16.1

where Qs = inflow to the sprinkler irrigation system (i.e. gross system capacity), L s-1 Cn = net system capacity, mm day-1 Ti = irrigation interval, days Ea = application efficiency, decimal fraction. Ai = area irrigated, ha, and To = time of operation per irrigation, days dg = gross depth of irrigation water applied, mm The irrigation interval is the time from the start of the irrigation until the beginning of the subsequent irrigation. The irrigation interval is often a nominal value representing normal practices during the peak water requirement period of the irrigation season. The time of operation is the amount of time that water is applied during the irrigation interval. There are times when the irrigation system may be shut down for maintenance, repositioning sprinkler laterals, farming operations, or convenience. This time can be referred to as the downtime. The downtime is the difference between the irrigation interval and the time of operation. The selection of the irrigation interval and downtime must be coordinated with the operator and should not be arbitrarily selected by the designer.

Design and Operation of Farm Irrigation Systems
1.0

563

NET SYSTEM CAPACITY, L/s/ha

0.9 WESTERN NEBRASKA

0.8

0.7

0.6

EASTERN NEBRASKA

0.5 25 50 75 100 125 150

ALLOWABLE SOIL WATER DEPLETION, mm

Figure 16.4. Net system capacity requirement for center pivots in Nebraska (adapted from von Bernuth et al., 1984).

The total system capacity cannot exceed the available water supply. If the capacity requirements are too high, the amount of area irrigated may have to be reduced, or deficit irrigation will be necessary. 16.3.3 Sprinkler Discharge The discharge required for a sprinkler can be determined by the density of sprinklers and the number of laterals within the field. For solid set and moved-lateral systems the density is determined by the spacing between the sprinklers along a lateral and the distance between sets along the main line (Figure 16.2). The representative area for a single sprinkler is the product of the sprinkler spacing along the lateral (SL) and the distance between laterals or the set width (Sm). The number of sprinklers on a lateral (N) is determined by the length of the field (FL), the number of laterals used along the field length (NL) and the sprinkler spacing (i.e., N = FL/NL SL). The number of sets per lateral (n) is determined by the width of the field (FW), the number of laterals along the width of the field (Ns) and the width of an individual set (n = FW /Ns Sm). The number of sprinklers per lateral and the number of sets per lateral must be integers. The irrigation interval depends on the number of sets per lateral (n), the operational time per set for a lateral (Ts), the time required to move the sprinkler later between sets (Tm), and downtime between successive irrigations: (16.2) Ti = n (Ts + Tm) + Td The combination of the operational time and the time required to move from one set to the next is often referred to as the set time. The set time must be selected in consultation with the owner/operator to match labor availability. The minimum downtime is the time required to reposition laterals from the last set in the field to the beginning set for the next irrigation. Additional time may also be required for maintenance and farming operations.

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The discharge from an individual sprinkler for such systems depends on (1) the depth of water that must be applied per irrigation, (2) the representative area for one sprinkler, and (3) the time that water is applied for an individual set, as
qs = 1 ? CnTi ?? S L Sm ? d g S L Sm ?? ?= ? 3600 ? Ea ?? Ts ? 3600Ts ? ?? ?

(16.3)

where qs = discharge from a sprinkler, L s-1 SL = spacing between sprinklers, m Sm = distance between laterals, m Ts = operational time per set for a single lateral, hours Part-circle sprinklers can be placed at the end of the lateral to provide uniform irrigation without applying water beyond the field boundaries. The discharge for an end sprinkler should be a fraction of the discharge of sprinklers along the lateral as based on the proportion of a full circle that is watered with the end sprinkler. Some solid set systems are designed so that the area between adjacent sprinklers forms an equilateral triangle. For this arrangement the distance between sprinklers is equal to the spacing along the lateral (SL) and the distance between laterals is Sm = 0.866 × SL. Relationships are developed in later sections for the discharge from individual sprinklers on other types of systems. The pressure available to the sprinkler device and the effective diameter of the nozzle determine the discharge from a sprinkler. For circular nozzles the relationship is given by
2 qs = 0.00111 Cd Dn P
-1

(16.4)

where qS = discharge per nozzle, L s Cd = discharge coefficient Dn = inside diameter of the nozzle, mm P = pressure at the base of the sprinkler device, kPa. The discharge coefficient, which incorporates characteristics of the sprinkler head and nozzle, is unique for sprinkler devices and nozzles. Heermann and Stahl (2006) developed equations to estimate the discharge coefficient for selected sprinkler devices: Cd = d0 + d1 Dn2 + d2P (16.5) where d0, d1 and d2 are empirical parameters. Typical parameter values are listed in Table 16.3 for selected sprinkler devices. The discharge from the range nozzle and the spreader nozzle should be added for sprinklers equipped with two nozzles. For some nozzles the orifice is not circular and the effective diameter of the nozzle should be used in Equations 16.4 and 16.5. 16.3.4 Diameter of Coverage The diameter of coverage, also called the wetted diameter, (Wd), is the size of the circular pattern that is wetted by an individual sprinkler. The diameter of coverage has a large influence on sprinkler and lateral spacing and thus the cost of a system. The diameter of coverage depends on the velocity of the sprinkler jet, the angle of the nozzle axis above the horizon and the size of droplets that form when the sprinkler jet breaks up. Up to a point, as pressure increases, the diameter of coverage increases and

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more uniform application may result. The pressure should be increased as the nozzle sizes increase to ensure that the spray breaks up adequately. Sprinkler manufacturers provide performance data that includes the discharge and diameter of coverage as a function of operating pressure and nozzle sizes as illustrated in Table 16.4. The diameter of coverage provided by the manufacturer generally specifies the height of the nozzle above the ground when the device was tested. Test results are usually for no-wind conditions and include the recommended operating pressure range for the listed configurations.
Table 16.3. Coefficients for the discharge coefficient (adapted from Heermann and Stahl, 2006). Minimum Model Pressure Company d1 d2 (kPa) d0 Number 245 0.954 1.859 × 10-5 5.061 × 10-3 Nelson Irrigation Corp. F32AS 245 0.954 Nelson Irrigation Corp. F32S 5.061 × 10-3 1.859 × 10-5 -3 245 0.976 -1.617 × 10 -2.567 × 10-6 Nelson Irrigation Corp. F33A -3 245 0.976 -1.617 × 10 -2.567 × 10-6 Nelson Irrigation Corp. F33AS -3 245 0.976 -1.617 × 10 Nelson Irrigation Corp. F33S -2.567 × 10-6 -4 196 1.011 -2.267 × 10 -3.441 × 10-4 Nelson Irrigation Corp. F33DN -3 245 1.011 -4.889 × 10 -5.737 × 10-6 Nelson Irrigation Corp. F43A -5 245 0.945 -3.363 × 10 -2.928 × 10-6 Nelson Irrigation Corp. F43AP -3 245 0.975 -1.390 × 10 -8.234 × 10-6 Nelson Irrigation Corp. F43P -3 294 0.996 -2.018 × 10 -1.357 × 10-6 Nelson Irrigation Corp. F70A -3 392 0.991 -1.002 × 10 Nelson Irrigation Corp. F80A 3.213 × 10-5 -3 147 1.035 -2.845 × 10 -1.359 × 10-4 Nelson Irrigation Corp. R30 196 0.294 Nelson Irrigation Corp. R3000 0 0 294 0.969 Nelson Irrigation Corp. R30D4 0 0 294 0.957 Nelson Irrigation Corp. R30D6 0 0 98 0.986 -2.692 × 10-3 -9.696 × 10-5 Nelson Irrigation Corp. SPR1 196 0.966 -7.768 × 10-3 Rain Bird Corp. 20 7.394 × 10-5 -2 245 1.059 -2.341 × 10 -4.154 × 10-6 Rain Bird Corp. L20 -2 245 1.084 -1.585 × 10 -6.466 × 10-6 Rain Bird Corp. L2020 -3 245 0.995 -3.149 × 10 -5.449 × 10-6 Rain Bird Corp. 30 -3 245 1.009 -9.980 × 10 -8.572 × 10-6 Rain Bird Corp. L30 -3 245 1.081 -4.962 × 10 -6.456 × 10-5 Rain Bird Corp. L3030 -3 294 0.952 Rain Bird Corp. 35 2.904 × 10 -4.927 × 10-6 -3 245 1.000 -2.656 × 10 4.358 × 10-5 Rain Bird Corp. 40 -3 392 0.984 -3.061 × 10 7.541 × 10-5 Rain Bird Corp. 65 -3 392 1.002 -3.780 × 10 6.085 × 10-5 Rain Bird Corp. 70 -3 245 0.960 -1.278 × 10 3.300 × 10-5 Rain Bird Corp. 85 -3 98 0.922 -9.870 × 10 1.005 × 10-4 Rain Bird Corp. 8X -4 245 0.963 -6.183 × 10 Senninger Irrigation Inc. S4006 1.269 × 10-5 -3 245 0.990 -2.502 × 10 -7.896 × 10-6 Senninger Irrigation Inc. S5006 59 0.983 -6.662 × 10-4 -8.100 × 10-5 Senninger Irrigation Inc. SSPR 59 0.865 Valmont Industries, Inc. SPR06 0 0 98 0.816 Valmont Industries, Inc. SPR10 0 0

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Chapter 16 Design and Operation of Sprinkler Systems

Table 16.4. Performance data for an example sprinkler. Nozzle Sizes (mm) 4.37 × 2.38 4.76 × 2.38 4.76 × 3.18 5.16 × 3.18 5.56 × 3.18 Nozzle Diam. Diam. Diam. Diam. Diam. Pressure Flow cover Flow cover Flow cover Flow cover Flow cover (kPa) (L s-1) (m) (L s-1) (m) (L s-1) (m) (L s-1) m (L s-1) (m) 173 0.35 26.8 0.40 27.4 0.47 27.4 0.52 27.7 0.58 28.4 207 0.38 28.0 0.44 29.0 0.51 29.0 0.58 29.9 0.64 30.5 242 0.41 29.0 0.47 29.9 0.56 29.9 0.63 30.8 0.70 31.4 276 0.44 29.6 0.51 30.5 0.60 30.5 0.67 31.4 0.75 32.3 311 0.47 30.2 0.54 31.1 0.63 31.1 0.71 32.0 0.80 32.9 345 0.50 30.5 0.57 31.7 0.67 31.7 0.75 32.6 0.84 33.5 380 0.52 30.8 0.60 32.3 0.70 32.3 0.79 33.2 0.88 34.1

5.95 × 3.18 Diam. Flow cover (L s-1) (m) 0.64 28.7 0.71 30.8 0.76 32.0 0.82 32.6 0.87 33.5 0.92 34.1 0.96 34.8

Several types of nozzles are available for sprinklers. Conventional straight bore (with round orifices) brass or plastic nozzles produce the maximum pattern radius and good patterns at medium to high pressures. Noncircular orifice nozzles (with diffuse jets) produce good patterns at reduced pressures but the wetted radius is usually smaller than for straight bore nozzles. Kincaid (1982) developed an equation to predict the diameter of coverage based on the nozzle pressure and discharge from the sprinkler. Heermann and Stahl (2006) used the diameter of the nozzle and the sprinkler pressure to predict the wetted radius for a range of sprinklers:
2 2 Wr = r0 + r1 Dn P + r2 Dn P

( ) ( )

2

(16.6)

where Wr= radius of coverage, m Dn = nozzle diameter, mm P = nozzle pressure kPa, r0, r1, r2 = empirical constants. Values for the empirical parameters are listed in Table 16.5 for selected devices. 16.3.5 Time of Operation The time of operation per set (Ts) should be selected to satisfy crop water requirements throughout the irrigation interval while avoiding deep percolation. The operational time must also be acceptable to the operator. The appropriate set time depends on the rate of water application. The average application rate is the average rate that water is applied to the crop/soil surface. The application rate depends on the discharge and the representative area for an individual sprinkler. The average application rate for solid set and periodically moved systems is given by
Ra = 3600 qS S L Sm

(16.7)

where Ra = the average application rate, mm hour-1. The rate of application is important since runoff may occur when the application rate exceeds the infiltration rate. The recommended maximum and minimum applica-

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tion rate for typical sprinkler layouts for periodically moved and solid set systems are listed in Table 16.6. These values represent average conditions and should be adjusted when local information is available or where runoff is prevalent.
Table 16.5. Coefficients for the wetted radius for selected sprinklers (adapted from Heermann and Stahl, 2006). Company Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Nelson Irrigation Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Rain Bird Corp. Senninger Irrigation Inc. Senninger Irrigation Inc. Senninger Irrigation Inc. Valmont Industries, Inc. Valmont Industries, Inc. Model Number F32AS F32S F33A F33AS F33S F33DN F43A F43AP F43P F70A F80A R30 R3000 R30D4 R30D6 SPR1 20 L20 L2020 30 L30 L3030 35 40 65 70 85 8X S4006 S5006 SSPR SPR06 SPR10 r0 7.66 11.16 9.95 9.95 10.74 9.67 9.25 9.36 12.34 12.62 16.87 9.15 6.40 8.54 7.62 3.73 10.54 8.30 9.48 10.75 10.23 10.27 10.64 12.00 13.09 16.52 16.69 6.47 11.02 11.49 3.83 3.52 2.97 Coefficients r1 1.0188 × 10-3 4.4340 × 10-4 6.7152 × 10-4 6.7152 × 10-4 6.5956 × 10-4 4.2715 × 10-4 4.2364 × 10-4 3.9817 × 10-4 4.5998 × 10-4 2.5634 × 10-4 1.1757 × 10-4 0 0 0 0 7.7219 × 10-4 5.2870 × 10-4 9.5893 × 10-4 4.0092 × 10-4 6.4496 × 10-4 1.8772 × 10-4 1.6917 ×10-4 6.9710 × 10-4 5.1893 × 10-4 3.6063 × 10-4 2.7829 × 10-4 1.9212 × 10-4 3.4306 × 10-4 5.0444 × 10-4 3.8105 × 10-4 4.1793 × 10-4 5.2760 × 10-7 7.6539 × 10-4 r2 -5.9017 × 10-8 -8.8466 × 10-9 -2.0504 × 10-8 -2.0504 × 10-8 -1.7156 × 10-8 -1.1985 × 10-8 -7.8742 × 10-9 -6.0400 × 10-9 -7.2575 × 10-9 -1.9555 × 10-9 -3.0453 × 10-10 0 0 0 0 -4.2217 × 10-8 -3.3920 × 10-8 -8.8110 × 10-8 -2.6504 × 10-8 -1.6816 × 10-8 -4.2494 × 10-9 -3.1560 × 10-9 -1.9120 × 10-8 -9.1826 × 10-9 -3.0844 × 10-9 -1.2432 × 10-9 -6.1428 × 10-10 -6.4353 × 10-9 -1.3677 × 10-8 -3.3509 × 10-9 -1.3894 × 10-8 0 -2.9334 × 10-8

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Chapter 16 Design and Operation of Sprinkler Systems

0-5 Soil Texture Coarse sand 50 Fine sand 40 Loamy fine sand 35 Sandy loam 25 Fine sandy loam 20 Very fine sandy loam 15 Loam 13 Silt loam 13 Sandy clay loam 10 Clay loam 8 Silty clay loam 8 Clay 5

Table 16.6. Maximum application rate and depth of infiltration based on soil type and management criteria (partially from Keller and Bleisner, 1990). Maximum Application Rate Available Maximum Depth of Infiltration (mm) for a root zone (mm h-1) Water1 m deep with allowable Holding Soil Slope (%) depletions of: Capacity 5-8 40 32 28 20 16 12 10 10 8 6 6 4 8-12 30 24 21 15 12 9 8 8 6 5 5 3 >12 20 16 14 10 8 6 5 5 4 3 3 2 (mm m-1) 50-70 75-85 85-100 110-125 130-150 145-165 150-170 160-200 140-170 150-180 140-180 130-180 35% 21 28 32 42 49 53 56 63 53 56 56 53 50% 30 40 45 60 70 75 80 90 75 80 80 75 65% 39 52 59 78 91 98 104 117 98 104 104 98

The depth of water applied per irrigation (i.e., the volume of water per unit area of land) for solid set and moved-lateral systems can be computed from
d g = RaTS = 3600 qS TS S L Sm

(16.8)

The depth of water applied and the corresponding application rate for other types of sprinkler systems will be presented in later sections. The net depth of application (dn = Ea dg) should not exceed the soil water depletion at the time of irrigation; yet, the net depth must be large enough to satisfy the crop water use during the irrigation interval. To satisfy these constraints the time of operation per set must be consistent with the irrigation interval and the net system capacity. If the depth required for the net capacity exceeds the depletion at the time of irrigation more laterals are generally required along the field width. For design purposes the depletion is computed as AD = MAD RD TAW (16.9) where AD = allowable depletion, mm MAD = management allowed depletion, decimal fraction RD = root depth during peak water use period, m TAW = total available water per unit depth of soil, mm of water per m of soil. Representative values for the parameters in Equation 16.9 are available in Chapter 3 and Table 16.6. The net depth of water must then fit within the following range:
CnTi ≤ d n = 3600 qS TS Ea ≤ AD = MAD RD TAW S L Sm

(16.10)

where dn is net application depth (mm). This equation can be revised to give the maximum and minimum limits for the operational time per set:

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n Tm + Td 86400 qS Ea ?n Cn S L Sm

≤ TS ≤

MAD RD TAW S L Sm 3600 qS Ea

(16.11)

The results show that the ability of the soil to hold the net application provides the maximum set time while the time required to satisfy the net capacity provides the minimum set time (Figure 16.5). For conditions between the limits the system can be shut off for more than the one day of downtime as used in the figure. These results illustrate that the amount of downtime increases as the sprinkler discharge increases. If there are no feasible combinations the number of laterals along the field width should be increased. While the equation for the time of operation is complex, it summarizes the considerations required for a given area and soil. The equation is easily analyzed with a spreadsheet program. Solution often involves a trial and error procedure. 16.3.6 Drop Size Distribution The size of sprinkler droplets profoundly affects the performance of sprinkler irrigation systems. The analyses by Edling (1985), Kohl et al. (1987), Kincaid and Longley (1989), and Thompson et al. (1993) show that small droplets evaporate faster than large droplets (Figure 16.6). Individual droplets are often assumed to be spherically shaped. Therefore the surface area is given by (πd2) while the mass of the droplet is given by (πρd3/6) where d is the diameter of the droplet and ρ is the density of water.
20
Sprinkler Spacing 12.2 m Lateral Spacing 18.3 m Applic. Efficiency 0.75 Root Depth 1.2 m MAD 0.50 Sets / Lateral 20 Time to Move Lateral 1h Downtime / set 24 h

16

TIME OF OPERATION PER SET, hours

12

TOTAL AVAILABLE WATER, mm/m 200

8 6 7 8

160

120

4

NET SYSTEM CAPACITY, mm/day

0 0.4 0.5 0.6 0.7 0.8 0.9 1.0

SPRINKLER DISCHARGE, L/s

Figure 16.5. Relationship of acceptable time of operation to sprinkler discharge when net capacity water requirements form the minimum operational time and the soil water holding capacity constrains the maximum operational time.

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Chapter 16 Design and Operation of Sprinkler Systems

Impact Sprinklers 20 38?C Air Temperature 20% Relative Humidity 20?C Water Temperature 15

EVAPORATION LOSS, %

10

5

0 0.2 0.4 0.6 0.8 1.0 1.5 2.0 3.0 4.0 5.0

DROPLET DIAMETER, mm

Figure 16.6. Illustration of the effect of droplet size on relative evaporation rates.

Accordingly, the ratio of the surface area to the mass of the droplet varies inversely with the diameter of the droplet. Hence, a higher portion of the water in a small droplet is exposed to the atmosphere which results in larger relative evaporation rates. Devices that produce large drops should reduce evaporation loss and would seem to increase the application efficiency. Research has also shown that small drops are very prone to drift in windy conditions. The drag force caused by the difference in velocity between the droplet and the air is proportional to the projected area of the droplet. The momentum is proportional to the mass of the droplet. Therefore, drag forces are more significant for small drops than large drops. The large influence of drag forces causes small drops to decelerate rapidly in still air. Since momentum is more significant for large drops, they are less affected by drag and decelerate more slowly than small drops. These processes result in a variation of droplet sizes with distance from the sprinkler device. In still air, small drops fall closer to the sprinkler while large drops travel further. In windy conditions the additional drag force from the wind transports droplets downwind. This process is called drift. Drift can reduce irrigation efficiency when the water is deposited outside of the field. Uniformity can be reduced if drift results in significant distortion of the application pattern for a prolonged period. The momentum of large drops resists drift in windy conditions; however, it negatively affects the soil surface. Levine (1952) and others have shown that the impact energy contained in water droplets leads to breakdown of soil aggregates and the formation of a seal on the soil surface which reduces infiltration and can lead to runoff. Levine (1952) showed that sandy soils were less affected by droplet impact than finer textured soils. The kinetic energy of a drop can be expressed as πρd3v/12 where v is the velocity of the droplet.

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Stillmunkes and James (1982) summarized literature showing that sealing is related to the amount of kinetic energy per unit area at impact and the accumulation of the energy over time. The kinetic energy per unit area increases with droplet diameter, but reaches a plateau where the kinetic energy per unit area is nearly constant as the drop size increases. Similar results were presented by Kohl et al. (1985). The kinetic energy per unit area is proportional to the diameter of the droplet while the drag coefficient as presented by Seginer (1965) is inversely proportional to the diameter of the droplet. These interactions are such that there is relatively little difference in the velocity of droplets as the size of the drop exceeds 2 or 3 mm (Figure 16.7). Stillmunkes and James (1982) concluded that the kinetic energy per unit area is insensitive to the drop size if the drops are larger than 3 mm and the kinetic energy per unit area is dependent on the application rate and the length of time that water is applied. The equation from Stillmunkes and James (1982) for the kinetic energy per unit area is given by

Ke ρdv 2 ρRTv 2 = = a 2 2 where Ke/a = kinetic energy per unit area ρ = density of water d = depth of water applied v = droplet velocity. R = average application rate T = exposure time
10 9 8 7 VELOCITY, m/s 6 5 1 4 3 2 1 0 0 1 2 3 4 5 6 0.5 FALL HEIGHT, m 20 8 6 5 4 3 2

(16.12)

7

DROP DIAMETER, mm

Figure 16.7. Effect of drop size and height of fall on the velocity of droplets (adapted from Stillmunkes and James, 1982).

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Chapter 16 Design and Operation of Sprinkler Systems

Data from several sources were used by von Bernuth and Gilley (1985) to estimate the relative infiltration rate for bare soils compared to soils protected with mulch or crop cover (Ir):
0. ? I r = 1 ? 0.0354 d50683 v1.271 Sa 0.353 Si0.237

(16.13)

where Ir = rate of infiltration for bare soil relative to protected soil, decimal fraction d50 = volume median drop size, mm v = velocity of the volume mean size droplet, m s-1 Sa = sand content of the soil, % Si = silt content of the soil, %. The effect of the drop size and velocity on infiltration is illustrated in Figure 16.8. While the evaporation and drift of small drops are significant, the relative volume of water in a particular size range of drops must be considered. For example, the mass of water in a 5-mm diameter drop is 1000 times the volume in a 0.5-mm droplet. The drop size distribution must be considered to determine if losses could be significant.
1.0 2 4 6 8 Droplet Velocity, m/s 0.6

RELATIVE INFILTRATION RATE

0.8

0.4

0.2 SANDY LOAM 0.0 0 1 2 3 4 5 6 7

DROP SIZE, mm
1.0

RELATIVE INFILTRATION RATE

2 0.8 4 6 8 0.4 Droplet Velocity, m/s

0.6

0.2 SILT LOAM 0.0 0 1 2 3 4 5 6 7

DROP SIZE, mm

Figure 16.8. Effect of droplet impact energy on infiltration rates.

Design and Operation of Farm Irrigation Systems
1.0

573

RELATIVE FREQUENCY OF DROP SIZE

0.8

STATIONARY PAD - Smooth Plate

0.6

STATIONARY PAD - Medium Grooved Plate

STATIONARY PAD - Coarse Grooved Plate 0.4 IMPACT SPRINKLER - Straight Bore Nozzle - 4 mm Diameter - 400 kPa

0.2

0.0 0 1 2 3 4 5 6

DROP SIZE, mm

Figure 16.9. Examples of drop size distributions for a stationary pad sprinkler device with three styles of plates and an impact sprinkler with a straight bore nozzle.

This has led to methods to determine the distribution of drop sizes from sprinkler devices operated at varying pressures and heights. Various methods have been used to measure the size of water droplets (see Solomon and Bezdek, 1980; Eigel and Moore, 1983; Kohl and DeBoer, 1984; and Dadiao and Wallender, 1985 for descriptions of the various techniques). Examples of drop size distributions are illustrated in Figure 16.9. Bezdek and Solomon (1983) analyzed various mathematical functions to describe drop size distributions. They concluded that the upper-limit lognormal distribution was adequate for many experimental data sets. The disadvantage of this distribution is that the solution is difficult to derive from experimental data. Until recently, the limitation on considering drop size distributions in design has been the scarcity of experimental data for commonly used sprinklers. Kincaid et al. (1996) developed methods to estimate the distribution of drop sizes for fourteen sprinkler devices. They analyzed four models of impact sprinklers that were equipped with straight bore, flow control, or square nozzles. They also examined ten spray head devices equipped with jets that impinged onto fixed or moving plates. An exponential model was used for the drop size distribution. Li et al. (1994) had indicated that the exponential model was comparable in accuracy to the upper-limit lognormal model but much easier to use. The exponential distribution model is given by
η ? ? ? ? ? ? ? ?? 0.693? d ? ? ? Pv = 100?1 ? exp ? d ? ?? ? ? 50 ? ? ? ? ? ? ?

(16.14)

where Pv = percent of the total drops that are smaller than d d = drop diameter, mm

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d50 = volume mean drop diameter, mm η = dimensionless exponent. Data were developed for the values of d50 and η for each nozzle and sprinkler combination for a range of operating pressures and nozzle sizes. The exponential model accurately represents the drop size distribution for drops larger than 3 mm and overestimated percentage volumes for smaller drop sizes. Kincaid et al. (1996) developed a procedure to estimate the parameters required for the exponential model. The procedure used the effective diameter of the nozzle and the nozzle pressure. They showed that the ratio of the nozzle diameter to the pressure head at the nozzle could be used to describe the volume mean drop diameter and the empirical parameter η. The relationships that they developed are given by d50 = ad + bd Ω and η = an + bn Ω (16.15) where ? = ratio of the nozzle diameter to the pressure at the base of the sprinkler device ad, bd, an, bn = regression coefficients. Results were grouped into seven categories as describe in Table 16.7. These results provide a more general method to estimate the effects of the design and operation of irrigation systems on the drop sized generated. Kincaid (1996) presented data on the kinetic energy of spray droplets. Relationships were developed to predict the kinetic energy per unit mass based on the ratio of the nozzle diameter to the pressure head:
? N e2 ? Ek = e0 + e1? d ? + U 1.5 (16.16) ? H e3 ? ? n ? where Ek = kinetic energy, kJ kg-1 Nd = diameter of the nozzle, mm Hn = nozzle pressure head, m U = wind speed, m s-1 e0,e1,e2,e3 = regression constants. Values for the regression constants are summarized in Table 16.8. Kincaid (1996) also correlated the kinetic energy to the mean volumetric drop size and the percentage of the drops that exceed a diameter of 3 mm:

Ek = 2.79 + 7.2 d50

and

Ek = 10.41 + 0.249 P3

(16.17)

where P3 = the percentage of the drops that are smaller than 3 mm.
Table 16.7. Coefficients for estimating drop size distribution parameters. Type of Sprinkler Device ad bd an bn Impact sprinkler, small round nozzles (3-6 mm) 0.31 11900 2.04 -1500 Impact sprinkler, 1.30 2400 1.82 300 large round nozzles (9-15 mm) Wobbler 0.78 1870 2.08 -630 Rotator, 4 groove 1.07 3230 1.70 -830 Rotator, 6 groove 0.81 1480 2.07 -1300 Concave, 30 groove plate 0.82 620 2.68 -750 Flat smooth plate 0.66 680 2.74 -920

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Table 16.8. Coefficients for estimating energy from sprinkler devices. Sprinkler Device e1 e2 e0 Impact, large round nozzles 14.1 45.1 0.5 Impact, small round nozzles 6.9 132 0.5 Rotators, 4 groove plates 12.1 50.7 0.5 Rotators, 6 groove plates 8.9 38.0 0.5 Spinners, 6 groove plate 6.9 36.9 0.5 Low drift nozzle (LDN) 10.4 0.57 2 Medium groove stationary plate 6.2 0.45 2 Smooth groove stationary plate 5.4 0.40 2

e3 1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.5

The wind speed has a major impact on the kinetic energy, increasing the energy per unit mass fivefold for wind speeds of 10 m s-1 compared to still air. Moldenhauer and Kemper (1969) showed that the infiltration rate decreased by one order of magnitude when the cumulative drop energy per unit area exceeded 500 J m-2. Spray devices that produce small drops with 5 J kg-1 could apply 100 mm of water before reduction occurs. For devices that produce larger drops with impact energies of 20 J kg-1 the threshold would be reached with 25 mm of water. These results allow designers to evaluate the potential runoff problems for various sprinkler devices. The concepts presented to this point can be applied to the design of sprinkler systems.
16.3.7 Sprinkler Placement Sprinklers must be properly placed and aligned. Occasionally sprinklers are not placed high enough to provide an unobstructed path for the sprinkler jet. The canopy that interferes with the water jet reduces the diameter of coverage and leads to poor water distribution. For row crops, sprinklers should be at least 0.5 m above the tallest mature crop that will be irrigated. For orchard crops the sprinkler should be placed to provide the wetted soil zone without causing degradation of fruit quality by wetting leaves and fruits. If the sprinkler is to be used for frost control, the sprinklers must be high enough to provide coverage of the crop canopy. On center pivots and lateral move systems the sprinkler devices may be placed below the lateral. The sprinklers should be placed so that the water stream does not impact on structural components of the machine. If the sprinkler devices are placed below the top of the crop canopy, they should be placed close enough along the pipeline to provide plants with equal access to water. This often requires that devices be positioned above alternate furrows. The vertical alignment of the sprinkler riser is also important. Nderitu and Hills (1993) showed that the diameter of coverage is reduced if the sprinkler riser is tilted from vertical. They showed that the precipitation profiles were nearly the same when the sprinkler riser was within 10° of vertical. However, the application uniformity decreased when the riser was tilted by 20°. Sprinklers that are firmly supported produced higher uniformities than unsupported risers.

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Chapter 16 Design and Operation of Sprinkler Systems

16.4 APPLICATION UNIFORMITY
16.4.1 Single-Leg Distribution The uniformity of application from a sprinkler system depends on the distribution of water from individual devices. The distribution is measured with catch containers placed around a sprinkler as shown in Figure 16.10. The sprinkler is operated long enough to measure the depth of water applied at distances from the sprinkler. Measurement containers should be large enough and deep enough to provide an accurate measurement (Kohl, 1972). Fisher and Wallender (1988) showed that the measurement accuracy was directly related to the diameter of the catch container. If the containers are used in an open field, evaporation should be estimated. Various methods are available including using oil in the containers to suppress evaporation (see Heermann and Kohl, 1980). If the irrigation system will operate in calm and windy conditions, it is best to measure the distribution under both wind conditions.
SPRINKLER LOCATION COLLECTOR LOCATION

AREA WETTED BY A SINGLE SPRINKLER

DIAMETER OF COVERAGE

DEPTH OF WATER APPLIED

ELLIPTICAL PATTERN

TRIANGULAR PATTERN

RADIAL DISTANCE FROM SPRINKLER

WETTED RADIUS

Figure 16.10. Arrangement to measure the single-leg water distribution of a sprinkler. Elliptical and triangular distribution patterns for the single-leg distribution are shown.

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The water application pattern about a single sprinkler is the single-leg distribution. The most commonly used equations are for a triangular and an elliptical distribution (Figure 16.10). Equations for the distributions are given by
d (r ) = 3q S To πWr3

(Wr ? r )

for a triangular pattern and (16.18)

d (r ) =

3q S To 2πWr3

Wr2 ? r 2 for an elliptical pattern

where d(r) = the depth of water applied at a radial distance r from the sprinkler, Wr = the radius of coverage or wetted radius of the sprinkler To = the duration of the sprinkler operation.
16.4.2 Overlapping (Stationary Laterals) Several sprinklers should apply water to a location in the field to achieve an acceptable uniformity of application. The total depth of application can be estimated by overlapping the depth determined from the single-leg distribution for individual sprinklers. An illustration of the procedure is shown in Figure 16.11. This example is based on an impact sprinkler with 4.76 × 3.18 mm nozzles operated at 350 kPa as listed in Table 16.4. An elliptical distribution and an irrigation set time of 10 hours were assumed for the example. These conditions produce the single-leg distribution given by
d (r ) = 3 16 2 ? r 2 . A distribution of catch containers is assumed to compute the uniformity. In this case the containers are placed on a 3 m × 3 m grid. The first container is located half the grid spacing from the sprinkler. To compute the depth of water applied at each container the radial distance from a sprinkler to the container (r) is com2 2 puted by r = x + y where x is the horizontal distances from the lateral and y is the vertical distance to the sprinkler. The first four values for each container in Figure 16.11 are arranged by sprinkler starting with the upper-left sprinkler and continuing to the lower-right sprinkler. The top and bottom rows of containers receive water from upstream and downstream sprinklers that are not shown in the figure. The fifth and sixth rows of the data shown in the figure represent the contribution for those sprinklers. The total depth of water applied is shown as the last value in the column for each container. For this example the maximum depth applied was 145 mm while the minimum was 89 mm. The coefficient of uniformity was computed to be 87 using procedures described in Chapter 4. Overlapping provides a means to evaluate sprinkler spacing in the design of moved-lateral or solid set systems. Distribution data are available from the manufacturer or testing organizations for many sprinkler devices. Actual data from a single-leg test conducted for normal wind conditions provide more representative information for assessing the uniformity. The effect of overlapping applications can also be computed for moving systems. The procedure is somewhat more involved and is discussed in a later section for those systems.

578
0 0 1.5 4.5

Chapter 16 Design and Operation of Sprinkler Systems
DISTANCE FROM LEFT LATERAL, m
7.5 10.5 13.5 16.5 18

DISTANCE FROM TOP SPRINKLER, m

1.5

48 0 37 0 27 0 112 46 0 43 0 89

46 27 35 0 24 0 132 45 24 41 15 125

43 37 30 20 15 0 145 41 35 37 30 143

37 43 20 30 0 15 145 35 41 30 37 143

27 46 0 35 0 24 132 24 45 15 41 125

0 48 0 37 0 27 112 0 46 0 43 89

4.5

7.5

43 0 46 0 89

41 15 45 24 125

37 30 41 35 143

30 37 35 41 143

15 41 24 45 125

0 43 0 46 89

10.5

12

37 0 48 0 27 0 112

35 0 46 27 24 0 132

30 20 43 37 15 0 145

20 30 37 43 0 15 145

0 35 27 46 0 24 132

0 37 0 48 0 27 112

SPRINKLER LATERAL CATCH CONTAINER

CU = 87

Figure 16.11. Distribution of water from overlapping the single-leg distribution for each sprinkler at each container location.

16.4.3 Wind Effects Wind has a pronounced effect on the distribution of water from sprinklers. The work by Christiansen (1942), Vories and von Bernuth (1986), and Seginer et al. (1991a) and others showed how the application pattern of a single sprinkler is distorted in the wind. The general effects of wind are illustrated in Figure 16.12. The pattern is transported downwind as would be expected; however, the diameter of coverage perpendicular to the wind is reduced. The narrowing of the pattern perpendicular to the wind has an impact on the layout of sprinkler laterals. As illustrated the narrowing of the diameter of coverage can lead to poor uniformity if laterals are not placed closer together when the wind is parallel to the lateral. To compensate for the smaller diameter, sprinklers must be located closer together in the perpendicular direction. It is more economical to space sprinklers closer together along the lateral rather than space laterals closer together. Thus, the general recommendation is to orient laterals perpendicular to the prevailing wind and to space sprinklers closer together along the lateral than the spacing between the laterals. General guidelines have been developed for the maximum spacing to maintain acceptable uniformities (Table 16.9). Models have been developed to predict the distribution of water about a sprinkler (Vories et al., 1987). The models treat water droplets as ballistic objects and the equations of motion which include the effects of gravity and the drag on the drops are

Design and Operation of Farm Irrigation Systems
SPRINKLER

579

WIND DIRECTION Wd PATTERN WITHOUT WIND PATTERN WITH WIND LATERAL WIND DIRECTION

LATERAL PERPENDICULAR TO WIND

LATERAL PARALLEL TO WIND

Figure 16.12. Effect of wind on sprinkler distribution and resulting water application uniformity. Note that orienting laterals perpendicular to the prevailing wind direction is generally the most economical arrangement. Table 16.9. Maximum sprinkler and lateral spacing as a percentage of the effective wetted diameter for sprinklers operating at the average pressure along the lateral. Wind Condition Sprinkler Spacing Lateral Spacing No wind 45 65% 8 km h-1 40 60% 35 50% 8-16 km h-1 30 30% > 16 km h-1

solved. The increased drag resulting from high wind speeds causes droplets to move down wind. Seginer et al. (1991b) showed that the distribution could be simulated if the drag coefficient was adequately described. The formulation they used for the drag coefficient was sensitive to the type of sprinkler used. The adjustment to the drag coefficient requires outdoor measurements for acceptable accuracy. The coefficient of uniformity was quite sensitive to the adjustment made to the drag coefficient. Han et al. (1994) developed a mathematical model of the effects of wind on the single-leg distribution. They used an ellipse to describe the effect of wind on the horizontal shape of the distribution pattern and shape patterns across four principal sections of the pattern to predict the depth of application. They also conducted tests of

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Chapter 16 Design and Operation of Sprinkler Systems

numerous combinations of sprinkler devices, nozzle types, operating pressures and wind speeds. While their model is more empirical than the work by Seginer et al. (1991b) it does provide a means to predict the three dimensional distribution of water about a sprinkler in windy conditions.

16.5 SOLID SET SYSTEMS
One method to minimize labor and automate sprinkler irrigation is to permanently, or for a single season, install laterals at intervals across the field. This type of system is called a solid set system (Figure 16.13). Solid set systems are adaptable to a wide range of soils, crops, topography and field shapes. Being relatively expensive, they are commonly used for high-value crops (orchards, turf, and seedling establishment) to save labor and for environmental control. Solid set systems can be temporary or permanent. Temporary systems use aboveground aluminum or plastic laterals placed in the field at the beginning of the season, left in place for at least one irrigation and removed prior to harvest. Permanent systems use buried plastic, asbestos cement, or coated steel pipe for main and laterals with risers or riser outlets aboveground. Controllers allow complete flexibility in operation of solid set systems. Sophisticated solid state controllers are available to control normally open diaphragm valves on sprinkler laterals. Two types of controls are commonly used. For one method, lowvoltage (24 VAC) electrical solenoids are used as pilot valves to control water pressure to a diaphragm. The second method uses a hydraulic system with small tubing to supply air or water pressure directly to the diaphragm to close the valve. Solid set systems are frequently designed with a control valve at the inlet to the lateral. The valves are normally closed and are opened by supplying an electrical excitation for the length of time that water is to be applied. This allows each lateral to operate independently. A controller is used to determine how long each lateral is operated. When multiple laterals are operated simultaneously, the combination is called a circuit or a zone. Solid set systems provide excellent control of the amount of water applied. Some characteristics of solid set systems are summarized in Table 16.1. The disadvantages of to solid set systems include: The high cost to install and maintain—More laterals are required than for a periodically moved system, which increases costs substantially. The electronic valves and the controller also increase the costs. Costs increase substantially when the main line, submains and laterals are buried. All of the working parts also require maintenance for proper operation. The inflexibility due to installation of the system at specific locations in the field—If production practices change, such as changes in implement width or row spacing, it is very difficult to modify the layout of the solid set system to facilitate the new management practices. Its inconvenience to farm around—The systems are difficult to farm around unless perennial crops are planted and remain in the same location for prolonged periods. The advantages of the solid set system are that: The systems apply water uniformly across the field. The systems provide push-button control. Properly designed solid set systems can satisfy auxiliary needs such as frost control.

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Figure 16.13. Diagram of solid set sprinkler irrigation system.

16.5.1 Sprinkler Selection, Performance, and Spacing In general, solid set systems are designed to use low-flow, medium-pressure sprinklers. However, large sprinklers may be used if they are manually moved or individually valved. Sprinkler spacings will vary from 9 m by 9 m, to 73 m by 73 m. Nozzle sizes can be as small as 1.59 mm to as large as 36 mm, pressures range from 172 to 620 kPa. The sprinkler spacing depends upon the sprinkler and nozzle combination, operating pressure, desired coefficient of uniformity (CU), wind speed, and use of the system. For certain high-value crops, it may be desirable to design for a high CU. A crop of lesser value may not justify the cost of a design with a high CU. Since it is not possible to design for all wind conditions, the system should be designed for average conditions. A system designed for frost and freeze protection may not require as high a CU as a system for soil moisture control. A system that is used to supplement rainfall may not need as high a CU as one in which crop production totally depends upon irrigation.

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16.5.2 Designing for Constant Sprinkler Discharge The general design procedure for solid set systems where sprinkler nozzle and pipe sizes are uniform has been described. As indicated there is variation of flow along the lateral. The following procedure can be used to design individual sprinkler laterals, or groups of laterals and associated main line or submain sections where there is less variation once the sprinkler spacing and discharges have been specified. The flow within any section of pipe can be determined. The procedure is iterative for selection of pipe sizes and input pressure. The procedure can be used to design complete systems or subsections of large systems and includes the following steps: 1. Assuming the desired sprinkler spacing has been selected, the first step is to lay out a system of laterals and main line on a topographic map of the field, or measure the elevation of each proposed sprinkler location, as well as the location and elevation of the system inlet. The elevation of each sprinkler outlet position is determined. 2. Calculate the flow in each pipe section as the total of all sprinkler downstream of that section. Flows need to be recalculated whenever the sprinkler flow or the number of operating sprinklers changes. 3. Pipe sizes are selected for main line and lateral sections. Initially, a large, uniform pipe size can be selected for main line and laterals, and then pipe sizes can be reduced in certain areas as the design is optimized. 4. Establish an assumed pressure head at an initial point in the system, the inlet being a convenient point. The initial pressure may be set low or equal to the minimum adequate sprinkler pressure. 5. Calculate pressures at all points by working upstream or downstream one pipe section at a time or by using the pressure distribution relationship previously presented. 6. Evaluate the pressure distribution. If some sprinkler pressures are inadequate, increase the input pressure and go back to Step 5 until the minimum sprinkler pressures are obtained. If all sprinkler pressures are within desired limits, the design may be acceptable, but some pipes may be oversized. If the range of pressures exceeds the desired limit, some pipes may be undersized, or elevation differences may be too large. 7. Reduce pipe sizes in selected areas, usually near the ends of laterals, or low elevation areas, and go back to Step 5. Repeat as necessary until pipe sizes and pressure distribution are optimized. When the pressure distribution has been calculated, the required nozzle size for each sprinkler can be calculated using the specified flow and calculated pressure. If the range of pressures is sufficiently narrow, one nozzle size can be used. Alternatively, pressure regulated sprinklers or flow-control nozzles can be used. If only a fraction of the laterals are to be operated at one time, the main line design should be checked with each set operating to ensure adequate pressure for all sets. The main line size(s) can usually be minimized by spreading the operating laterals as uniformly as possible over the whole main line. However, it may be desirable for cultural reasons to concentrate the operating laterals, in which case the set farthest from the inlet will usually dictate the main line design. The variability of topography within each group of sets must be considered. If elevation differences are great in the direction perpendicular to the lateral, pressure regu-

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lated sprinklers or flow-control nozzles are desirable. Also, if different numbers of laterals may be operated at different times, inlet pressure may change. The sprinkler flow, input pressure, pipe roughness, or any of the pipe sizes can be changed recomputed when using a spreadsheet. If the spacings are changed, the layout and thus the elevations would need to be changed accordingly. The laterals can be shortened by setting downstream pipe diameters to zero. In the case of uphill flows, large unavoidable pressure differences may be encountered. Pressure regulating valves can be located at lateral inlets or other points to reduce and limit the pressure to a specific value. Such controlled-pressure points can be used as the starting point for pressure calculations. Alternatively, individual pressure regulators can be used on sprinklers to limit the nozzle pressure and the inlet pressure can be increased to maintain the minimum pressure throughout the system. The following examples will illustrate the design procedure.
Example 1. A 160-m × 180-m field is to be irrigated as shown in Figure 16.14. A spacing of 17 m along the main line and 15 m along the laterals is selected. The sprinkler flow is 0.8 L s-1. The inlet is at the pump, located one main line section upstream of the first lateral. The pump elevation is 10 m. Figure 16.15 shows the calculations using a spreadsheet program. Numbers in bold type indicate the required input data. Part A gives the pipe roughness (Hazen-Williams C), sprinkler flow, spacings, pump (inlet) pressure and elevation, total flow and computed average and percent variation in sprinkler pressure.
WATER SOURCE AND PUMP
LATERAL 1 2
v v

MAINLINE
6
v

3
v

4
v

5
v

7
v

8
v

9
v

10
v

VALVE

SECTION

1 2 3 4 5 6

9

8

7
7 8 9 10

6 5 4
ELEVATION CONTOUR, m FIELD BOUNDARY

SPRINKLER

3

Figure 16.14. Plan view of field irrigated with a solid set sprinkler system.

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Chapter 16 Design and Operation of Sprinkler Systems

Figure 16.15. Design of mainline and laterals for a solid set system with 10 laterals, where: Pm is the pressure head at the main line outlet or lateral inlet, Pmin is the minimum pressure and DP is the pressure difference on a lateral, Ns is the number of sprinklers on a lateral (ON:OFF = 0,1), Ql is the total lateral flow, Qtot is the total flow in the mailine section, and EL is the elevation of the inlet of a main line section (m).

Design and Operation of Farm Irrigation Systems

585

Part B gives the number of sprinklers or pipe sections on each lateral, elevations and diameters of the main line section upstream of each lateral, and computed main line pressures, minimum lateral pressures, pressure difference on each lateral, flow in each main line section and lateral, and main line velocities. Laterals are designated as being on or off by entering 1 or 0 in the ON:OFF column (Figure 16.15). Individual laterals can be turned off to simulate smaller sets or movable lateral systems. Parts C to F give the elevations, pipe diameters, sprinkler pressure heads, and computed nozzle sizes in grid form corresponding to the grid layout of Figure 16.15. The sprinklers are located at the downstream end of each lateral section. Following the above procedure, the elevations and pipe sizes were input (Parts C and D). The flows in the main and laterals are computed by summing the sprinkler flows downstream of each section. The assumed input pump pressure of 50 m becomes the inlet pressure for the first mainline. All 10 laterals are to be operated simultaneously, so the flow in the first section is 80 L s-1. The diameter is 200 mm, and friction loss is 0.6 m. The outlet pressure for the first section is 50 – 0.6 + 10 – 9.9 = 49.5 m, which becomes the inlet pressure for the second main line section and also the inlet pressure for the first lateral. Pressures were calculated down the first lateral one section at a time, and the minimum was found to be 39.9 m (in section 8 since the laterals are running down slope). Successive main and lateral sections were calculated to the end of the last lateral. The overall minimum pressure head was 39.6 m in lateral 2. Pipe size selection and readjustment is the main iterative process in this procedure. In this example pipe sizes were adjusted so the pressure difference within the laterals is less than 20% of the minimum pressure. The pipe size on the first section of the first 2 laterals was reduced to decrease the lateral pressures and thus reduce the overall pressure variation to less than 20%. The computed nozzle sizes are thus nearly uniform, and one nozzle size can be selected. If variable nozzle sizes are needed to maintain uniform flows, the designer can select the available nozzle size closest to the computed size.
Example 2. This example uses the same field and lateral layout in Example 1, operated by running two adjacent laterals per set. Results are given in Figure 16.16 for laterals 9 and 10, the laterals farthest from the pump. The total flow at the pump decreases to 16 L s-1. The main line sizes are reduced accordingly. The lateral pipe sizes are the same as in Example 1. The average pressure, Pav, and variation Pvar, are for the operating laterals only.

16.5.3 Operation and Maintenance Guidelines The operating mode for a solid set sprinkler system depends upon the design and use of the system, available labor, water supply, and available capital. The system can be designed using the lateral or area (zone) design method. With lateral design individual laterals are controlled by valves and each lateral may be operated as desired. Normally, more than one lateral is operated simultaneously, but the operating laterals usually are widely separated in the field. The lateral design method minimizes the main or supply line pipe size, but it increases the number of valves required and also

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Chapter 16 Design and Operation of Sprinkler Systems

Figure 16.16. Design of mainline and laterals for a solid set system with 2 laterals o perating, where Pm is the pressure head at the main line outlet or lateral inlet, Pmin is the minimum pressure and DP is the pressure difference on a lateral, Ns is the number of sprinklers on a lateral (ON:OFF = 0,1), Ql is the total lateral flow, Qtot is the total flow in the mailine section, and EL is the elevation of the inlet of a main line section (m).

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the time to open and close valves when a manual valve system is used. With the area (or zone) design method, a contiguous portion of the field is irrigated at one time. Usually a submain is installed to supply water to that portion of the field. For frost and freeze protection, the entire system may be operated at one time. Depending upon the crop being protected, the application rate will be 2 to 5 mm per hour. In the eastern U.S., most orchard systems are designed to apply water over the crop. In the western U.S., both undertree and overtree systems are used; however, with saline water only undertree systems can be used successfully. If the system is being used strictly for irrigation, only a portion of the system is normally operated at one time. Where several hours are required for irrigation, control may be manual or automatic. For an irrigation system on a shallow-rooted crop grown on a coarse-textured soil, or in a container nursery operation where daily or frequent irrigation is required, it is best to automatically control the sequencing of the system. Where labor is very limited, automatic control may be desirable regardless of irrigation frequency, however this will increase the initial investment. Conversely, limited capital may require a totally manual system. A limited water supply, such as a well or stream, may mean that only a portion of the system can be operated at once.

16.6 PERIODICALLY MOVED LATERALS
Sprinkler systems in this category have laterals that are moved between irrigation settings. They remain stationary while irrigating. The lateral is drained prior to moving to the next set. Set-move laterals are used extensively because of their relatively low cost and adaptability to a wide range of crops, soils, topographies and field sizes. Equipment cost is largely dependent on the number of sets irrigated by each lateral. They are well suited to soils with high water holding capacity, deep-rooted crops, lowgrowing crops, supplemental irrigation, and deficit irrigation management. They can be classified as hand move or mechanical-move systems. Mechanical-move systems are similar to hand move except that pipe types and sizes are often dictated by mechanical rather than hydraulic considerations. The system consists of laterals, a pipeline with outlets for distributing irrigation water to sprinkler devices that are periodically moved across the field. The lateral is made of pipe sections that are from 50 to 150 mm in diameter and 6 to 18 m long (Figure 16.1). A coupler is installed on one end of the pipe section. The other end of the pipe section is inserted into the upstream coupler and fastened with either a hook that latches into the coupler or a ring assembly. New types of couplers are currently being developed that use different mechanisms to connect the sections of pipe. Gaskets are installed in the coupler to prevent leaks when the system is pressurized. Small pipes, called risers, convey water from the lateral to the sprinkler. Water is supplied to the laterals by main lines, or submains which branch from the main line. The most common system has a single center main line with one or more laterals which irrigate on both sides of the main line. If there are multiple laterals, they are spaced equally, so by the time any lateral reaches the starting position of the lateral ahead of it, the entire field has been irrigated once. The spacing of the sprinklers on the laterals and between subsequent sets of each lateral is such that the water distribution patterns from the sprinklers give almost complete overlap. Large systems often require more complex designs with multiple main lines, although simple systems are possible on rectangular fields up to at least 64 ha.

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Valve-tees are usually placed in the main line at the desired interval for spacing between lateral settings. The valve-tees are controlled by manipulating a valve-opening elbow that makes the connection between the main line and laterals. Common valve spacings are 12.2, 15.2, 18.3, and 24.4 m. Since common pipe lengths are 6.1, 9.1, 12.2, and 15.2 m, the desired valve spacing is obtained by using various combinations of lengths. For many systems buried main lines are preferred. The pipe should be placed safely below plow depth and should also be below frost depth, unless provisions are made for draining the pipeline. The advantages of periodically moved systems are that: investment costs are minimal; the systems offer a great deal of flexibility; the systems are easy to understand and operate; and the sprinklers and nozzles are generally the same size, which maximizes interchangeability. The disadvantages of periodically moved systems are: the high labor requirements; the relatively large applications of water each irrigation; the reduced uniformity when a uniform size of nozzle is used on long laterals or rough terrain; and the time required for laterals to drain before moving. The water application and other characteristics of the hand move, towline and side roll systems are very similar. Characteristics of the system are presented in Table 16.1.
16.6.1 Hydraulics The hydraulic design of moved laterals has been described in Chapter 15 and previously in this chapter. With these systems the sprinkler and nozzle sizes are generally constant and the pipe diameter for the lateral is uniform. Thus, the procedures developed in earlier sections apply. The general procedure is to layout the field boundaries, water supply and pump location. The main line is frequently laid down the center of the field to minimize pressure losses in long laterals. The spacing between sprinklers and laterals is selected to fit the field. With an initial layout the discharge for the sprinkler and lateral are determined. The minimum average pressure is determined for the selected size of lateral pipe. From these data the nozzle sizes can be determined. The diameter of coverage of the sprinkler must be large enough to provide adequate overlap. If all components are adequate the inlet pressure for each lateral position should be determined. The main line should be selected to provide the highest feasible uniformity. The cost of design alternatives should be determined. For the final design, product specifications and an operational plan should be developed and discussed with the client. Special considerations for each type of periodically moved system are discussed below. 16.6.2 Hand Move Laterals The earliest periodically moved systems involved laterals that were moved by carrying sections of pipe across the field. Between moves the lateral operates for a period time (the set time) and applies water to a portion of the field (a set) (Figure 16.17). This is called a hand moved system. An extensive amount of labor is required to move laterals from one set to the next set, which discourages frequent movement resulting in large applications of water per irrigation.

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Figure 16.17. Picture and operational sketch of a hand move irrigation system. Common pipe diameters range from 51 to 152 mm, and pipe lengths are 6.1, 9.2, and 12.2 m. The most popular aluminum lateral length is 9.1 or 12.2 m. Shorter lengths mean more walking during the move. Longer lengths are more difficult to transport and do not provide proper spacing for the common sprinkler sizes.

Hand move systems are the lowest equipment cost alternative for moved-lateral systems, but are labor intensive. Lack of available labor is the main reason farmers are tending toward center pivot or other automated systems. Most hand move sprinkler systems now use aluminum laterals, although plastic is available. Most lateral pipe couplers contain a chevron-type rubber gasket which seals under pressure, and are designed to latch automatically when the pipes are pushed together. Many couplers contain an optional adjustment for easier unlatching. Hook and latch type couplers can be fixed so they will unlatch automatically when the irrigator pushes and twists the pipe. Ball and socket couplers automatically latch when the pipe is under pressure, and

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release when the pressure is off. Drop-lock couplers have hooks that engage as the pipe is lowered to the ground. The automatic unlatch saves some walking, but could increase the hazard of an accidental unlatching. In contrast to the main line gaskets, the lateral gaskets are designed to release their tight contact with the pipe when the pressure on the water is reduced. This permits the water to drain from the pipe when the pressure has been turned off so the pipe can be moved easily to the next setting. Each coupler is threaded to receive a sprinkler riser pipe, usually 25 mm in diameter. If both the coupler and the riser are aluminum, it is customary to connect them with a zinc alloy fitting or Teflon tape to avoid thread seizure. The riser should extend at least to the top of the crop canopy, but the uniformity of water distribution is improved if it is extended another 0.5 m. The uniformity of water distribution can usually be improved by using an offset pipe with a 90° elbow every second irrigation. The length of the offset should be half of the spacing between lateral settings. Using the offset pipe permits the lateral to be placed midway between the positions used during the previous irrigation. Thus, considering two irrigations added together, a 12.2-m by 18.3-m spacing, for example, is effectively reduced to 12.2 m by 9.2 m. A good procedure for the irrigator to follow when moving the lateral from one setting to the next is to start by moving the valve opening elbow and the section of pipe connected to it. As soon as these pieces are in place at the new location, the valve is slightly opened so a very small stream of water runs out the end of the first pipe section. As each subsequent section of pipe is put into place, the small stream of water runs through it, flushing out any soil or debris that may have been picked up during the move. The last section of pipe with its end plug in place can be connected before the stream of water reaches the end and builds up pressure. Then the irrigator walks back along the lateral, correcting any plugged sprinklers, leaky gaskets, or tilted risers. After returning to the main line, the valve is opened further until the desired pressure is obtained. A quick check with a pitot gauge on the first sprinkler confirms the valve adjustment. To save time on each lateral move, there is a tendency to completely open the valve and fill the line as quickly as possible. This causes water hammer at the far end of the line, so a surge plug at that end may be needed. The sprinklers commonly used on hand move systems may have either one or two nozzles. Typically, individual sprinkler capacities range from about 0.06 to 0.63 L s-1. Operating pressures range from 240 to 415 kPa. Certain crops, such as orchards, require specially designed sprinklers. When the sprinklers are used over the tops of the trees, conventional models can be used. However, when they are used under trees, sprinklers with low water trajectory must be used. Lowering the trajectory reduces the uniformity, unless the spacing is reduced. Hedge-rowed trees present a difficult problem, especially if it is desirable to irrigate through the skirts of the rows.
16.6.3 Towline Systems Towline or skid-tow systems were developed to reduce labor required to reposition laterals. The most efficient layout for towlines is to divide the field in half so the lateral can be pulled in a zigzag fashion across the field (Figure 16.18). Towline sprinkler laterals have relatively rigid couplers fitted with skids or wheels so the line can be moved by pulling it from the end. The skids consist of a flat metal plate held on the

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Figure 16.18. Picture and operational sketch of a towline or skidtow system.

underside of the pipe by one or more clamps. In one type, the skid is placed under the coupler and clamped at both ends. This makes the skid take the major part of the end thrust at the coupler when the pipe is towed. If relatively long sections of pipe are used, a second skid may be needed under the center of each section to reduce abrasion from soil contact. Stabilizers, outriggers, or wheel-mounts were used to keep the lateral from tipping. Two or three outriggers along the line are needed to keep the pipe oriented with the skids on the bottom and the sprinklers upright. For wheel-type units, a pair of wheels mounted on a simple U-frame is clamped to each section of pipe. The wheels are oriented so the entire length of lateral pipe can be pulled endwise. The pipe itself stands only 0.3 to 0.5 m above the ground. The flexibility of the pipe and the articulation of the couplers permit the lateral to curve slightly while being moved to a new position. In one type, however, the lateral stays straight. The wheels are fixed so they shift to a 45° angle from the lateral when pulled in one direction, and then shift back to a 45° position the other way when pulled from the

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other end of the lateral. Thus, by pulling alternately from both ends, the entire length of lateral is shifted the desired distance to the next set position.A coupler and hitch are attached to each end of the lateral so that it can be towed in either direction. An end cap is used to plug the downstream end of the pipeline. Drain plugs are installed along the lateral to empty water from the pipeline before moving. Often a flexible hose is used to connect the lateral to the main line. If more than one lateral is used in a field, provisions need to be made to split the main line while moving the lateral. A telescoping coupler can be used for this purpose. When the lateral reaches the edge of the field, the lateral must be disassembled and transported to the starting location. If the surrounding property is amenable, the lateral can be towed to the starting position. The traditional way of moving a towline lateral is to snake it past the main line in an S-shaped curve to a new position on the other side (Figure 16.18). For the next setting, the lateral is pulled in the other direction past the main line in an opposite Sshaped curve. With this procedure, each move needs to advance the lateral only half the distance between adjacent sets. Towline systems are the least expensive of the mechanically moved systems. However, they are not used extensively because the moving process is tedious, requires careful operation, and damages many crops. Towline systems have been used successfully in some forage crops and in row crops. The moves are made easier if the main line is buried.
16.6.4 Side Roll Systems The side roll, or wheel-move, system is a third type of periodically moved lateral. With this system, wheels are mounted on the sprinkler lateral to carry the pipeline above the crop (Figure 16.19). A cart provides power to rotate the wheels. The cart and the water feed to the side roll can be positioned anywhere along the lateral. Frequently the pipeline is used as the axle for the torque; however, a separate drive shaft can be used to rotate the pipe. Multiple laterals are often used in a field. A special apparatus with a swivel and weight is used to keep sprinklers vertical when the pipeline rotation is not precise. The mainline may be above or below ground. Flexible hoses are often used to connect the side roll lateral to the mainline. Rigid couplers permit the entire lateral to be rolled forward by applying torque at the center while the pipe remains in a nearly straight line. Aluminum pipe having a 100 or 125 mm diameter is commonly used. To have sufficient strength, the aluminum pipe wall thickness should be at least 1.8 mm. A motorized drive unit, usually near the center of the lateral, provides torque to move the lateral and holds the pipe in place during operation. Normally, the drive unit contains a gasoline engine and a transmission with a reverse gear. Electric motors or hydraulic motors are also used. Typical lateral length is 400 m, but longer lengths are made with two drive units spaced about ? the lateral length apart, and connected by a drive shaft. Since the greatest torque is applied to the pipe near the drive unit, 125-mm pipe is sometimes used near the center of the lateral for greater strength. The pipe is flexible enough so that these systems can be used on rolling topography with mild slopes. The most popular sprinkler spacing (and pipe length) is 12.2 m. The wheels are usually placed in the center of each length of pipe, with sprinklers located halfway between the wheels. Thus, a standard 400-m lateral contains 32 pipe lengths and 36 wheels because four wheels are required for the drive unit. Sometimes an extra wheel is provided for the last pipe section at each end.

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Figure 16.19. Picture and operational sketch of a side roll irrigation system.

The rigid couplers can be quickly disconnected to shorten the lateral in the case of odd shaped fields. Often the sprinklers are provided with self-levelers so they will right themselves if the lateral is not stopped where the riser would be exactly upright, i.e. on variable topography, and to aid in aligning the laterals with main line valves. Labor requirement is approximately five minutes per lateral per move. Some side roll drive units can be controlled from the end of the lateral, eliminating the need to walk to the center drive unit. At least one manufacturer has developed an automated side roll system that can be programmed to drain and move itself and irrigate up to five sets. Water is supplied through a flexible hose. The wheel diameter must be large enough so the pipe will pass over the crop without damaging it, and the crop will not prevent the lateral from being rolled to the next position. Common wheel diameters commonly are 1.17, 1.47, 1.63, and 1.93 m. A spring-loaded drain valve also is located about midway between the wheels, near the pipe coupler and near the sprinkler. This valve opens automatically when the pressure is off, so the pipe will drain quickly and permit moving the lateral forward to the next set without much time loss. (Attempting to roll the pipeline when it is full of water will damage the equipment).

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The most popular side roll spacing along the main line is 18.3 m. Two popular operating schemes are used. In one, the lateral is connected to every outlet valve along the main line, and when the lateral reaches its destination and completes its last set, it is rolled back to the starting point. In the other, the lateral is connected to every evennumbered outlet valve on the main line while the lateral is moved across the field, and then connected to the odd- numbered valves while the lateral is moved back to the starting position. For the latter case the lateral interval between irrigations is longer at the two ends of the field than in the center. As with hand move laterals, there is a tendency to completely open the hydrant valve and fill the line as quickly as possible, causing a water hammer at the far end of the line. Therefore, a surge plug at the closed end is recommended. The use of offsets, especially for the 12.2 m by 18.3 m spacing, is also recommended. Side roll laterals are highly susceptible to wind damage when they are empty, and should be staked down during the off season. Special braces, which allow the lateral to roll in one direction only, help protect the laterals during the irrigation season. A lateral with 32 sprinklers is commonly designed with 100-mm diameter pipe, even when the water is introduced into it from one end and the friction loss is 55 to 60 kPa. However, if the water is admitted to it at the center of the lateral, the friction loss is reduced to about 1/5 as much. A 125-mm pipe would have only about 1/3 the friction loss of the 100-mm pipe when the water is admitted from one end. The best method will depend on the future price and availability of energy. End-feed laterals are usually preferred because a drive-through roadway can be maintained along the main line for easy accessibility to the valves.
16.6.5 Side Move with Trail Lines Side move laterals with trail lines are supported on wheel-mounted A-frames. The pipe does not serve as the axle of the wheels and can be higher above the ground. Each A-frame carriage is driven from a drive shaft that extends the length of the pipeline. The drive shaft can be turned from the center of the line or from one end. One version uses a continuous-move (center pivot) type lateral operating in stationary set-move mode. The wheels are powered by electric or hydraulic motors supplied from an onboard generator or hydraulic pump. The small diameter trail lines can each carry several sprinklers. Usually, short sprinkler risers are used as they are easier to keep upright than tall risers. Outriggers at the last sprinkler on each trail line are used to keep the risers upright; however, these may damage some crops. This system is sometimes called a “movable solid set.” It greatly reduces the number of moves necessary to cover the field, thus saving labor. Sprinkler spacings and nozzle sizes that give low application rates at an acceptable uniformity can be used. With the low rates, 24-h set times may be practical for some soils and crops, thus permitting a normal daytime work schedule for the irrigator. When a trail line system reaches the end of the field, the trail lines are uncoupled, and the lateral is moved to the opposite ends of the trail lines, which are then recoupled to irrigate on the way back across the field. The wheels on most side move systems can be turned 90°, permitting the lateral to be pulled endwise to another field.

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16.7 CENTER PIVOTS
In 1948 Frank Zybach invented the self-propelled irrigation system. His invention used towers with wheels to carry a pipeline around a pivot point in the field. Even though his invention has undergone numerous changes, the basic concept is still used. A span of pipe is supported by a tower and a truss system (Figure 16.20). Today most systems are propelled by electrically or hydraulically powered motors mounted on each tower. A system of switches on each tower energizes the motor when the tower needs to move. The depth of water application is generally controlled by selecting the speed of the last or end tower. For many electric systems a one-minute timer is used to control the velocity. If the timer is set to 100%, the motor on the end tower is energized the entire minute causing the end tower to move at a constant velocity equal to the maximum speed of the system. If the timer is set to 50% the motor is only energized for 30 seconds; thus, the end tower is stationary for 30 seconds and then moves for 30 seconds at the maximum speed for the end tower. Some hydraulic and electric systems provide continuous movement of the end tower at variable speeds to apply the desired depth of application. The interior towers are controlled by switches or valves mounted on the tower. One switch or valve is set to energize the motor if the downstream tower has moved far enough to exceed a start angle. The switch or valve energizes the motor and causes the tower to move at a constant velocity. The tower moves until the angle between adjacent spans exceeds a stop angle. For continuously moving systems the interior towers move continuously at varying speeds to maintain alignment. The interior towers may be designed to move at a faster velocity than the end tower allowing them to catch up with the end tower to maintain alignment. Controllers are presently available to change the speed and/or direction of rotation of the pivot lateral as the system circles the field. This is useful if different crops are planted under a pivot or if obstructions are located in the field. Small center pivots are also made that can be moved within a field, or from field to field. This allows for semi-automated irrigation of irregularly shaped fields and small tracts of land. Center pivots can be equipped with an end gun to increase the portion of a field that is irrigated (Figure 16.20). The end gun is a large sprinkler similar to that used on a traveler that is mounted on the end of the pivot lateral. The gun throws water a long distance thereby increasing the amount of land irrigated for a given length of lateral. A valve is attached to the end gun so that the end gun only operates in the corners of the field. When the pivot lateral reaches a preset angle of rotation, the valve opens and water is supplied to the end gun. In some cases a booster pump is attached to the valve to increase the pressure for the end gun. A corner system can be used to irrigate an even larger portion of a square field. A special span is attached to the end of a typical system (Figure 16.21). The corner span revolves about the end of the pivot lateral. The corner span is tucked behind the main lateral when the boundary of the field is close to the end of the pivot lateral. The sprinklers on the corner span begin to irrigate when the pivot rotates to an angle where the sprinkler package on the primary lateral does not reach to the field boundary. Sprinklers on the corner span are attached to a series of valves. As the pivot lateral rotates toward the corner of the field, the corner lateral extends and opens valves as the corner lateral is extended. An end gun can be attached to the corner lateral to throw

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Figure 16.20. Components and field layout for typical center pivot irrigation systems.

water further into the corner. Many corner systems use an underground cable and an antenna on the corner tower to follow a desired path. The cable can be positioned to irrigate irregular shapes and to move around permanent obstructions. Center pivots have many advantages including: Automated operation—Center pivots can be operated with minimal labor often making several revolutions without stopping. They can also be controlled remotely from farm vehicles or computers. Ability to apply small irrigation depths—Since the systems are automated, they can apply small irrigations to match crop needs without leaching nutrients. Very high uniformity—Since the lateral moves slowly and because there is a great deal of overlap of water application from successive sprinklers on the lateral, center pivots apply water very uniformly. Chemigation—Pivots can be managed to quickly and uniformly irrigate a field with small amounts of water, which provides the opportunity to apply fertilizer, herbicides, and insecticides.

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PRIMARY LATERAL PRIMARY LATERAL CORNER SPAN CORNER SPAN

CORNER CORNER TOWER TOWER
FIELD BOUNDARY

+ UN M G TE ND S E SY D IC AN S M BA R S A IU ER D N RA OR C

RADIUS OF BASIC SYSTEM

Figure 16.21. Picture and operational sketch for a center pivot equipped with a corner watering system.

Little annual setup is required—Once the system is constructed, the pivot can be operated anytime that water is needed. This is advantageous for germination of crops or for preparing seedbeds. Some disadvantages of center pivot systems are its: Cost—Depending on the reference, the cost of a center pivot system is moderately high. The cost per unit of land for a typical center pivot is approximately 20 to 30% of that for solid set and microirrigation systems. However, the cost exceeds that for systems requiring more labor such as hand move or towline systems. The cost per unit area decreases as the length of the lateral increases; however, several factors limit the maximum length of the lateral. High application rates—The rate of water application at the outer end of the lateral is quite high, which can cause runoff.

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Circular pattern—Center pivots irrigate about 80% of a square field. Lost production in the corners may be a consideration when land is expensive or highvalue crops are produced. The self-propelled irrigation system as named by inventor Frank Zybach in 1948 has transformed irrigation. Many lands are now efficiently irrigated that were once labeled as unsuitable for irrigated agriculture. Center pivot systems as developed by the irrigation industry have reduced labor requirements, improved water application efficiency and contributed to the economic viability of many regions. Nevertheless there have been failures with center pivots. Problems have arisen due to poor design and improper management. In some locations excessive development has led to the overdraft of water resources causing declines in ground water supplies. Without proper management leaching of agricultural chemicals can occur especially under shallow rooted crops grown on sandy soils. Other chapters discuss the planning and operation of irrigation systems to fit within environmental, economical and resource constraints. The focus in this section is on the design of center pivot systems to efficiently and uniformly apply irrigation water.
16.7.1 Sprinkler Discharge The discharge from each sprinkler on a center pivot must be determined to apply water uniformly. The area midway between the adjacent upstream and downstream sprinklers defines the representative area for a sprinkler on a pivot (Figure 16.22).
FIELD BOUNDARY

RS

R

SL

SPRINKLER SPACING

SL

SL

RADIAL DISTANCE TO SPRINKLER (R)

PIVOT LATERAL

SL R – SL / 2 R + SL / 2

Figure 16.22. Diagram of representative area for determining the discharge for sprinklers on a center pivot lateral.

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This area is area is given by

AR = 2π R SL

(16.19)

where AR = representative area for the sprinkler at distance R from the pivot point SL = local spacing between sprinklers on the pivot lateral, m. It is common for the spacing between sprinklers to vary in intervals along the pivot lateral. Often the spacing near the center of the pivot is double the spacing near the outer end of the pivot. The average discharge per unit area for the pivot is the ratio of the flow into the pivot system divided by the circular area irrigated by the primary system (i.e. where the end gun or corner system is not operating). Combining these relationships provides the discharge required for the sprinkler located at radial distance R from the pivot point: 2QS RS L qR = (16.20) 2 Rs where qR is the discharge required for a sprinkler located a distance R from the pivot point and QS is the total discharge into the center pivot system. The expression can be revised to include the gross system capacity as

qR = 2 × 10-4π Cg R SL

(16.21)

To design the sprinkler package the nozzle sizes for each sprinkler must be determined. This requires information on the distribution of pressure along the lateral. For level land the relationship developed by Chu and Moe (1972) can be used for the distribution: 3 5 ?? ? ? 15 ? R 2 ? R ? 1 ? R ? ?? ? ? ? + ? PR = PS + PL ?1 ? ? ? ? (16.22) ? 8 ? Rs 3 ? Rs ? 5 ? R s ? ?? ? ? ? ? ? ? ? ? where PR = the pressure in the lateral at point R PS = pressure at the distal end of the pivot lateral PL = pressure loss from the inlet into to the distal end of the pivot lateral. Keller and Bleisner (1990) and Scaloppi and Allen (1993) presented methods to compute the pressure distribution along pivot laterals. Their results produce the same pressure distribution along the lateral as that from Chu and Moe (1972). The relative distribution of pressure loss along the lateral is shown in Figure 16.23. The difference between the methods by Chu and Moe (1972) and Scaloppi and Allen (1993) is in the procedures used to compute the friction loss from the inlet to the distal end of the lateral (i.e., PL). Chu and Moe used the Hazen-Williams equation. Scaloppi and Allen used the Darcy-Weisbach equation. Scaloppi and Allen considered the variation in the velocity head in the lateral as well while Chu and Moe considered it to be negligible. In Chapter 15 the Darcy-Weisbach method was recommended for hydraulic calculations. We concur with that recommendation for applications using computer spreadsheets and/or programs to compute friction loss. Applying the DarcyWeisbach method for sprinkler laterals results in changing friction factors along the pipeline. This is difficult to represent in this chapter. Thus, the hydraulics for center pivot pipelines is illustrated using the Hazen-Williams equation for this chapter.

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FRACTION OF PRESSURE LOSS ALONG LATERAL

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

FRACTION OF DISTANCE ALONG LATERAL

Figure 16.23. Distribution of pressure loss along a center pivot lateral.

The pressure loss along pivot laterals is traditionally computed by the industry using the Hazen-Williams equation (see Chapter 15). Typical C values for center pivot materials are provided in Table 16.10. Since the discharge from sprinklers varies along the lateral for a center pivot the F value for pivot laterals is different than for other sprinkler laterals. Results from Chu and Moe (1972) show that the F value for pivots without end guns is 0.54. Pair et al. (1983) show that the F value is 0.56 when the end gun is operating. These values have been compared with a procedure that computes the pressure loss between each sprinkler along the pivot lateral as used by Kincaid and Heermann (1970). Comparisons show that the method by Chu and Moe (1972) is sufficiently accurate. Based on these procedures the pressure distribution along pivot laterals can be computed using Table 16.10. These results provide the friction loss along laterals that only have one diameter of pipe. Figure 16.23 shows that over half of the total loss along the lateral occurs within the first third of the lateral and that 80% of the pressure loss in the lateral occurs in the first half of the pipeline. Experience has shown that operating pressures and therefore operating costs can be reduced by using larger diameter pipe for the first portion of the lateral. Computing the friction loss for laterals that contain two diameters of pipe can be computed for R ≥ Rc as
? ? R 2 ? R ? 3 1 ? R ?5 ? PR = PS + PLs ?1 ? 1.875 ? ? ? ? + ? ? ? ? Rs 3 ? Rs ? 5 ? Rs ? ? ? ?? ?? ? ?? ??

(16.23)

and for R ≤ Rc:

? PC = PR + PLs ?1 ? 1.875 ? ? ? PR = PC + PLL ? 1.875 ? ?

? Rc 2 ? Rc ?3 1 ? Rc ?5 ? ? ? ? ? + ? ? ? Rs 3 ? Rs ? 5 ? Rs ? ?

?? ?? ? ?? ?? ? ? ? ?

3 5 ?R 2 ? Rc R 3 ? 1 ? Rc R5 ? ? ? C R ? ? ? ? 5 ? 5 ?? ? 3? 3? + ? 3 ? Rs Rs ? 5 ? Rs Rs ? ? RS RS ? ? ?? ? ? ?

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Table 16.10. Friction loss (m/100 m) in galvanized steel pipe for center pivot and lateral move systems. Results assume a C value of 140 for the Hazen-Williams equation and a single pipe size. To compute losses for a pivot multiply values times the lateral length and the appropriate F factor. System Outside Diameter of Pipe (mm) Discharge -1 102 114 127 141 152 168 178 203 219 254 (L s ) 5 0.57 10 2.06 1.12 15 4.36 2.38 20 7.42 4.06 25 11.2 6.13 30 15.7 8.60 35 20.9 11.4 40 26.8 14.6 45 33.3 18.2 50 22.1 60 31.0 70 80 90 100 120 140 160 180 200 220 240 260 F factor for pivots: Adjustment for pipes with different roughness

1.39 2.37 3.58 5.02 6.68 8.56 10.6 12.9 18.1 24.1 30.9

1.38 2.09 2.92 3.89 4.98 6.19 7.53 10.6 14.0 18.0 22.4 27.2 38.1

1.42 1.99 2.65 3.40 4.22 5.13 7.20 9.57 12.3 15.3 18.5 26.0 34.6

1.21 1.61 2.06 2.56 3.12 4.37 5.81 7.44 9.25 11.3 15.8 21.0 26.9 33.4

1.22 1.56 1.94 2.36 3.31 4.41 5.64 7.02 8.53 12.0 15.9 20.4 25.3 30.8 36.7

Without end gun: 0.54 C value 100 110 Multiplier 1.86 1.56

1.00 1.21 1.70 1.16 2.26 1.55 2.89 1.98 3.59 2.47 1.18 4.37 3.00 1.43 6.12 4.20 2.01 8.14 5.59 2.68 10.4 7.16 3.43 13.0 8.91 4.26 15.8 10.8 5.18 18.8 12.9 6.18 22.1 15.2 7.26 25.6 17.6 8.42 With end gun: 0.56 120 130 140 150 1.33 1.15 1.00 0.88

where RC = location along the lateral where the pipe diameter changes PLS = pressure loss from the inlet to the distal end for the small diameter pipe PLL = pressure loss from the inlet to the distal end for the large diameter pipe. An example of the use of this procedure is included in the example shown in Figure 16.24. The example illustrates that the investment in larger pipe for the first 40% of the lateral would reduce the inlet pressure by approximately 20% while using larger pipe along the entire lateral would only reduce the pressure by 29%. A cost analysis is required to determine which alternative is optimal. The above computations for pressure distribution along the lateral neglected the effects of elevation changes. If there is a uniform slope along the lateral the pressure can be adjusted appropriately and the proper nozzle sizes can be selected to provide the desired discharge. However, the performance of the pivot across a sloping field varies once a specific set of nozzles is installed. Often design along a flat slope is a reasonable compromise for sloping fields. For many fields irrigated with pivots the terrain is

602
450 400

Chapter 16 Design and Operation of Sprinkler Systems

168 mm DIAMETER PIPE MIXED PIPE SIZES

PRESSURE IN LATERAL, kPa

350 300 250 200 150 100 50 0 0 50 100 150

203 mm DIAMETER PIPE

80 L/s FLOW RATE

200

250

300

350

400

450

500

RADIAL DISTANCE FROM PIVOT, m

Figure 16.24. Distribution of pressure along center pivot laterals with external diameters of 168 and 203 mm, and for a mixed lateral with 200 m of 203-mm and 300 m of 168-mm pipe.

neither uniform nor flat. Since the design of sprinkler packages is usually accomplished with computer programs that start at the end of the lateral and determine pressure loss and nozzles sizes for each outlet along the lateral, the elevation at each location can be used in the calculation. Pressure regulators are commonly used if the elevation differences in the field lead to pressure variations that would reduce the uniformity of application below an acceptable level. The procedures from Chu and Moe (1972) are useful in understanding the hydraulics of sprinkler laterals and for simple analysis of pivots. However, center pivot systems are typically designed using computer programs that start at the distal end of the pivot and sequentially compute the pressure available at each upstream sprinkler outlet (Heermann and Stahl, 2006). In these programs the friction loss in the portion of the lateral between outlets is computed using either the Darcy-Weisbach or HazenWilliams equations for a pipe without outlets. The pressure losses through fittings used to connect the sprinkler to the lateral are also computed. This has become more important as the operating pressure for sprinkler devices continues to decrease to save energy and as the sprinkler device is installed further from the pivot later using drops. Using the sprinkler discharge equation for the required sprinkler discharge gives the required flow at each outlet. With this information and the available pressure at the base of the sprinkler device, the most appropriate nozzle size can be determined. Since a finite number of nozzle sizes are available, it is not possible to exactly match the required discharge. Many designers maintain the cumulative error between the actual total flow in the system and the required flow. At each outlet the nozzle is selected that closely matches the required discharge for the outlet and that minimizes the cumulative flow error. These types of programs provide very detailed specification of system characteristics and allow for consideration of the terrain of the land.

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603

16.7.2 Uniformity of Application As with other types of sprinkler systems the uniformity of application beneath center pivots can be computed by considering the overlapped depth of application from individual sprinklers. This analysis provides guidance for the most appropriate spacing of sprinklers devices along the lateral. The procedure is more complex for pivots since individual sprinklers are not stationary but travel in circular paths about the field. Bittinger and Longenbaugh (1962) developed the framework for analyzing the distribution of water from moving sprinkler systems. They considered the case of sprinklers moving in a straight line and in a radial pattern. They showed that sprinklers that travel on an arc produce a skewed application pattern that is somewhat difficult to analyze. However, the effect of the skewed pattern is negligible when the distance from the pivot point to the sprinkler is more than five times the wetted radius of the sprinkler. For center pivots the inaccuracy of uniformity calculation incurred from assuming linear travel of sprinklers will be small, thus only the solutions for linear travel are presented here. The framework for the analysis by Bittinger and Longenbaugh, which considered triangular and elliptical water application patterns for an individual sprinkler, is illustrated in Figure 16.25. The precipitation rates for the triangular and elliptical application rates are given by Ppt (Wr ? s ) for triangular patterns with 0 ≤ s ≤ Wr P= Wr

and

P=

Ppe Wr

Wr2 ? s 2 for elliptical patterns with 0 ≤ s ≤ Wr

(16.24)

where P = precipitation rate at distance s from the sprinkler, mm h-1 Pp = peak precipitation rate at the sprinkler location, mm h-1 Wr = wetted radius of the sprinkler, m s = distance from the observation point to the sprinkler, m t, e = subscripts denoting triangular and elliptical patterns. The peak application rate for the triangular and elliptical patterns can be computed from the discharge of the sprinkler located at radial distance R and the wetted radius of that sprinkler:
Ppt =

3q R πWr2

for triangular patterns and Ppe =

3q R 2πWr2

for elliptical patterns

(16.25)

The solutions for the depth of water applied at a point from an individual sprinkler as the pivot lateral moves over the point were developed by Bittinger and Longenbaugh (1962). Heermann and Hein (1968) used this procedure to compute the depth of water along a radial line from the pivot point to the end of the center pivot. The depth applied at a point is the summation of water applied by all sprinklers that apply water to the point and is given by for triangular patterns: d =
? 2 Wri Ppti ? ? 1 ? u 2 ? u 2 ln? 1 + 1 ? ui i i ? ω i =1 Ri ? ? ui ? ? ?

1



N

?? ?? ?? ? ?? ?

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Chapter 16 Design and Operation of Sprinkler Systems
TRIANGULAR APPLICATION RATE

AVERAGE

ELLIPTICAL

DISTANCE FROM SPRINKLER (s)

Wr

PATH OF SPRINKLER

OBSERVATION POINT

ET ANC IST LD IA RAD

ER NKL PRI OS

(R)

Wr

s x u Wr

PIVOT POINT

PIVOT LATERAL

SPRINKLER

AREA WETTED BY A SPECIFIC SPRINKLER

Figure 16.25. Illustration of parameters used to compute uniformity of water application for center pivot systems.

and for elliptical patterns: d =

π N Wri Ppei 1 ? ui ∑ Ri 2ω i =1

(

2

)

(16.26)

where d = depth of water applied at a point u = ratio of the distance from the sprinkler to the point relative to the wetted radius of the sprinkler,

Design and Operation of Farm Irrigation Systems

605

ω = angular velocity of the pivot lateral, Ri = radial distance from the pivot point to sprinkler i, N = number of sprinklers that apply water to the point of interest. The angular velocity can be determined from the system capacity and the average depth of water applied (da): 2πCg ω= (16.27) da
In using these equations a series of points along a radial line is selected that represents the density of observations desired for computing the uniformity. At each point Equation 16.26 is used to compute the aggregate depth of application. Heermann and Hein (1968) used this technique to compute the coefficient of uniformity for pivots as given by ? ? ∑d j X j ?? ?? ? ? j ?? ? ∑? X j d j ? ∑ X j ?? ? j ? ?? ? j ? ? (16.28) UC p = 100 ?1 ? ? ? ∑d jX j ? ? j ? ? ? ? ? ? ? ? where UCp = the uniformity coefficient for center pivots Xj = distance from the pivot point to the point that the depth is computed dj = depth of water applied at point j. Field results and simulation models have shown that the uniformity of application is very high for well-designed and operated center pivots. It is common to find uniformities greater than 90 for center pivots. This is higher than most other types of sprinkler systems or alternate irrigation methods. The previous analysis assumed that the angular velocity was constant. Of course center pivot lateral do not rotate at a constant angular velocity. Some pivots are designed so that the towers move at a constant velocity for a period of time and then the tower is stationary until a signal is received for the tower to move again. Other pivots are designed to move continuously but not at a constant velocity. If a pivot tower is stationary at one point the area watered during that time receives a larger application than areas irrigated when the lateral is moving. If sprinkler devices are used that have a small wetted radius, the start-stop motion of the later can lead to nonuniformity, as illustrated by Hanson and Wallender (1985). With this program they were able to simulate the performance of the sprinkler package for any terrain and for the adjustments of the pivot system that control the start-stop nature of the pivot. This procedure allows for efficient evaluation of many design alternatives. The development presented is based on triangular or elliptical application patterns for a single sprinkler. Devices that have been introduced recently have a different application pattern that those analyzed by Bittenger and Longenbaugh (1962). The procedure presented here should be combined with the single-leg distribution for the packages that are of concern to assess the performance of sprinkler packages.

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Chapter 16 Design and Operation of Sprinkler Systems

At this point the nozzle size for each sprinkler along the lateral can be computed for alternative sprinkler packages. The operating pressure at the pivot point can be estimated for the field terrain and each sprinkler package. Ultimately, the uniformity of application can be computed (Heermann and Stahl, 2006). However, two additional features, runoff and evaporation losses, have a bearing on the suitability of a design.
16.7.3 Runoff Avoidance For center pivots to operate efficiently the applied water must be available for crop use. If the water runs off sloping lands or evaporates while in the air, the design efficiency will not be achieved. Keller and Bleisner (1990) provide a simple diagram of the suitability of center pivots based on the soil texture (Figure 16.26). While this helps with the general suitability, it is not adequate for design of the center pivot. The water application rate beneath the outer spans of a center pivot is very high. The application rate may exceed the ability of the soil to infiltrate water. Some of the water applied at rates exceeding the infiltration rate can be stored temporarily on the soil surface. This is called surface or retention storage. Once the local surface storage is filled, the excess application begins to flow across the field. Some infiltration occurs as the water flows across the field. Ultimately, the runoff water either accumulates in low areas in the field leaves the field. In either case the distribution of water that infiltrates is different than the distribution of water applied and some water is lost as runoff or deep percolation. Thus, both uniformity and efficiency are reduced when runoff becomes significant. Kincaid et al. (1969) investigated the potential for runoff from center pivot irrigation systems. They developed a procedure to adjust the infiltration rate measured with systems that pond water on the soil surface for conditions that occur under center pivots. Dillion et al. (1972) were the first to develop design procedures that considered the infiltration characteristics of the soil. A combination of these procedures was used by Gilley (1984) to develop guidelines for the selection of sprinkler packages based on
PE PE RF RC OR MA EN RG MA TC INA LA NC L Y E

LIM ITE D

90 80 70

10 20 30 40

AN TIC IPA EX TE CE DC LL EN EN T TE RP IVO GO O T

60

ILT TS EN RC PE

CLAY SILTY CLAY

D

50

50

40

60

SANDY CLAY

30
SANDY CLAY LOAM

CLAY LOAM

SILTY CLAY LOAM

70

20

80
LOAM

10

LO AM Y

SANDY LOAM
SA ND

SILT LOAM SILT

90

SAND

90

80

70

60

50

40

30

20

10

PERCENT SAND

Figure 16.26. Anticipated performance of center pivot systems based on soil texture (adapted from Keller and Bleisner, 1990).

Design and Operation of Farm Irrigation Systems
70 INFILTRATION RATE 60 PEAK APPLICATION RATE

607

RATES, mm / hour

50 SURFACE STORAGE APPLICATION RATE

40

POTENTIAL RUNOFF

30

20

10

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

TIME, hours

Figure 16.27. Illustration of the runoff potential when the application rate of the pivot exceeds the infiltration rate of the soil.

the potential for runoff. With this method an elliptical pattern is assumed for the sprinkler package. The combined distribution of water from the overlap of individual sprinklers provides an elliptical application rate perpendicular to the lateral (Figure 16.27). This application rate is given by
P (t ) = Pp tp 2 t t p ? t2

(16.29)

where P(t) = rate of water application as a function of the time (t) that water has been applied Pp = peak application rate for the elliptical pattern of the sprinkler package tp = time after initial wetting that the peak application rate is reached. With this distribution for the sprinkler package the peak application rate can be computed as ? Q ? R Pp = 4 ? S ? ? π R 2 ? Wr S ? ? (16.30) This demonstrates that the peak application rate for the sprinkler package depends entirely on the design of the system. Once a flow rate is established for the irrigation system and the sprinkler package is installed on the pivot, the peak application rates for points along the system are established. The quantity within the parentheses in Equation 16.30 is the gross system capacity. The peak application rate increases linearly with the system capacity and the distance from the pivot point and inversely with the wetted radius of the sprinkler package. The time required to reach the peak application rate (tp) can be computed from:

608
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Chapter 16 Design and Operation of Sprinkler Systems

INFILTRATION RATE

75

RATES, mm/hr

50

25 20 mm 30 mm 40 mm

APPLICATION DEPTH 60 mm

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

TIME, hr

Figure 16.28. Illustration of the effect of the application depth on potential runoff.

tp =

Wr d g ? Q s 2π R ? ? π R2 S ? ? ? ? ?

(16.31)

Thus, the time to reach the peak application rate is linearly related to the wetted radius of the sprinkler package and the gross depth of water applied, and inversely related to the distance from the pivot point and the system capacity. The total time that water is applied to a point is twice the time required to reach the peak application rate for the elliptical pattern which is symmetrical about the peak rate. Thus, the time that water is applied to a point is affected by design variables (i.e., Wr, R, QS, and RS) and by management (dg). The example in Figure 16.28 shows how the potential for runoff increases for increasing water application depths. Gilley (1984) used the infiltration rates provided by the intake family of curves provided by the Natural Resources Conservation Service to estimate the maximum depth of water that could be applied before runoff began. The surface storage volumes from Dillion et al. (1972) were used to incorporate the effects of slope on the runoff process (Table 16.11). Results from Gilley are presented in Figure 16.29 for the four soils that he analyzed. These results provide a means to begin assessment of the suitTable 16.11. Allowable surface storage as a function of slope (adapted from Dillion et al., 1972). Slope Range, % Allowable Surface Storage, mm 0-1 12.7 1-3 7.6 3-5 2.5 >5 0.0

Design and Operation of Farm Irrigation Systems
50 MAXIMUM APPLICATION DEPTH, mm 0.1 INTAKE FAMILY 40
MAXIMUM APPLICATION DEPTH, mm 50 0.3 INTAKE FAMILY 40

609

30

30
12.5 7.5

20

20

12.5 7.5 0 2.5 SURFACE STORAGE, mm

10

10
0

2.5

SURFACE STORAGE, mm

0 0 50 100 150 PEAK APPLICATION RATE, mm/hr 200 250

0 0 50 100 150 200 250 PEAK APPLICATION RATE, mm/hr

50 MAXIMUM APPLICATION DEPTH, mm MAXIMUM APPLICATION DEPTH, mm 0.5 INTAKE FAMILY 40

50 1.0 INTAKE FAMILY 40

30
12.5

30

12.5

20
7.5

20
2.5

7.5

10
0

2.5 SURFACE STORAGE, mm

10

0

SURFACE STORAGE, mm

0 0 50 100 150 200 250 PEAK APPLICATION RATE, mm/hr

0 0 50 100 150 200 250 PEAK APPLICATION RATE, mms/hr

Figure 16.29. Maximum depth of application to avoid runoff as a function of the peak application rate for the sprinkler package installed on a center pivot for four NRCS intake families and surface storage values shown in Table 16.11.

ability of sprinkler packages on fields with various soils and slopes. The method by Gilley was extended to consider an entire field by Wilmes et al. (1994). Von Bernuth and Gilley (1985) and Martin (1991) adapted the method from Hachum and Alfaro (1980) to use the Green-Ampt model instead of the family method to simulate infiltration. These developments allow for the consideration of surface sealing and initial soil water distribution on the performance of sprinkler packages. The variables given above for the description of the performance of center pivot sprinkler packages shows that five system variables are involved. Considering these variables shows that runoff increases as the system capacity, depth of application, and the length of the lateral increase. The runoff is inversely related to the wetted radius of the sprinkler package. Problems can occur if the length of the pivot lateral is excessive, especially for soils with low infiltration rates and steep slopes. Unfortunately, some operators attempt to rectify runoff problems by reducing the system capacity. This increases the potential for water stress during dry periods and encourages management that applies excess water during time of the year when water demands are low in an effort to guard against periods of inadequate supply. The methods such as those by Heermann et al. (1974), von Bernuth et al. (1984), and Howell et al. (1989) should be employed to determine the system capacity required for a specific location and soil. These methods included the utilization of stored soil water to carry crops through periods of high evaporative demand. Users should not arbitrarily reduce capacity to avoid runoff problems.

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Chapter 16 Design and Operation of Sprinkler Systems

The sprinkler package and the depth of application are the most feasible adjustments once a system is in place. The depth of application per irrigation can be easily adjusted by operators to manage applications to reduce runoff. This can be an iterative procedure based on field observation of runoff problems. The second most feasible method to solve runoff problems is to replace the sprinkler package (perhaps only on the outer portions of the pivot) to sprinkler devices that provide a larger wetted radius for the same pressure as the previous package. Boom systems are available to spread water over a wider distance to also mitigate problems. There have also been a series of special tillage systems developed to increase the surface storage to reduce runoff. One implement that has been used creates implanted reservoirs. The machine used to create the implanted reservoirs uses a subsoiler followed by a paddlewheel assembly that creates the reservoirs. Research by Oliveria et al. (1987), Kranz and Eisenhauer (1990), Cuelho et al. (1996), and others has helped to quantify the benefits of systems that develop implanted reservoirs. The implanted reservoirs provide 5 to 10 L of storage per reservoir. For the reservoir density and row spacing used by Cuelho et al. (1996) the storage volume is equivalent to the water use for two days during the middle of the season if water is applied to every furrow and all furrows are specially tilled. The ability to sustain the reservoirs throughout the growing season varies greatly depending on the cohesiveness of the soil, the slope in the field and rainfall patterns. Others have used basins within the furrows to store water. The basins are generally from 1 to 3 m long. Generally basins can store more water than implanted reservoirs but they are not well suited to sloping soils. In the end, the designer must weight the potential savings of using low-pressure sprinkler devices that have smaller wetted radii than devices requiring more pressure against the costs of special tillage to store water that may run off. In some locations there is an additional benefit of special tillage in reduction of runoff and erosion from rainfall.
16.7.4 Devices to Irrigate the Corners Many center pivots are equipped with special sprinklers attached to the end of the lateral to increase the amount of land irrigated in the corners of the field. The end gun is operated when the lateral reaches an angle where the end gun throw will stay within the boundaries of the property (Figure 16.30). The central angle (β) during which the pivot operates was presented by von Bernuth (1983):

β = cos ?1 ?

?R ? S ? ?R ? ? E?

(16.32)

where RE is the total radial length irrigated when the end gun operates. The area irrigated in each corner (AE) is given by
? 2 2 ?π AE = RE ? RS ? ? cos ?1 (β )? ?4 ?

(

)

(16.33)

These relationships show that there is a trade-off between the radial length of the area irrigated with the end gun and the central angle that is irrigated. When the coverage of the end gun is short, the central angle is larger but the area gained per unit rotation of the lateral is small. The area irrigated in one corner relative to the area irrigated with the primary system is shown in Figure 16.31. These results show that the area in

Design and Operation of Farm Irrigation Systems
FIELD BOUNDARY AREA IRRIGATED WITH ENDGUN

611

END GUN RADIUS

RE

β

RADIUS OF AREA IRRIGATED WITH ENDGUN ON

PIVOT POINT

RADIUS OF AREA IRRIGATED WITH ENDGUN OFF

Rs

Figure 16.30. Illustrations of the parameters used to describe the amount of land irrigated in pivot corners.
0.6 6

End Gun Discharge / System Discharge

0.5

5

D

0.4

4

0.3

3

Area
0.2 2

0.1

1

0.0 0 0.05 0.10 0.15 0.20 0.25

0

End Gun Radius / System Radius

Figure 16.31. Discharge required for an end gun as a percentage of the flow for the main system and the area in one corner of the field relative to the area in the main field.

Area in One Corner as % of System Area

r ha isc

ge

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Chapter 16 Design and Operation of Sprinkler Systems

the corners is maximized when the length of the end gun coverage (RE –RS) is approximately 18% of the system radius (RS). The discharge for an end gun depends on the length of the coverage by the end gun:
q E = QS

(R

2 E

2 ? RS

2 RS

)

(16.34)

where qE is the discharge required for the end gun. The discharge for the end gun is approximately 35% of the discharge required for the primary system if the optimal area is irrigated (Figure 16.31). While the results in Figure 16.31 show that the area is maximized at a ratio of 18% of the system length, it is difficult to find end guns that provide the wetted radius required at the needed discharge. The area in the corner does not change appreciably for radii larger than 12% of the system length; therefore, the radius of the end gun can be less than the optimal with little loss in area. Solomon and Kodoam (1978) provided data on the pattern for end sprinklers. They show that the depth of application from typical end guns decreases near the edge of the pattern wetted by the end gun. Because the application tapers off at the end, it may be necessary to apply some water beyond the edge of the field to ensure that crops planted there receive an adequate water supply. In windy climates this is especially important. Application of water beyond the boundary of the planted cropland reduces the application efficiency of the end gun below that for the primary system and affects the central angle if the overthrow cannot be applied to the adjacent land. Many center pivots used today utilize a low-pressure sprinkler package for the primary system. Although end guns have been developed that require less pressure than earlier, end guns often require more pressure for adequate operation than is available at the end of the lateral for many sprinkler packages. A booster pump is generally needed on these systems to provide for adequate operation. The booster pump is usually located near the end of the lateral, often at the last tower. As special supply line is provided from the lateral to the booster pump and ultimately to the end gun. The power required for the end gun is computed from the discharge for the end gun and the difference in pressure between that for the sprinkler lateral at the distal end and that required for the end gun. Any pressure losses in the supply system to the end gun must also be considered. As the above information illustrates the discharge through the end gun represents a substantial portion of the flow for the primary system. This results in more pressure loss along the lateral while the end gun operates, which can cause the discharge from the sprinkles on the lateral to decline. Ultimately, two systems curves need to be constructed to determine how the center pivot interacts with the pump used to supply irrigation water. A pump should be selected that provides for efficient operation for both conditions. It would be most desirable for the difference in the discharge for the primary system when the end gun operates and when it is off to be small. To irrigate a larger portion of the area in the corners of a field a corner span can be attached to the end of the primary lateral. When the lateral has rotated to the proper angle, the corner span begins to move into the corner of the field. As the corner span rotates into the corner, a series of sprinklers turns on. More sprinklers operate as the corner span rotates further into the corner. The hydraulics of the corner system is very complex requiring computer modeling to predict the performance of corner machines.

Design and Operation of Farm Irrigation Systems

613

16.8 LATERAL MOVE SYSTEMS
The components of center pivot systems have been used to develop a system that travels in a straight line. These systems are called linear or lateral move machines. The towers, pipe material and the truss systems are very similar to center pivots. The difference is that instead of pivoting about a permanent base where water is supplied, the water supply to the lateral is available across the field. Water application amounts and the frequency of irrigation for lateral move systems are similar to that for center pivots. Thus, guidelines and limitations for design and operation of pivots generally apply for lateral move systems. Water can be supplied to the lateral move by one of three methods. A supply canal can run parallel to the direction of travel of the lateral move (Figure 16.32). A pump mounted on the supply tower lifts water from the canal and pressurizes the water for the system. Although a portable dam can be attached to the suction system to block flows on sloping fields; the canal system is limited to fields that are relatively flat in the direction of travel. A second choice is to drag a hose across the field similar to that for a traveler (see Section 16.10). Water is supplied from the main pump to a riser along the travel path of the supply tower. A hose is attached to the riser and to the inlet on the supply tower. As the machine moves it pulls the hose across the field. Many

Figure 16.32. Examples of lateral move systems that are supplied by a ditch and a hard-hose system.

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Chapter 16 Design and Operation of Sprinkler Systems

times the valve is placed in the middle of the field and the supply hose is long enough to irrigate the entire field without stopping. If the field is too long, several risers may be needed. Some lateral moves were designed to automatically connect to valves connected to an underground main line. This type of system is equipped with carts that lead and trail the lateral. The carts automatically connect to valves installed on risers from the buried main line. Water is supplied by one or both carts. This system is expensive and complex; thus, few of these systems have been produced. The lateral move requires a guidance system to control the direction of travel. Three types of systems have been used. One uses an aboveground cable that runs across the field parallel to the direction of travel. An assembly on the cart follows the cable and keeps the lateral move on course. A second option uses a signal from a lowvoltage buried cable with an antenna guidance system on the control tower. The third option uses a trench cut across the field to provide the direction. A guide follows the trench to steer the lateral move system across the field. The guidance and alignment system on lateral move machines is interfaced to maintain proper alignment and to ensure that the system follows the intended path. The speed of the last tower is controlled by the irrigator to apply the desired depth of water. The alignment of individual towers for lateral move systems works similar to that for center pivots. However, this does not ensure that the lateral move progresses parallel to the guidance cable. The guidance assembly is designed to reduce the speed of the interior towers if the lateral move begins to travel away from the guidance cable. If the lateral move system is progressing on an angle toward the guidance cable, the velocities of the interior towers are increased causing the lateral move to change the direction of travel. Lateral move systems have characteristics similar to pivots. They also have the following advantages: a larger portion of a square field is irrigated than for pivots; a rectangular field can be irrigated; and the rate of water application is less than for pivots, leading to fewer problems with runoff. The disadvantages of lateral move systems are that: the cost per unit area irrigated is substantially higher than for pivots; more labor is required to move the system to the starting point or to reverse the system so it can irrigate in the opposite direction; the hose used to supply the system can be difficult to move and attach; and aboveground guidance and supply systems interfere with farm operations. Utilization of lateral move systems lags significantly behind the use of center pivots. The higher investment costs and increase labor required to arrange the water supply and to reposition the lateral move system deter wider use. However, the systems do offer substantial promise. Low energy precise application (LEPA) systems and other types of sprinkler packages that are capable of applying water below crop canopies are well suited to lateral move systems. Such systems essentially become moving microirrigation systems. If the development of site-specific irrigation develops, lateral move systems are logical choices for supplying irrigation water, crop nutrients, and other agricultural chemicals. Thus, there are good reasons to assume that the use of lateral move irrigation systems will grow.

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16.8.1 Layout Since the cost per unit length of the lateral move usually exceeds the linear cost of the water supply system, investment costs are minimized by laying out the system so that the lateral pipeline is parallel to the shortest side of a rectangular field. This orientation results in longer travel distances that produce longer irrigation intervals. The length of the field, water supply capacity, and the depth of application determine the irrigation interval. If the lateral move is repositioned to a starting location after each irrigation the downtime required to move the lateral and water supply system must be included in the irrigation interval. In some cases irrigators simply reverse the direction of travel at one edge of the field and irrigate in the opposite direction for the subsequent irrigation. There is some downtime to reposition the water supply system for this mode of operation but it is not as long as required to return the lateral move to the starting position. With this type of operation the depth of water applied and the irrigation interval at the ends of the field is essentially twice that at the center of the field. Care must be taken to avoid deep percolation and/or water stress at the edges of the field when operating in such a manner. Lateral move systems are not capable of negotiating steep lands or severely rolling terrain. The maximum recommended slope along the lateral can be as high as 6%, but should generally be less than 2%. The type of water supply system and the direction of operation determine the maximum slope in the direction of travel. For canal-fed systems the maximum slope is about 0.5%, while it is 1% for canal-fed systems with a movable dam in the canal and 3% for hose-supplied systems. The pressure loss in the lateral can be minimized by placing the water supply system in the center of the field. However, this may present obstacles to farming operations that are less severe if the water source is placed along the boundary of the field. The consequences of each alternative should be discussed with the producer. As with all sprinkler systems the main line must be designed after the lateral move system is designed. The head-discharge curve for matching pumps to the sprinkler system can be developed by treating the lateral move as a single large sprinkler or the pressure distribution function can be used directly. The head-discharge relationship depends on the losses in the conveyance system, supply carts, and canal suction system. The losses in various components for typical systems are shown in Figure 16.33. Information should be obtained for specific models for final design. 16.8.2 Water Application Lateral move systems have characteristics similar to center pivots and periodically moved laterals. The discharge from sprinklers should be constant similar to moved laterals. The discharge is given by Q S qS = S L FW (16.35)

where the field width (Fw) is parallel to the lateral. The pressure distribution along the lateral is similar to that for moved laterals, yet the pipe materials are the same as for center pivots. The friction per unit length along the lateral can be computed from Table 16.10; however, the F value for lateral move systems should be determined as for moved laterals.

616
50 45 40 35 30 25 20 15 10 5 0 0 10 20
64 76

Chapter 16 Design and Operation of Sprinkler Systems
89 102 114 127 NOMINAL DIAMETER OF HOSE,

H EA D LOSS, m

HARD HOSE

152

LOSS IN SUPPLY CART

30

40

50

60

70

80

90

100

INFLOW, L/s
50 45 40 35
127 64 76 89 102 114

HEAD LOSS, m

30 25 20 15 10 5 0
0 10 20 30 40 50 60 70 80 90 100

NOMINAL DIAMETER OF HOSE, mm SOFT HOSE @ 1000 kPa

INFLOW, L/s

Figure 16.33. Head loss in hard and soft hoses used on lateral move and traveler irrigation systems. Head loss in the supply cart for travelers is also shown.

The pressure and required discharge for a sprinkler are used to select the sprinkler package and the nozzle sizes. The lateral move is different than moved laterals in that the size of nozzle may vary along the lateral as the pressure changes. If pressure regulators are used along the entire machine, the nozzle sizes would be constant. The application rate is uniform along the lateral of a lateral move system because the representative area is the same for all sprinkler The application rate can be computed using the elliptical or triangular patterns as previously presented. The peak application rate for a lateral move system is given by 4QS Pp = (16.36) πWr FW and the time to the peak application rate is tp = Wr/v (16.37)

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where v is the average linear velocity of the lateral move and Wr is the radius of coverage for the sprinkler package. For equal net system capacities and application efficiencies, the peak application rate for a lateral move is equal to the application rate at a point 65% of the way along the lateral of a pivot. The depth of water applied per irrigation is given by
dg = QS vFW

(16.38)

Runoff is less of a problem with lateral move systems than for center pivots because field slopes are less than sometimes found for center pivots and the peak water application rate is less than for pivots. The runoff relationships for center pivots in Figure 16.29 can be utilized to determine if runoff is a potential problem. Lateral move systems provide the opportunity of very high uniformity of application. Part-circle sprinklers can be used to improve the uniformity at field boundaries. The effect of sprinkler spacing on the uniformity can be estimated using the overlapping procedures developed for center pivots. Automated sprinkler systems are increasingly replacing surface irrigation systems. When systems are replaced irrigation management must change. A frequent problem is that irrigators attempt to apply the same depth of water per irrigation with automated sprinkler systems as was applied with surface systems. This negates the potential of the automated system and is generally unsuccessful. Runoff and traction problems generally occur with such management. Applying smaller depths per irrigation capitalizes on the potential of automated sprinkler systems to provide high application efficiencies. The maximum depth of water applied with lateral move systems should be less than 50 mm with 25 mm per application being typical.
16.8.3 Water Supply System Design considerations vary depending on the type of water supply used for the lateral move. For systems that drag a supply hose, considerations involve the friction loss in the hose and other elements of the water supply system and the force required to drag the hose across the field. The pipe roughness for the Hazen-Williams equation (i.e., the C value) is approximately 150 for the hard or soft hoses used to supply water. The inside diameter of soft hoses varies with the pressure inside the pipe and additional information is required for operating pressure other than shown in Figure 16.33. There are limits on the shortest bending radius for each hose. Short bends cause the soft hose to kink, which temporarily blocks the flow, but there is no long-term damage to the hose. The hard hose can be damaged if it is bent too severely. The force required to drag the hose limits the maximum diameter and length of hose and may require a special design of the tower that pulls the hose. Hose lengths of 200 m are common as that length matches well with the property dimensions common in the U.S. Hoses longer than 200 m are not common and should only be specified if in consultation with manufacturers. The normal towers used to pull the hose are not usually capable of pulling 200 m of 203-mm diameter hose. Even for smaller diameter hose, the supply tower can be equipped with four wheels that are supplied power rather than the normal two. This improves traction for wet or slippery surfaces.

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Systems that are supplied from a ditch require that the channel capacity, system capacity, water supply source, and the onboard pump and power unit be matched. Once the topography is known the ditch can be designed using procedures described in earlier chapters on conveyance systems. The minimum depth of the ditch is usually about 1 m. The minimum bottom width is about 0.3 to 0.6 m. Erosion, weed control, and trash in the ditch are issues that must be considered in the design and operation. If the soils at the site are highly permeable, the canal may need to be lined with flexible membranes or concrete. Either type of lining significantly increases the cost of the system. The sideslope of the canal should not be so steep that erosion occurs or that humans and wildlife cannot climb from the canal should they enter the waterway. Outflow structures may be required in locations where storm water enters the canal. Special features are needed for canal supply systems to ensure that the ditch does not overtop if the lateral move stops. Controls are also needed to stop the lateral move if the water supply is interrupted. Systems that automatically connect to valves attached to buried main lines require special considerations in design. When the water flow changes between the supply carts some water hammer may develop in the supply system. The amount of pressure surge depends on the flow and the time required for valves to open and close. Pressure surges problems are most prevalent on long lateral moves that require a large inflow. Pipeline protection is essential for these systems. The supply system for these lateral moves is very complex and is usually designed by the manufacturer.

16.9 LOW ENERGY PRECISION APPLICATION (LEPA) SYSTEMS
LEPA irrigation systems are either center pivots or lateral move systems that are modified with extended length drop tubes and application devices designed to apply small frequent irrigations at or near ground level to individual furrows (Lyle and Bordovsky, 1981). The primary purpose of LEPA systems is to minimize evaporation from spray droplets, foliage, and soil surfaces. The LEPA concept involves soil surface management to increase surface storage (Lyle and Bordovsky, 1983). Selected tillage methods and/or crop residue management are used to increase retention of both rainfall and irrigation. Nozzle pressure requirements for LEPA are low since wide dispersal of water is not necessary. Much of the pressure head at the nozzle (located near ground level) is obtained by the elevation head differential between the lateral and the nozzle. This results in reduced lateral design pressures as compared to overhead sprinkler packages and reduced energy requirements. LEPA systems discharge water beneath the plant canopy and therefore offer an opportunity to irrigate with poor-quality water that might cause leaf burn when applied through sprinkler systems. LEPA systems are also advantageous for crops susceptible to fungal diseases that may thrive with frequent wetting of the foliage.
16.9.1 General Considerations There are numerous considerations in the design, installation and management of LEPA systems that are not required for sprinkler systems (Lyle, 1994). Since water is applied as a narrow band or stream it is important that the LEPA drop tubes and discharge devices be positioned so that each plant within a field has equal opportunity for irrigation water delivery. This is best accomplished if water is applied in the furrow

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between crop rows. It is, therefore, recommended that plant row position for both lateral move systems and center pivots be established using the system’s tower tire tracks as a guide for row crop establishment. This guideline results in circular rows for center pivots and linear rows for lateral move systems. It is highly desirable that the system span length be an even multiple of the row spacing and also of equipment width to facilitate consistency in row establishment and applicator placement along each span. For alternate furrow application, the drop tubes and applicators should be positioned in the “soft” furrows or those not compacted by tractor or equipment wheel traffic so that infiltration is maintained. This requires that equipment gage wheel location in relation to tractor wheels be such that every other furrow remains free of traffic. LEPA systems are designed to be essentially independent of soil intake rates. LEPA systems are best suited to soils that maintain structural integrity throughout the season to retain surface storage formed by enhancing tillage practices. Topography (slope) is the primary limiting factor in choosing LEPA irrigation. Although system speed can be adjusted to accommodate slopes of up to 2% without irrigation surface redistribution, slopes should be limited to 1% for circular rows in climates with high-intensity rainfall to minimize rainfall runoff and potential soil erosion. In all instances, normal practices to prevent erosion from rainfall runoff, such as terracing and/or grassed waterways, should be used in conjunction with LEPA.
16.9.2 Surface Storage and Water Application Devices The establishment of improved soil surface storage and its maintenance throughout the irrigation season is a function of both tillage methods and type of applicator chosen for irrigation. The combination should maintain seasonal surface water storage capable of holding the entire water volume of each irrigation pass without surface water redistribution. Recommended surface modification practices include basin tillage, reservoir tillage, interfurrow chiseling or subsoiling, or any combination of these practices with thick standing stubble or residue. These practices may also beneficially increase the infiltration rate. To insure surface containment of applied irrigation, LEPA systems should be operated at a sufficiently high speed so that the application volume is less than or equal to the surface storage. For some situations and locations this might require daily irrigation. The surface storage should be minimally diminished during LEPA irrigation. In addition, the design and installation of the nozzle and regulator system should deliver maximum uniformity for all operating conditions within the field and throughout the irrigation season. Ideally, LEPA applicators should have a narrow profile to minimize crop contact and drag. It is desirable for applicators to also possess spray capability in addition to single furrow discharge, although this is not the primary mode of operation. Examples of spray application needs include temporary use for seed germination, herbicide application, and when close-seeded crops are included in cropping rotations. Currently there are two basic LEPA applicator designs that minimize erosion to furrow dikes or other enhanced soil storage conditions (Figure 16.2). One consists of a nozzle and shroud assembly which causes water to be discharged as a continuous sheet or bubble. The other design uses removable drag socks, designed to minimize furrow dike erosion as water is delivered directly to the soil surface.

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Nozzle selection and hydraulic design are similar to that of other pressurized irrigation systems but careful consideration of elevation head gain and friction losses in each drop tube between the lateral and the nozzle is required to accurately determine the pressure available to the applicator.
16.9.3 Drop Tube Design The drop tube should be sufficiently long to position the application device when irrigating from 10 to 45 cm above the soil surface depending on type of applicator and topography. The diameters of all materials making up the drop tube should be sufficient to supply necessary operating pressure to an applicator or pressure regulator with an overhead lateral pressure at the end of the system of no greater than 70 kPa. A pressure of 21 kPa above the regulator rating is normally necessary at the regulator inlet for correct operation of LEPA systems. Drop tube diameters normally range from 15 to 20 mm. Drop tubes are attached to the overhead pipeline outlets by means of furrow arms (extended length gooseneck connectors) which are normally 30 to 50 cm in length. Although pivots and lateral move systems may be ordered with outlets spacings the same as the desired furrow spacing, they are often not in satisfactory alignment. The furrow arm is therefore rotated to center the drop over the furrow. When determining friction loss between the pipeline and nozzle, entrance and friction losses in the furrow arm and associated fittings as well as the various components of the drop tube must be calculated using appropriate head loss equations. Several material options are available for drop tube construction and configuration. The major (largest) portion of the drop tube, normally the upper portion, must be sufficiently rigid to prevent significant movement in high winds during preplant irrigation and prior to full canopy establishment. Rigidity should also be sufficient to insure that the LEPA applicator remains within the crop canopy after full canopy development. The entire drop tube should exhibit sufficient flexibility to allow travel over terraces or soil mounds, or encounters with other objects without breakage. The upper rigid section of drop tubes may consist of galvanized steel, UV-protected PVC, or extruded polyethylene. Galvanized steel may also be used for a lower section (approximately 70 cm in length) located above the applicator for weight and rigidity to help maintain applicator position within the canopy. Slip weights of galvanized metal, concrete, or poly material may also be slipped over light and/or flexible materials to make up the lower section of drop tubes. A short section (0.5 to 0.67 m) of flexible reinforced or non-reinforced vinyl-covered PVC hose or tubing should be used to couple the more rigid upper and lower section to provide necessary flexibility to the drop. It also allows adjustment of applicator height above the soil surface by modifying the hose length after filling the system with water. 16.9.4 Evaluation and Performance The coefficient of uniformity is used for evaluating overhead irrigation systems but it is not applicable to the individual furrow application of LEPA systems. Instead, LEPA nozzle package design and performance should be evaluated by nozzle discharge uniformity. The nozzle discharge uniformity describes the uniformity of LEPA nozzle discharge rate converted to equivalent application depth along the length of the system. The nozzle discharge uniformity is calculated for pivots with the HeermannHein equation and with the Christiansen’s equation for lateral move systems. How-

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ever, the depth of applied water in the equation is replaced with equivalent application depth, which equals the individual nozzle discharge (measured by timed volumetric catchment) divided by coverage area per nozzle. LEPA nozzle package design should result in nozzle discharge uniformities of 96 or greater. A measured drop in uniformity, to 94, due to elevation and/or flow rate changes in the field is the lowest value recommended. Modification of design parameters (operating pressure, pipe size, pressure regulator use, etc.) should be considered if uniformity drops below 94 over significant portions of an irrigated field.

16.10 TRAVELERS
Another method to automate and save labor is the traveler irrigation system. Traveler systems consist of a large sprinkler, commonly referred to as a “gun,” mounted on a moving cart (Figure 16.34). Water is supplied to the cart by a hose. With early designs the cart was connected to a cable and winch system which was attached to an anchor. The winch, which was powered by water pressure, retracted the cable pulling the cart across the field. Many current designs use the water supply hose to pull the cart across the field (Figure 16.34). A large spool is anchored at one end of a travel lane. The spool rotates winding up the hose and pulling the cart toward the spool. Whether connected by cable or hose, the spool assembly is designed to provide a nearly constant travel velocity. This requires that the spool rotate faster when small amounts of cable or hose have been retracted. The traveler system is operated as shown in Figure 16.34. The cart is positioned at one edge of the field and is pulled to the center. The system then shutdowns and the cart is moved to the opposite side of the field. Once a strip is irrigated, the system is moved to the next lane. Occasionally, the system can be pulled across the entire field to avoid reconnecting the hose at the middle of the field. The gun on the cart discharges a large volume of water and produces a large wetted radius. Therefore, the travel lanes are often spaced up to 100 m apart. The disadvantages of traveler systems include: High pressure requirements—Travelers require higher pressures than other sprinkler systems. The pressure at the cart can exceed 700 kPa. Pressure loss in the hose and water supply components add to the pressure requirement. Therefore, traveler irrigation systems have high operating costs. Labor required to move the cart—Traveler systems should be considered semiautomated since the cart, hose, and spool must be manually moved. Loss of land in the travel lanes—The carts generally have a low clearance, so if tall crops are grown, the lane must be planted to a low-growing crop. Nonuniform application at the edges and center of the field—The gun is operated to irrigate part of a circular pattern (Figure 16.34). The exact angle depends on the lane spacing, the overlap of adjacent passes of the traveler, and the design of the gun. However, it is common to require more than 180° rotation, as shown in Figure 16.34. If the traveler cannot apply water beyond the boundaries of the property, there are sections along the edges of the field that may receive less water than the bulk of the field. A series of dry diamond-shaped regions may also occur near the center of the field. If too much overlap occurs at the center there can be areas of excess irrigation.

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HOSE REEL AND CART

SUPPLY HOSE

TRAVELING GUN AREA WETTED IN ONE PASS

ZONE WETTED WHEN STATIONARY TRAVELING GUN

SUPPLY HOSE

HOSE REEL/CART

TRAVEL LANE

FIELD BOUNDARY

MAINLINE WELL & PUMP

Figure 16.34. Picture and operational sketch of traveler or big-gun irrigation system.

Poor uniformity in windy conditions—Since the traveler throws water a long distance, wind can distort the application pattern. To maintain uniformity in locations with high winds the travel lanes must be closer together, which increases labor requirements and leads to longer times between irrigations. Large droplets—Traveler systems often produce large droplets with a high velocity producing a large amount of energy when reaching the soil/crop surface. The impact energy can exacerbate runoff and soil compaction. Therefore, travelers are best suited to cropping and farming systems that provide plant or residue material to absorb the energy in the droplets before reaching the soil surface. There are several advantages for traveler systems, including their:

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Flexibility—Since the lanes can vary in length, traveler systems can be used to irrigate nearly any shape of field. The systems can be moved from field to field to irrigate several tracts of land. This is especially attractive in semi-humid areas where the system may not be necessary for one field for the entire season or where cropping rotations require that irrigation be shifted from one field to another. Wide range of irrigation depths—Since the sprinkler moves automatically, the depth of water applied per irrigation can be set to the desired amount. If a small irrigation is needed the system can be set to move quickly across the field. Conversely, for deep-rooted crops the cart can move slowly. Factory assembly—The system is assembled during manufacturing, thus the system can be quickly installed. This allows traveler systems to be operational with a minimum amount of preparation. Traveler irrigation systems are mostly used in semi-arid and semi-humid climates where the needs for irrigation are not as large and consistent as for arid lands. The systems are more popular in locations where the size and shape of fields are not amenable to center pivot or lateral move systems, or where multiple fields can be irrigated from the same water source. The high pressure requirements for travelers cause operating costs to be high. The major considerations for travelers are the hydraulic design of the water supply system and the arrangement of travel lanes to match field boundaries while achieving acceptable uniformities of application. Users should be very careful around traveler systems. Travelers operate at high pressures and large amounts of tension develop in the hose or cable system used to move the traveler. Both of these conditions contribute to the danger. Special care should be taken to avoid applying water onto electrical power lines. Kay (1983) recommends travel lanes be located at least 30 m from electrical power lines.
16.10.1 Field Layout As with any irrigation system, the initial design step is to lay out the field boundaries and arrange the system on the landscape. Topographical information is needed to ensure that the desired uniformity is achieved. Small elevation changes in the field are of less concern with travelers because of the high operating pressure. Slopes only slightly affect the ability of the traveler to maintain the uniform speed of travel required for acceptable uniformities. Therefore, the topography within a field is not of major concern to the operation of the traveler. The pressure required to reach the highest elevation in the field should be the primary interest. The arrangement of travel lanes is the principal consideration in laying out the system. The maximum length of the supply hose is generally 200 m. If the field is wider than 200 m the traveler will generally have to be pulled toward the center of the field requiring that the main line be positioned through the center of the field. The distance between lanes should be the same across the field. Also, the lanes must be placed close enough together to provide the overlap needed for acceptable uniformity. The spacing of the travel lanes depends on the wind speed and the diameter of coverage of the sprinkler. Recommended maximum spacings are given in Table 16.12. Iteration may be needed to find a travel lane spacing that provides uniform irrigation while fitting within the field dimensions. It is generally recommended that the travel lanes be oriented perpendicular to the prevailing wind direction. The first travel lane may be

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Table 16.12. Maximum spacing of lanes for traveler systems (adapted from Addink et al., 1980 and Kay, 1983). Wind Speed, m s-1 Diameter of 0 0 to 2.5 2.5 to 5 >5 Coverage, m (80% of Wd) (70% of Wd) (60% of Wd) (50% of Wd) 50 40 35 30 25 60 48 42 36 30 70 56 49 42 35 80 64 56 48 40 90 72 63 54 45 100 80 70 60 50 120 96 84 72 60 140 112 98 84 70 160 128 112 96 80 180 144 126 108 90

located a half of the spacing from the boundary of the field if the water from the sprinkler can be applied beyond the edge of the field. Otherwise the first travel lane should be a full spacing from the field boundary. The sprinkler can be positioned at the edge of the field if the water can be applied beyond the edges of the field that are perpendicular to the travel lanes. Otherwise the sprinkler cart should be positioned to avoid throwing water beyond the property boundaries. If multiple travelers are used in one field it is desirable to supply water to the center of the field and to place a traveler on each half of the field. This minimizes the difference in pressure available to each sprinkler.
16.10.2 Hydraulic Design The discharge from the sprinkler on the travelers should be equal to the flow required for the field divided by the number of travelers used in the field. The guns used on travelers can be equipped with either tapered or ring nozzles. Tapered nozzles generally produce larger diameters of coverage but do not provide as much breakup of the sprinkler jet. The depth of water applied per irrigation can be determined from
dg = qS vWT

(16.39)

where WT is the width of the travel lane (i.e. the distance between travel lanes) and v is the linear velocity of the sprinkler cart. The discharge from the sprinkler and the spacing between travel lanes determines the pressure required for the sprinkler. The pressure must be high enough to provide the discharge and diameter of coverage required. The total pressure loss in the systems consists of losses in the main line, fittings, supply hose, and the sprinkler and hose carts. The pressure loss in the supply hose can be determined from Figure 16.33. The diameter of the soft hose increases as the pressure of the system increases. Thus, if the pressure is different than shown in Figure 16.33 the head loss will vary accordingly. There is additional loss within the sprinkler and reel carts. Losses increase when the hose is wrapped around the reel rather than

Design and Operation of Farm Irrigation Systems

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when fully extended. Oakes and Rochester (1980), Rochester et al. (1990), and Rochester and Hackwell (1991) provided methods to compute the friction loss for varying sizes of travelers. The head loss in the components of the traveler was computed from (16.40) where hl = the loss k = the loss coefficient V2/2g = the velocity head. The loss coefficients in the sprinkler cart and hose-reel cart were found to be 1.76 and 3.91, respectively. The coefficient for the loss due to coiling on the reel was represented by a bend coefficient which was found to be 0.09 m-1. The bend coefficient is multiplied by the length of hose coiled on the reel. The loss coefficients vary with the size of the system and the design of the equipment; therefore, they should be used as an initial estimate. Portable pipe or underground pipelines with risers and valves can be used for the main line used to supply water to the traveler. The friction loss in the main line is similar to that for any other type of sprinkler system. Special care is required to protect pipelines against pressure surges with traveler systems. Travelers require high operating pressures so pumps used to supply the water are usually designed for high pressures. When more than one traveler is supplied by the same main line or if the supply hose should kink or some other form of obstruction occurs, the pressure in the conveyance system can increase rapidly. The pipeline should be protected to avoid damage under such conditions. In many cases it is best to select pumps where the shutoff head is less than the pressure rating of the supply system. Water hammer can also be a problem since the pressure builds quickly when water reaches the sprinkler. The elasticity of the hose helps to minimize some of the pressure surge problem but water hammer should be evaluated in designing the supply system.
16.10.3 Uniformity The uniformity of application with traveler systems depends on a constant velocity of travel, constant sprinkler discharge and proper spacing to provide adequate overlap. Travelers are now specially designed to vary the speed of the reel used to retract the sprinkler cart so that variations in travel speed along the travel lane are not severe. Long supply hoses exert considerable resistance when fully extended as compared to when only a portion of the hose must be moved. The increased resistance can cause variations in the speed of travel. Thus, designers should check with manufacturers for the maximum length of hose for local conditions. The pressure at the inlet to the sprinkler varies with the distance of travel of the sprinkler cart, thus calculations should be made for varying amounts of retracted hose. The uniformity can be estimated using the overlapping procedure for a constantly moving sprinkler. The single-leg distribution of the guns used on travelers will not usually fit the elliptical or triangular functions that were presented, thus the procedure must be modified for the appropriate distribution (Rochester et al., 1989). The overlapping process can be used to assist in selecting lane spacings. The single-leg distribution for average wind conditions should be used if available. However, the overall field uniformity and application uniformity are difficult to estimate due to the variability along field boundaries and at the center of the field.

hl = kV 2/2g

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The operator must check to ensure that the sprinkler device is operating at the appropriate pressure. Low pressure leads to an inadequate breakup of droplets which produces a doughnut-shaped water application pattern. Excessive pressure causes the sprinkler jet to break into to small drops that do not travel as far as intended and that are subject to evaporation and drift.

16.11 AUXILIARY USES OF SPRINKLER SYSTEMS
Sprinkler irrigation systems can perform several auxiliary uses in addition to supplying crop water requirements. Permanently installed systems can be operated quickly to meet these additional demands. If the system must be positioned in the field and periodically moved the ability to perform auxiliary uses is diminished. The application of effluent and agricultural chemicals are two common uses. Since the sprinkler system is designed for high uniformity and operates during the rapid growth period of crops, sprinkler systems provide excellent capabilities for these applications. However, these applications are regulated in most states to ensure protections of soil and water resources. In addition, special hydraulic designs are required to guard against the backflow of chemicals into the water source. Details of the use of sprinkler systems for chemigation are discussed in more detail in Chapter 19. Due to the high heat of fusion for ice, sprinkler systems can be used for frost and freeze protection in some applications. Generally water must be applied frequently to the crops during periods of frost or freeze danger. This may require a higher flow rate for the field than needed for crop water requirements. The requirement also eliminates those systems that cannot irrigate the entire field in a very short time. Very careful management is required to ensure that the objectives are accomplished for frost or freeze protection. Successful practices vary considerably depending on the water source, wind speeds, and other local conditions. Local guidelines should be followed for success. Care must also be taken to avoid damage to the irrigation system when applying water during cold periods. Ice may form on structural members of the irrigation system and the added weight may cause components to fail. In some locations wind erosion, before plants are large enough to shield the soil surface, is a major concern. Irrigation during such periods may increase the cohesion between soil particles, which increases aggregate stability and reduces erosion. Care must be taken, however, because the impact from water droplets can dislodge soil particles and contribute to increased wind erosion when the soil dries. Irrigation with small applications is usually adequate to stabilize the soil surface for a period of time. The efficiency of water use for erosion control is low. The small application does not wet the soil to a very large depth which leads to evaporation of water from the soil surface with little storage of water in the soil profile. In many ways irrigating to control wind erosion is a last-ditch effort, as it is better controlled through residue management and other production practices.

16.12 SAFETY
Producers, service technicians, and other individuals that work around sprinkler irrigation equipment must be very careful. Sprinkler irrigation equipment is often connected to high-voltage electrical supplies, has numerous moving parts, requires high water pressures, and operates in a wet and slippery environment. The systems are occasionally used to apply chemicals that could be toxic. While many standards and operational guidelines have been developed for the proper design, manufacturing, instal-

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lation, and operation of this equipment, not all systems adhere to the intended practices. Anyone designing or operating these systems must be aware of the appropriate laws, codes, and engineering standards that apply to the equipment and the intended use. The standards from ASABE are continually updated and should be consulted routinely for proper practices. Local ordinances and other regulations should be determined before the system is designed.

16.13 SUMMARY
This chapter describes the fundamentals of sprinkler irrigation, performance of sprinkler systems including uniformity and efficiency of application, types and characteristics of sprinkler systems currently used, and design and management procedures for specific types of sprinkler systems. Information is provided to enhance design and management of sprinkler systems which are the most rapidly growing form of irrigation today.

LIST OF SYMBOLS
AD AE Ai AR Cd Cn d d50 dg Dn Ea Ek FL FW Hn Ir Ke/a MAD n N Nd NL NS P P(i) P3 PL PLL PLS Pp PR PS
allowable depletion area irrigated in a corner of a center pivot area irrigated representative area for a sprinkler on a center pivot discharge coefficient net system capacity depth of water applied or effective diameter of water droplet volume mean drop diameter gross depth of irrigation water applied inside diameter of a nozzle application efficiency kinetic energy length of the field width of the field nozzle pressure head rate of infiltration for bare soil relative to protected soil kinetic energy per unit area management allowed depletion number of sets per lateral number of sprinklers on a lateral diameter of the nozzle number of laterals along the field length number of laterals along the width of the field pressure in a sprinkler lateral or at a sprinkler nozzle rate of water application as a function of time percentage of drops smaller than 3 mm pressure loss from the inlet into to the distal end of the pivot lateral pressure loss from the inlet to the distal end for the large diameter pipe pressure loss from the inlet to the distal end for the small diameter pipe peak precipitation rate at the sprinkler location pressure in the center pivot lateral at a point R from the pivot point pressure at the distal end of the pivot lateral

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Chapter 16 Design and Operation of Sprinkler Systems

Pv qE qs Qs R Ra RC RD RE s Sa Si SL Sm T TAW Td Ti Tm To tp Ts u U UCp v Wr WT

β ρ ω ?

percent of the total drops that are smaller than a specified size discharge required for an end gun discharge from a sprinkler flow into the sprinkler system (equal to the gross system capacity) average application rate or radial distance from pivot point average application rate location along the lateral where the pipe diameter changes root depth during peak water use period total radial length irrigated when the end gun operates distance from the observation point to the sprinkler sand content of the soil silt content of the soil sprinkler spacing along the lateral distance between laterals along the main line (equal to the set width) exposure time total available water per unit depth of soil downtime between successive irrigations irrigation interval time required to move the sprinkler later between sets time of operation per irrigation time after initial wetting that the peak application rate is reached. operational time per set for a lateral distance from a sprinkler to a point relative to the wetted radius of the sprinkler wind speed the uniformity coefficient for center pivots droplet velocity or linear velocity for a traveler system radius of coverage or wetted radius of the sprinkler distance between lanes for a traveler system angle of operation for center pivots and end guns density of water angular velocity of a pivot lateral ratio of the nozzle diameter to the pressure at the base of the sprinkler

REFERENCES
Addink, J. W., J. Keller, C. H. Pair, R. E. Sneed, and J. W. Wolfe. 1980. Design and operation of sprinkler systems. In Design and Operation of Farm Irrigation Systems, 621-660. M. E. Jensen, ed. St. Joseph, Mich. ASAE. Bezdek, J. C., and K. Solomon. 1983. Upper limit lognormal distribution for drop size data. J. Irrig. Drain. Eng. 109(1): 72-88. Bittinger, M. W., and R. A. Longenbaugh. 1962. Theoretical distribution of water from a moving irrigation sprinkler. Trans. ASAE 5(1): 26-30. Christiansen, J. E. 1942. Irrigation by sprinkling. Univ. Calif. Agr. Exp. Sta. Bull. 670. Chu, S. T., and D. L. Moe. 1972. Hydraulics of a center pivot system. Trans. ASAE 15(5): 894-896. Cuelho, R. D., D. L. Martin, and F. H. Chaudry. 1996. Effect of LEPA irrigation on storage in implanted reservoirs. Trans. ASAE 39(4): 1287-1298.

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Dadiao, C., and W. W. Wallender. 1985. Drop size distribution and water application with low-pressure sprinklers. Trans. ASAE 28(2): 511-514, 516. Dillon, Jr., R. C., E. A. Hiler, and G. Vittetoe. 1972. Center pivot sprinkler design based on intake characteristics. Trans. ASAE 15(5): 996-1001. Edling, R. J. 1985. Kinetic energy, evaporation and wind drift of droplets from low pressure irrigation nozzles. Trans. ASAE 28(5): 1543-1550. Eigel, J. D., and I. D. Moore. 1983. A simplified technique for measuring raindrop size and distribution. Trans. ASAE 26(4): 1079-1084. Fisher, G. R., and W. W. Wallender. 1988. Collector size and test duration effects on sprinkler water distribution measurement. Trans. ASAE 31(2): 538-541. Gilley, J. R. 1984. Suitability of reduced pressure center pivots. J. Irrig. Drain. Eng. 110(1): 22-34. Hachum, A. Y., and J. F. Alfara. 1980. Rain infiltration into layered soils: Prediction. J. Irrig. Drain. Eng. 106(4): 311-319. Han, S., R. G. Evans, and M. W. Kroeger. 1994. Sprinkler distribution patterns in windy conditions. Trans. ASAE 37(5): 1481-1489. Hanson, B. R., and W. W. Wallender. 1986. Bidirectional uniformity of water applied by continuous-move sprinkler machines. Trans. ASAE 29(4): 1047-1053. Heermann, D. F., and P. R. Hein. 1968. Performance characteristics of self-propelled center pivot sprinkler irrigation system. Trans. ASAE 11(1): 11-15. Heermann, D. F., and R. A. Kohl. 1980. Fluid dynamics of sprinkler systems. In Design and Operation of Farm Irrigation Systems, 583-618. M. E. Jensen, ed. St. Joseph, Mich.: ASAE. Heermann, D. F., H. H. Shull, and R. H. Mickelson. 1974. Center pivot design capacities in eastern Colorado. J. Irrig. Drain. Eng. 110(2): 1127-141. Heermann, D. F., and K. M. Stahl. 2006. CPED: Center Pivot Evaluation and Design. Available at: www.ars.usda.gov/Services/docs.htm?docid=8118. Howell, T. A., K. S. Copeland, A. D. Schneider, and D. A. Dusek. 1989. Sprinkler irrigation management for corn-southern Great Plains. Trans. ASAE 31(2): 147160. Kay, M. 1983. Sprinkler Irrigation Equipment and Practice. London, UK: Batsford Academic and Educational Ltd. Keller, J., and R. D. Bliesner. 1990. Sprinkle and Trickle Irrigation. New York, N.Y.: Van Nostrand Reinhold. Keller, J., F. Corey, W. R. Walker, and M. E. Vavra. 1980. Evaluation of irrigation systems. In Irrigation: Challenges of the 80’s. Proc. Second Nat’l Irrigation Symp., 95-105. St. Joseph, Mich.: ASAE. Kincaid, D. C. 1982. Sprinkler pattern radius. Trans. ASAE 25(6): 1668-1672. Kincaid, D. C. 1996. Spraydrop kinetic energy from irrigation sprinklers. Trans. ASAE 39(3): 847-853. Kincaid, D. C., and D. H. Heermann. 1970. Pressure distribution on a center pivot sprinkler irrigation system. Trans. ASAE 13(5): 556-558. Kincaid, D. C., D. F. Heermann, and E. G. Kruse. 1969. Application rates and runoff in center pivot sprinkler irrigation. Trans. ASAE 12(6): 790-794. Kincaid, D. C., and T. S. Longley. 1989. A water droplet evaporation and temperature model. Trans. ASAE 32(2): 457-463.

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Chapter 16 Design and Operation of Sprinkler Systems

Kincaid, D. C., K. H. Solomon, and J. C. Oliphant. 1996. Drop size distributions for irrigation sprinklers. Trans. ASAE 39(3): 839-845. Kohl, R. A. 1972. Sprinkler precipitation gage errors. Trans. ASAE 15(2): 264-265, 271. Kohl, R. A., and D. W. DeBoer. 1984. Drop size distributions for a low pressure spray type agricultural sprinkler. Trans. ASAE 27(6): 1836-1840. Kohl, K. D., R. A. Kohl, and D. W. DeBoer. 1987. Measurement of low pressure sprinkler evaporation loss. Trans. ASAE 30(4): 1071-1074. Kohl, R. A., R. D. von Bernuth, and G. Heubner. 1985. Drop size distribution measurement problems using a laser unit. Trans. ASAE 28(1): 190-192. Kranz, W. L., and D. E. Eisenhauer. 1990. Sprinkler irrigation runoff and erosion control using inter-row tillage techniques. Applied Eng. Agric. 6(6): 739-744. Levine, G. 1952. Effects of irrigation droplet size on infiltration and aggregate breakdown. Agric. Eng. 33(9): 559-560. Li, J., H. Kawano, and K. Yu. 1994. Droplet size distributions from different shaped sprinkler nozzles. Trans. ASAE 37(6): 1871-1878. Lundstrom, D. R., and E. C. Stegman. 1988. Irrigation scheduling by the checkbook method. Extension Circular No. AE-792. Fargo, N.D.: North Dakota State Extension Service. Lyle, W. M. 1994. LEPA defined: More control with less consumption. Irrig. J. 44(6): 8,11. Lyle, W. M., and J. P. Bordovsky. 1981. Low energy precision application (LEPA) irrigation system. Trans. ASAE 24(5): 1241-1245. Lyle, W. M., and J. P. Bordovsky. 1983. LEPA irrigation system evaluation. Trans. ASAE 26(3): 776-781. Martin, D. L. 1991. Effect of frequency on center pivot irrigation. In Proc. Nat’l Conf. Irrigation and Drainage, 38-44. ASCE. Moldenhauer, W. C., and W. D. Kemper. 1969. Interdependence of water drop energy and clod size on infiltration and clod stability. Soil Sci. Soc. America Proc. 33: 297301. Morgan, R. M. 1993. Water and the Land: A History of American Irrigation. Washington, D.C.: The Irrigation Association. Nderitu, S. M., and D. J. Hills. 1993. Sprinkler uniformity as affected by riser characteristics. Applied Eng. Agric. 9(6): 515-521. Oakes, P. L., and E. W. Rochester. 1980. Energy utilization of hose towed traveler irrigators. Trans. ASAE 23(5): 1131-1138. Oliveira, C. A. S., R. J. Hanks, and U. Shani. 1987. Infiltration and runoff as affected by pitting, mulching and sprinkler irrigation. Irrig. Sci. 8: 49-64. Pair, C. H., W. H. Hing, K. R. Frost, R. E. Sneed, and T. J. Schiltz. 1983. Irrigation. 5th ed. Washington, D.C.: The Irrigation Association. Rochester, E. W., C. A. Flood, Jr., and S. G. Hackwell. 1990. Pressure losses from hose coiling on hard-hose travelers. Trans. ASAE 33(3): 834-838. Rochester, E. W., S. G. Hackwell, and K. H. Yoo. 1989. Pressure vs. flow control in traveler irrigation evaluation. Trans. ASAE 32(6): 2029-2034. Rochester, E. W., and S. G. Hackwell. 1991. Power and energy requirements of small hard-hose travelers. Applied Eng. Agric. 7(5): 551-556.

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Scaloppi, E. J., and R. G. Allen. 1993. Hydraulics of center pivot laterals. J. Irrig. Drain. Eng. 119(3): 554-567. Seginer, I. 1965. Tangential velocity of sprinkler drops. Trans. ASAE 8(1): 90-93. Seginer, I., D. Kantz, and D. Nir. 1991a. The distortion by wind of the distribution patterns of single sprinklers. Agric. Water Mgmt. 19: 341-359. Seginer, I., D. Nir, and R. D. von Bernuth. 1991b. Simulation of wind-distorted sprinkler patterns. J. Irrig. Drain. Eng. 117(2): 285-306. Solomon, K., and J. C. Bezdek. 1980. Characterizing sprinkler distribution patterns with a clustering algorithm. Trans. ASAE 23(4): 899-906. Solomon, K., and M. Kodoma. 1978. Center pivot end sprinkler pattern analysis and selection. Trans. ASAE 21(5): 706-712. Steiner, J. L., E. T. Kanemasu, and R. N. Clark. 1983. Spray losses and partitioning of water under a center pivot sprinkler system. Trans. ASAE 26(4): 1128-1134. Stillmunkes, R. T., and L. G. James. 1982. Impact energy of water droplets from irrigation sprinklers. Trans. ASAE 25(1): 130-133. Thompson, A. L., J. R. Gilley, and J. M. Norman. 1993. A sprinkler water droplet evaporation and plant canopy model: I. Model development. Trans. ASAE 36(3): 735-741. USDA-NASS (U.S. Department of Agriculture National Agricultural Statistics Service). 2003. Farm and Ranch Irrigation Survey and the 2002 Census of Agriculture. National Agricultural Statistics Service (NASS), Agricultural Statistics Board, USDA. von Bernuth, R. D. 1983. Nozzling considerations for center pivots with end guns. Trans. ASAE 26(2): 419-422. von Bernuth, R. D., and J. R. Gilley. 1985. Evaluation of center pivot application packages considering droplet induced infiltration reduction. Trans. ASAE 28(6): 1940-1946. von Bernuth, R. D., D. L. Martin, J. R. Gilley, and D. G. Watts. 1984. Irrigation system capacities for corn production in Nebraska. Trans. ASAE 27(2): 419-424, 428. Vories, E. D., and R. D. von Bernuth. 1986. Single nozzle sprinkler performance in wind. Trans. ASAE 28(6): 1940-1946. Wilmes, G. J., D. L. Martin, and R. J. Supalla. 1993. Decision support system for design of center pivots. Trans. ASAE 37(1): 165-175.


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