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Global and Planetary Change 60 (2008) 365 – 380 www.elsevier.com/locate/gloplacha

Recent changes of water discharge and sedimen

t load in the Zhujiang (Pearl River) Basin, China
Shurong Zhang a,?, Xi Xi Lu a , David L. Higgitt a , Chen-Tung Arthur Chen b , Jingtai Han c , Huiguo Sun c
b

Department of Geography, National University of Singapore, Arts Link 1, Singapore 117570 Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan c Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Received 11 October 2006; received in revised form 26 March 2007 Available online 6 May 2007

a

Abstract The paper is concerned with identifying changes in the time series of water and sediment discharge of the Zhujiang (Pearl River), China. The gradual trend test (Mann–Kendall test), and abrupt change test (Pettitt test), have been employed on annual water discharge and sediment load series (from the 1950s–2004) at nine stations in the main channels and main tributaries of the Zhujiang. Both the Mann–Kendall and Pettitt tests indicate that water discharge at all stations in the Zhujiang Basin showed no significant trend or abrupt shift. Annual water discharges are mainly influenced by precipitation variability, while the construction of reservoirs/dams in the Zhujiang Basin had little influence on water discharge. Sediment load, however, showed significant decreasing trends at some stations in the main channel of the Xijiang and Dongjiang. More stations have seen significantly decreasing trends since the 1990s. The decreasing sediment load in the Zhujiang reflects the impacts of reservoir construction in the basin. In contrast, the Liujiang, the second largest tributary of the Xijiang, has experienced a significant upward shift of sediment load around 1991 likely caused by exacerbated rock desertification in the karst regions. The annual sediment load from the Zhujiang (excluding the delta region) to the estuary has declined from 80.4 × 106 t averaged for the period 1957–1995 to 54.0 × 106 t for the period 1996–2004. More specifically, the sediment load declined steadily since the early 1990s so that in 2004 it was about one-third of the mean level of pre-90s. Water discharge and sediment load of the Zhujiang would be more affected by human activities in the future with the further reservoir developments, especially the completion of the Datengxia hydroelectric project, and an intensification of the afforestation policy in the drainage basin. ? 2007 Elsevier B.V. All rights reserved.
Keywords: gradual trend; abrupt change; water discharge; sediment load; the Zhujiang (Pearl River); time series analysis

1. Introduction There is an increasing concern for water resource management in the international community due to the more dominant influence of anthropogenic forcing over natural drivers on river systems in the Anthropocene era (Crutzen and Stoermer, 2000; Meybeck, 2001; Varis and

? Corresponding author. Postal address: Department of Geography, National University of Singapore, Arts Link 1, Singapore 117570. Tel.: +65 65164422; fax: +65 67773091. E-mail address: g0306366@nus.edu.sg (S. Zhang). 0921-8181/$ - see front matter ? 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2007.04.003

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Vakkilainen, 2001; Meybeck, 2003). Many studies have been initiated to examine how river systems are affected by climate change and human activities. Detecting trends of long time series of water discharge and sediment load is a fundamental technique for understanding the relative importance of natural climate change and anthropogenic disturbances as well as their complex interactions on fluvial systems (Walling, 1995, 1997). It can also be helpful for better water resource management in the future (Walling and Fang, 2003; Kundzewicz, 2004). It has been reported that changes of water discharge and sediment load can cause various effects on river system itself as well as the estuary and coastal shelf environment. Geomorphological evolution of river channels can be greatly affected by changes in water discharge and the relationship between sediment load and sediment-transport capacity of flow, such as the Rhone River in France (Petit et al., 1996; ArnaudFassetta, 2003), Wisloka River in southern Poland (Wy?ga, 1997), Piave River in northeast Italy (Surian, 1999, 2006), and Cache Creek and Stony Creek in California, USA (Collins and Dunne, 1990; Kondolf, 1997). Yang et al. (2003) reported that the development of the Yangtze Delta has been, and will be, seriously influenced by the decline of sediment supply from the Yangtze River. Chen (2000) pointed out that changes of river inputs (mainly water flow, sediment load as well as nutrient flux) to the oceans caused by river basin development, notably the construction of dams, have more subtle effects which go far beyond the delta and estuaries including the transformation of the coastal shelf ecosystem and the starvation of fish populations. Hence, knowledge of riverine transports of water discharge and sediment load is of great significance not only for the river system itself but also for the delta and estuarine as well as coastal environments. Water discharge and sediment load of most Chinese rivers have experienced great changes due to climate change and anthropogenic impacts in the drainage basin (Lu, 2004). Both water and sediment discharge of the Huanghe (Yellow River) have shown progressive decrease during the last 50 years, which is partly due to climate change (particularly, reduced precipitation) and more primarily, due to a series of human activities (particularly reservoir construction, water abstraction, and soil conservation) (Milliman, 1997; Yang et al., 1998; Xu, 2003; Walling, 2006; Wang et al., 2006). More studies have been carried out to examine the variations of water discharge and sediment load of part or the whole Changjiang (Yangtze River) Basin and possible influences of human activities were explored and discussed in these studies (Lu and Higgitt, 1998;

Chen et al., 2001a,b; Yang et al., 2002; Lu et al., 2003; Zhang and Wen, 2004; Yang et al., 2005; Zhang et al., 2006). It has been reported that the completion of the Three Gorges Dam on the Yangtze River, the world largest dam, has already caused significant decrease in sediment discharge (Yang et al., 2003, 2006; Chu and Zhai, 2006), and primary production and fish catch in the East China Sea (ECS) (Chen, 2000; Gong et al., 2003). The Zhujiang (Pearl River), is the second largest Chinese river in terms of annual water discharge, and plays a key role for fresh water supply to large cities in the Zhujiang Delta Region, such as Macau, Hong Kong, Zhuhai, and Zhongshan. The Zhujiang is the second largest river system contributing water discharge, sediment load and nutrient materials to the South China Sea (SCS) which is a major fishing ground. However, there are few reports in the international domain about current changes of water discharge and sediment load in this large drainage basin. Changes of water discharge and sediment load at Gaoyao hydrological station of the Xijiang in the Zhujiang Basin were briefly described by Walling (2006). Decreasing sediment load was reported to coincide with decreasing trends of some major ions and total dissolved solids (TDS) in the Dongjiang, one of the three main rivers in the Zhujiang Basin (Zhang et al., 2007). However, the above studies only showed and analyzed hydrological data from a few downstream stations in the main rivers rather than in the whole Zhujiang Basin. In this study, annual water discharge (WD) and sediment load (SL) during the past 50 years (the 1950s–2004) at nine stations in the Zhujiang Basin were investigated. Five stations are along the main channel of the Xijiang, two stations along the two largest tributaries of the Xijiang (namely Yujiang and Liujiang), one station along the Beijiang main channel, and one station along the Dongjiang main channel. The spatial distribution of these stations makes a full coverage of the Zhujiang Basin in the basin-wide scale. The main objective of this study was to detect the recent changes, both gradual and abrupt, of water discharge and sediment load in the Zhujiang (Pearl River) Basin during the past 50 years. Also the possible causes for changes of water discharge and sediment load in the Zhujiang Basin, from both natural and anthropogenic aspects, were explored and discussed. 2. The Zhujiang (Pearl River) Basin The Zhujiang (Pearl River) is the second largest Chinese river in terms of mean annual water discharge

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(336 km3, (Pearl River Water Resources Committee (PRWRC), 1991)), which is less than the Changjiang (899 km 3 ) but much higher than the Huanghe (36.6 km3). It is a compound water system, including three principal rivers: the Xijiang, Beijiang, and Dongjiang, and some small rivers draining the Zhujiang (Pearl River) Delta (Fig. 1). Out of the total water discharge of the Zhujiang (336 km3), 238 km3 is from the Xijiang, 39.4 km3 from the Beijiang, 23.8 km3 from the Dongjiang, and 34.8 km3 from the delta region (Pearl River Water Resources Committee (PRWRC), 1991). The Zhujiang Basin is situated between 21.31°– 26.49° N and 102.14°–115.53° E with a drainage area of 0.45 × 106 km2. Generally, the elevation in the basin decreases from northwest (Yunnan–Guizhou Plateau) to southeast delta area. It covers a region of subtropical to tropical monsoon climate straddling the Tropic of Cancer. The mean annual temperature across the basin ranges from 14 to 22 °C and the mean annual precipitation ranges from 1200 to 2200 mm. The Xijiang River, the main stem of the Zhujiang, originates in the Maxiong Mountain of Yunnan Province in southwest China, and flows southeastward through Guizhou, Guangxi and Guangdong Provinces, to enter the South China Sea (SCS) through the Pearl River Delta in Guangdong Province. The total length is 2214 km. The main channel of the Xijiang, like many Chinese rivers, has distinctive names for particular sections:

Nanpanjiang, Hongshuihe, Qianjiang, Xunjiang, and Xijiang (moving in a downstream direction). The five principal tributaries of the Xijiang (in the downstream direction) are the Beibanjiang, Liujiang, Yujiang, Guijiang, and Hejiang, and among them, Yujiang and Liujiang are the two largest tributaries of the Xijiang. Both the Beijiang and Dongjiang originate in Jiangxi Province and flow south through Guangdong Province. All the Xijiang, Beijiang and Dongjiang waters flow to the Zhujiang Delta and empty through eight large distributaries into the SCS. 3. Data set and methodology 3.1. Data set Hydrological data series (1950s–2004) of annual water discharge (WD) and sediment load (SL) of nine stations in the Zhujiang Basin were collected from the hydrological yearbooks of the People's Republic of China. Nine hydrological stations investigated in the study include five stations along the main channel of Xijiang River (Xiaolongtan (No. 1), Qianjiang (No. 2), Dahuangjiangkou (No. 3), Wuzhou (No. 4) and Gaoyao (No. 5)), two stations along the two largest tributaries of the Xijiang, i.e. Liuzhou (No. 6) in the Liujiang River and Nanning (No. 7) in the Yujiang River, one station along the Beijiang main channel (Shijiao (No. 8)), and

Fig. 1. the Zhujiang Basin and the locations of hydrological stations and major reservoirs (Hydrological stations: 1. Xiaolongtan; 2. Qianjiang; 3. Dahuangjiangkou; 4. Wuzhou; 5. Gaoyao; 6. Liujiang; 7. Nanning; 8. Shijiao; 9. Boluo. Reservoirs: ①Xijin Reservoir; ②Datengxia Reservoir (under design); ③Feilaixia Reservoir; ④Xinfengjiang Reservoir; ⑤ Fengshuba Reservoir).

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one station along the Dongjiang main channel (Boluo (No. 9)). The locations and general information of the hydrological stations are presented in Fig. 1 and Table 1. From Table 1, it can be seen that multi-year averaged water and sediment distributions are spatially uneven. The upstream part of the Xijiang has the highest sediment yield and is the dominant source of sediment in the Zhujiang river system. For example, the drainage basin upstream of Qianjiang (No. 2) contributes more than half of total sediment load by the Zhujiang with only 31% of drainage area and 23.4% of water discharge. The Beijiang and Dongjiang have relatively abundant water resources per unit area (higher runoff and higher contribution of water discharge) but lower sediment yield and lower sediment contributions compared with the Xijiang. 3.2. Methodology and preliminary data analysis Both the Mann–Kendall (MK) test (Mann, 1945; Kendall, 1975) for gradual trend analysis and the Pettitt test (Pettitt, 1979) for abrupt change detection were employed in this study to examine time series of annual water discharge and sediment load in the Zhujiang Basin. The non-parametric Mann–Kendall (MK) statistical test has been widely used to assess the significance of trends in hydro-meteorological time series due to its

robustness against non-normally distributed, censored and missing data as well as its comparable power to parametric competitors (Serrano et al., 1999; Yue et al., 2002). The null hypothesis H0 is that a sample of data Xi (i = 1, 2 …, n) is independent and identically distributed. The alternative hypothesis H1 is that a monotonic trend exists in Xi. The null hypothesis H0 is rejected at the significant level α if Z N Z(1?α/2), where Z(1?α / 2) is the value of the standard normal distribution with a probability of α / 2. However, the MK test is not robust against autocorrelation or serial correlation, which may be statistically significant in some of the hydrological series, such as annual mean stream flow. The influence of autocorrelation on trend detection capability in hydrological series has been extensively examined and many methods have been proposed to eliminate or reduce the influence of autocorrelation in the MK test, such as the pre-whitening method (MK–PW) by Von Storch (1995) and Kulkarni and von Storch (1995), and the ESS approach (MK–VCA) by Hamed and Rao (1998), Bayley and Hammersley (1946) and Lettenmaier (1976). In this study, a modified pre-whitening method, trend free pre-whitening (MK-TFPW) proposed by Yue et al. (2002), was applied in data series with significant autocorrelation to eliminate the effect of serial correlation.

Table 1 General information of hydrological stations along the mainstream and main tributaries of the Zhujiang No. Rivers/ tributaries Station name Data period Basin area (103 km2) 15.4 128.9 288.5 327.0 351.5 WD (109 m3/a) 3.8 66.6 171.3 204.0 219.9 SL (106 t/a) 4.9 42.2 57.8 63.3 67.5 Runoff (mm/km2/a) 246.6 516.9 593.8 623.8 625.5 SY (t/km2/a) 320.7 327.5 200.4 193.7 191.9 Percentage (%) b Basin area 3.7 31.0 69.5 78.8 84.7 WD 1.3 23.4 60.1 71.5 77.1 SL 6.6 56.3 77.1 84.4 90.0

Xijiang main channel 1 Nanpanjiang 2 Hongshuihe 3 Xunjiang 4 Xijiang 5 Xijiang

Xiaolongtan Qianjiang Dahuangjiangkou Wuzhou Gaoyao

1953–2004 1954–2004 1954–2004 1954–2004 1957–2004

Xijiang main tributary 6 Liujiang Liuzhou 7 Yujiang Nanning Beijiang 8 Beijiang Dongjiang 9 Dongjiang The Zhujiang a 5+8+9

1954–2004 1954–2004

45.4 72.7

39.9 37.1

5.1 9.2

877.9 509.8

111.5 126.5

10.9 17.5

14.0 13.0

6.8 12.3

Shijiao

1954–2004

38.4

41.7

5.4

1087.0

141.5

9.2

14.6

7.2

Boluo

1954–2004

25.3

23.0

2.4

907.4

96.6

6.1

8.0

3.3

1957–2004

415.2 a

285.2 a

75.0 a

Note: WD: water discharge; SL: sediment load; SY: sediment yield. a The sum of the Xijiang at Gaoyao (No. 5), Beijiang at Shijiao (No. 8), and Dongjiang at Boluo (No. 9), not including the delta region. b The percentage of Basin area/WD/SL accounting for that of the Zhujiang.

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Table 2 Results of Mann–Kendall (MK) test and Mann–Kendall-Trend Free Pre-Whitening (MK-TRPW) for water discharge and sediment load series (data in bold means significant at the significant level of 0.10) No. 1 2 3 4 5 6 7 8 9 5+8+9 Station name Xiaolongtan Qianjiang Dahuangjiangkou Wuzhou Gaoyao Liuzhou Nanning Shijiao Boluo The Zhujiang Parameter Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment Z ? 0.505 ? 1.426 ? 0.357 ? 2.989 0.747 ? 1.137 0.495 ? 3.363 0.258 ? 1.467 0.763 0.753 ? 0.227 ? 0.390 0.634 ? 0.770 0.106 ? 4.516 0.009 ? 1.568 Tau ? 0.049 ? 0.156 ? 0.035 ?0.289 0.073 ? 0.111 0.049 ?0.325 0.027 ? 0.147 0.075 0.074 ? 0.021 ? 0.037 0.062 ? 0.079 0.011 ?0.437 0.002 ? 0.165 ρ 0.614 0.154 0.722 0.002 0.456 0.256 0.620 0.000 0.796 0.142 0.445 0.452 0.820 0.696 0.526 0.440 0.916 0.000 0.992 0.126 Trend Decreasing Decreasing Decreasing Decreasing Increasing Decreasing Increasing Decreasing Increasing Decreasing Increasing Increasing Decreasing Decreasing Increasing Decreasing Increasing Decreasing Increasing Decreasing n 52 41 51 51 51 51 51 51 48 48 51 50 51 51 51 47 51 51 48 44

The preliminary analysis for checking autocorrelation was conducted by examining the autocorrelation coefficients of the time series. The autocorrelation coefficient is calculated using the following equation by Haan (2002):
1 n?k n?k P t?1 1 n?1

? ? ?Xt ? X ??Xt?k ? X ?
n P t?1

rk ?

?1? ? ?Xt ? X ?

Pn ? ? t?1 Xt , n is the sample size and k is the lag. where X n The critical value for a given confidence level (e.g., 95%) is calculated following the equation by Salas et al. (1980): p??????????????????? ?1F1:96 n ? k ? 1 ?2? rk ?95%? ? n?k If the autocorrelation coefficients fall into the 95% confidence interval at different lags, the independent null hypothesis cannot be rejected. Otherwise, the alternative hypothesis of dependence is accepted at the 5% significance level. The results of serial correlation analysis indicated only sediment load series at Qianjiang (No. 2), Wuzhou (No. 4), and Boluo (No. 9) had significant serial correlations at the lag = 3, 3, and 1 respectively. The MK-TRPW procedure was used to limit the influence of autocorrelation on trend detection in the above series, and pre-whitening method with models AR (3), AR (3),

and AR (1) was applied to remove autocorrelation from the above three series respectively. MK test was applied directly to other series without serial correlation. The Pettitt test is used to test one unknown change point by considering a sequence of random variables X1, X 2, …, XT, which have a change-point at τ (Xt for t = 1,2,…, τ have a common distribution function F1(x) and Xt for t = τ + 1,…, T have a common distribution function F2(x), and F1(x) ≠ F2(x)) (Pettitt, 1979). The null hypothesis H0: no change or τ = T is tested against the alternative hypothesis Ha: change or 1 ? τ b T using ? ? the non-parametric statistic Kt ? max1VtVT jUt;T j ? max?KT ; KT ? 8 t T < 1?if h N 0? P P Sgn?Xt ? Xj ?; Sgn?h? ? 0?if h ? 0?d K ? max U for where Ut;T ? : ?1?if hb0? i?1 j?t?1 ? downward shift and KT ? ?min1VtVT Ut;T for upward + or ? shift. The significant level associated with KT  KT is  2 ?6KT determined approximately by q ? exp T 3 ?T 2 (Pettitt, 1979). When ρ is smaller than the specific significance level, e.g. 0.10 in this study, the null hypothesis is rejected. The time t when the KT occurs is the change point time. In this study, Pettitt test was applied to the data series unaffected by serial correlation or the new data series where autocorrelation had been removed from original data series through pre-whitening, such as new series of sediment load at Qianjiang (No. 2), Wuzhou (No. 4), and Boluo (No. 9).
? T 1VtVT t;T

4. Results The results of the Mann–Kendall (MK) test for gradual trends of water discharge and sediment load

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series in the Zhujiang Basin are shown in Table 2. There are no significant trends detected for water discharge series at any of the nine stations. For sediment load, significant decreasing trends were detected at Station Qianjiang (No. 2) and Wuzhou (No. 4) in the Xijiang main channel and Boluo (No. 9) in the Dongjiang main channel. Water discharge at most stations in the Zhujiang Basin show slightly increasing trends except at stations in the upper Xijiang, such as Xiaolongtan (No. 1) and Qianjiang (No. 2), and at Nanning (No. 7) in the Yujiang, located in the southern part of the Xijiang

Basin (Fig. 2a). Sediment load, however, shows decreasing trends at most stations except Liuzhou (No. 6) in the Liujiang (Fig. 2b). The results of Pettitt test for abrupt changes of water discharge and sediment load series are shown in Table 3. At the significance level of 0.10, there are no significant abrupt changes of water discharge happening in the Zhujiang Basin during the study period. Sediment load at stations downstream Xiaolongtan (No. 1) in the main channel of the Xijiang, and Boluo station (No. 9) in the Dongjiang, as well as sediment load of the Zhujiang,

Fig. 2. The change trends of water discharge (a) and sediment load (b) in the Zhujiang Basin.

S. Zhang et al. / Global and Planetary Change 60 (2008) 365–380 Table 3 Results of Pettitt test for water discharge and sediment load series (data in bold means significant at the significant level of 0.10) No. Rivers/tributaries Station name Parameter Pettitt test KT Xijiang main channel 1 Nanpanjiang 2 3 4 5 Hongshuihe Xunjiang Xijiang Xijiang Xiaolongtan Qianjiang Dahuangjiangkou Wuzhou Gaoyao Water Sediment Water Sediment Water Sediment Water Sediment Water Sediment 206 140 ? 148 408 ? 206 270 ? 166 372 ? 136 209 p 0.169 0.189 0.378 0.000 0.152 0.039 0.295 0.002 0.374 0.098 Shift Downward Downward Upward Downward Upward Downward Upward Downward Upward Downward

371

T 1974 1974 1963 1991 1992 1991 1992 1988 1992 1997

Xijiang main tributary 6 Liujiang 7 Yujiang

Liuzhou Nanning

Water Sediment Water Sediment

? 200 ? 225 192 128

0.170 0.092 0.195 0.483

Upward Upward Downward Downward

1992 1991 1986 1986

Beijiang 8

Beijiang

Shijiao

Water Sediment

? 150 160

0.369 0.235

Upward Downward

1991 1998

Dongjiang 9 The Zhujiang 5+8+9

Dongjiang

Boluo

Water

? 142 ? 126 184

0.409

upward

1972

Water Sediment

0.430 0.097

Upward Downward

1992 1998

have significant abrupt downward changes, and most of these changes appeared in the 1990s (Fig. 3). However, sediment load at Liuzhou (No. 6) in the Liujiang, the second largest tributary of the Xijiang, has shown a significant abrupt upward change around 1991 (Fig. 3). The mean levels of sediment load shifted downward or upward by more than 30% at these stations with significant abrupt changes. If the time series of sediment load is divided into two segments based on their change points (at the significant level of 0.10), the gradual trends of sediment load before and after their change points can be further examined. The results of Mann–Kendall test for those sediment load series before and after their change points are shown in Table 4. Compared with the results of Mann–Kendall test for sediment load records in the whole study period of the 1950s–2004, the results of trend analysis for separate segments before and after change points can suggest more interesting history of sediment change. More times series of sediment load were found to show increasing or

decreasing trends at the significant level of 0.10. All stations in the Xijiang main channel downstream of Xiaolongtan (No. 1) have significant decreasing trends after their respective change points. Moreover, significant increasing trends of sediment load before the change points around were detected at Qianjiang (No. 2) and Dahuangjiangkou (No. 3). The opposite change trends were found for new time series of sediment load after and before abrupt change points at all stations in the Xijiang main channel although some trends are not statistically significant. The total sediment load of the Zhujiang (excluding the delta region) also follows the similar trends due to the dominant contribution of the Xijiang in the Zhujiang water system. The opposite trends before and after change points suggest that there were different controlling factors of sediment load in different periods. Sediment load records both before and after change point at Boluo (No. 9) in the Dongjiang have decreasing trends, but only the decreasing trend after change point 1987 is significant.

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5. Discussion 5.1. Possible causes for changes of water discharge and sediment load 5.1.1. Climate change Studies of climate change in China have showed a decreasing trend of precipitation in the upper Zhujiang

Basin but an increasing trend in the lower Zhujiang Basin during the last 40 years (Ren et al., 2004). The above tendency of precipitation is consistent with the recent variations of water discharge in the Zhujiang Basin although the variations are not statistically significant (Fig. 2a). The consistency of trends between water discharge and precipitation indicate that annual water discharge in the Zhujiang Basin is mainly

Fig. 3. The shift of mean level of sediment load at stations with significant change point at the significant level of 0.10.

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Table 4 Results of the Mann–Kendall (MK) test for times series of sediment load before and after significant change points (data in bold means significant at the significant levelof 0.10) No. Rivers/ tributaries Station name Change point (T) Qianjiang Dahuangjiangkou Wuzhou Gaoyao 1991 1991 1988 1997 Pre-T Post-T Pre-T Post-T Pre-T Post-T Pre-T Post-T Z Time series Mann–Kendall test Tau 0.264 ? 0.626 0.210 ? 0.516 0.162 ? 0.412 0.046 ? 0.643 ? 0.15 ? 0.099 ? 0102 ? 0.307 ρ 0.022 0.002 0.069 0.012 0.182 0.024 0.683 0.036 Trend Increasing Decreasing Increasing Decreasing Increasing Decreasing Increasing Decreasing

Xijiang main channel 2 Hongshuihe 3 4 5 Xunjiang Xijiang Xijiang

2.289 ? 3.066 1.818 ? 2.518 1.334 ? 0.066 0.408 ? 2.103 ? 1.321 ? 0.438 ? 0.821 ? 1.742

Xijiang main tributary 6 Liujiang

Lizuzhou

1991

Pre-T Post-T

0.186 0.661

Decreasing Decreasing

Dongjiang 9

Dongjiang

Boluo

1987

Pre-T Post-T

0.412 0.082

Decreasing Decreasing

The Zhujiang 5+8+9

1998

Pre-T Post-T

0.461 ? 1.502

0.051 ? 0.524

0.644 0.133

Increasing Decreasing

controlled by climate change (i.e. precipitation variation). However, the impact of climate change (particularly precipitation variation) on sediment load is difficult to detect at most stations in the Zhujiang Basin due to other potential anthropogenic impacts, such as basin water conservancy projects and land use change. The station with sediment load most likely impacted by climate change is Xiaolongtan (No. 1) in the upper Xijiang, where both water discharge and sediment load show slightly decreasing trends and sediment load response does not show any change in the double mass plot (Fig. 4a). At most stations the variations of sediment load are opposite to precipitation variations (Fig. 2b), which suggests that there are external controlling mechanisms influencing sediment load rather than natural climate change. 5.1.2. Reservoirs/dams constructions Documentary sources from PRWRC show that a total number of 387 large and medium reservoirs with a total storage capacity of 46.7 billion m3 have been constructed by 2005 in the Zhujiang Basin (including the delta region). The water storage index defined as the ratio of the reservoir water storage capacity and river water discharge is 0.15 for the Zhujiang. However, there are great spatial variations in reservoir constructions in the three main rivers of the Zhujiang, namely the Xijiang, Beijiang and Dongjiang (Table 5). The water

storage index is as high as 0.78 in the Dongjiang, but is only around 0.10 in the Xijiang and Beijiang. In the Xijiang main channel, most of reservoirs/dams were constructed in the 1990s and located in the section between Xiaolongtan (No. 1) in the Nanpanjiang section and Qianjiang (No. 2) in the Hongshuihe section. The sediment-reducing impacts of reservoir constructions can be detected at stations in the Xijiang main channel downstream Xiaolongtan (No. 1) (Figs. 4(b) and 5(e)). In the downstream direction, the change of sediment response in the Xijiang main channel reflected by the cumulative double mass plot appeared around 1991, 1993 to 1996, which may indicate the buffering impacts of large tributaries of the Xijiang, such as Liujiang and Yujiang. However, the annual water discharge series at stations downstream Qianjiang (No. 2) are not affected by constructions of reservoirs/dams, but show slightly increasing trends due to climate variations. Currently, the Datengxia hydroelectric project in the Qianjiang section, the largest water conservancy project in the Xijiang is under design and is expected to play its important role in basin flood regulation, electricity generation, navigation as well as irrigation. The sediment-reducing impacts in the Xijiang would be more apparent when the Datengxia project becomes operational in the future. The two major tributaries of the Xijiang, the Liujiang and Yujiang, have some

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reservoirs/hydropower stations, but most of them are of small/medium scales. Currently there is only one large reservoir — Xijin Reservoir in the Yujiang (constructed in 1964) located downstream of the hydrological station at Nanning (No. 7) and there is no large reservoir in the Liujiang. Consequently, no sediment load reduction can

be visibly detected in the cumulative double mass plots at these two stations (Figs. 4(f) and 5(g)). In the Beijiang, Feilaixia Reservoir was constructed in 1999 for regulating floods and farmland irrigation. The impacts of reservoir construction on sediment load in the Beijiang can also be detected in the double mass

Fig. 4. The variations of water discharge, sediment load and the associated double mass plots at 9 hydrological stations as well as in the whole Zhujiang excluding the delta region. (a) Xiaolongtan (b) Qianjiang (c) Dahuangjiangkou (d) Wuzhou (e) Gaoyao (f) Liujiang (g) Nanning (h) Shijiao (i) Boluo (j) the Zhujiang (excluding the delta region).

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Fig. 4 (continued ).

plot, where an obvious break in sediment load happened around 1999 (Fig. 4(h)). The Dongjiang is the most regulated river among the three main rivers with large scale multipurpose water resource development since the 1960s, such as flood control, electricity generation, farm irrigation and water supply for the large cities in the Pearl River Delta region (Guangzhou, Shenzhen,

Dongguan, Hong Kong etc.). Xinfengjiang Reservoir (constructed in 1960 with a storage capacity of 13.98 billion m3), and Fengshuba Reservoir (constructed in 1974 with a storage capacity of 1.94 billion m3) in the Dongjiang Basin are the two largest reservoirs in the Guangdong Province and have significantly affected sediment load in the Dongjiang (Fig. 4(i)).

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Table 5 Summary information of large and medium reservoirs constructed in the Zhujiang Basin (from PRWRC website http://www.pearlwater.gov.cn (in Chinese)) Rivers Large reservoir a Number Xijiang Beijiang Dongjiang The Pearl River Delta Sum
a b c

Medium reservoir b Number 212 41 35 60 348 Storage capacity (109 m3) 6 1.2 0.8 1.6 9.6

Total Number 236 47 39 65 387 Storage capacity (109 m3) 20.6 5 18 3.1 46.7 Storage indexc c 0.09 0.12 0.78

Storage capacity (109 m3) 14.7 3.8 17.2 1.5 37.2

24 6 4 5 39

The capacity of large reservoirs is over 108 m3. The capacity of the medium reservoirs is from 107 to 108 m3. Storage index is defined as the ratio of the reservoir water storage capacity and river water discharge.

5.1.3. Land use change The impacts of land use change on water discharge and sediment load, mainly afforestation and deforestation, have been known for small river basins (e.g. Chen et al., 2004). On the other hand, the effects of land use change may be difficult to identify or detect in large basins due to the existence of other human disturbances (such as reservoirs/dams construction, water extraction) and hydrological time lags (Lu, 2004). In the Zhujiang

Basin, the most obvious impact of land use change on hydrological regime is observed in the Liujiang (Liuzhou (No. 6)), the second largest tributary of the Xijiang, where the impacts of reservoirs/dams constructions are relatively minor. An obvious break around 1982 in the double mass plot can be observed likely due to rock desertification in the karst regions in the Liujiang basin (Fig. 4(f)). “Rock desertification” refers to the processes of transforming a karst area into a rocky landscape almost devoid of soil and vegetation, which is a serious ecological problem in the karst regions of southwestern China, covering Yunnan, Guizhou and Guangxi provinces (Yuan, 1997). An investigation carried out by the State Forestry Administration, PR China in 2004–2005, reported that the area suffered from rock desertification in the Zhujiang Basin is 48,650 km2, which accounts for 11.0% of the total basin area (data from Rock Desertification Bulletin in Karst Regions, published online http://www.forestry.gov.cn/). Moreover, the area affected by rock desertification increased at a rate of 0.8%–3.2% during the period 1990–2002 although a wide range of ecological environmental recovery measurements have been carried out in these regions. Due to the overwhelming sediment trapping influence of reservoirs/dams constructed after 1990 in other parts of the Xijiang Basin, the sedimentincreasing impacts of rock desertification cannot be clearly discerned beyond the Liujiang (Table 2). However, when we divide the period of 1950s–2004 into two separate segments based on the change points detected by the abrupt change tests, the sediment-increasing impacts of rock desertification can be identified before change points at stations in the Xijiang main channel (Table 4). 5.2. Sediment budget in the Xijiang main channel in different periods In order to examine the change of sediment supply along the Xijiang main channel in different periods, three

Fig. 5. Sediment load changes in the Xijiang main channel (a) as well as the sediment budget between the two neighboring stations (b). The stations represented by numbers are the same as in Fig. 1 and Table 1. Sediment budget between the two neighboring stations here is determined by subtracting sediment load at the upstream station from that at the downstream station.

S. Zhang et al. / Global and Planetary Change 60 (2008) 365–380

377

time series of sediment load were extracted from the original records, that is, sediment load series of pre-1970, 1971–1990, and post-1990 (Fig. 5). Fig. 5a shows that sediment load at Xiaolongtan (No. 1) in the upper Xijiang has a slightly continuous decline since the 1950s, which is consistent with the trend of water discharge and precipitation. Stations downstream of Xiaolongtan (No. 1) experienced an increase of sediment load in the period of 1971–1990 but a decrease of sediment load post-1990 which corresponds with reports of increased soil erosion caused by land use change in the 1980s and the subsequent sediment-trapping impacts of reservoirs post-1990. This is especially prevalent in the Hongshuihe section between Xiaolongtan (No. 1) and Qianjiang (No. 2). Calculation of the net sediment budget between neighboring stations for the three periods, also illustrates changing patterns of sediment supply along the Xijiang main channel (Fig. 5b). 1) Sediment supply from the section between Xiaolongtan (No. 1) and Qianjiang (No. 2) had the highest value in the period of 1971–1990, but decreased significantly post-1990, which is similar to the change pattern of sediment load at stations downstream of Xiaolongtan (No. 1). This also reflects land use change in the 1980s and dominant impacts of reservoir constructions post-1990 in the Hongshuihe river section. 2) The highest value of sediment supply from the section between Qianjiang (No. 2) and Dahuangjiangkou (No. 3) is found post-1990, which suggests the impacts of rock desertification in the Liujiang post-1990. 3) Sediment supply from the section between Dahuangjiangkou (No. 3) and Wuzhou (No. 4) has decreased continually since the 1950s. 4) Sediment supply from the section between Wuzhou (No. 4) and Gaoyao (No. 5) shows a dramatic increase post-1990, which may be due to increasing sediment supply both in the main channel and from small tributaries in the lower Xijiang associated with various engineering activities, including road constructions, mining, and urbanization. 5.3. Decline in sediment load to the estuary The total fluxes of the Zhujiang to the estuary (excluding the Pearl River Delta) in terms of water discharge and sediment load were expressed by the sum of the Xijiang at Gaoyao (No. 5), Beijiang at Shijiao (No. 8), and Dongjiang at Boluo (No. 9) (Table 1). The recent sediment load series of the Zhujiang exhibited a decreasing trend (though not significant at the significant level of 0.05). The break point in the double mass plot occurred around 1996, which was similar with most stations in the lower Xijiang. The average annual sediment

load was 80.4 × 106 tonnes for the period 1957–1995, and decreased to 54.0 × 106 tonnes for the period 1996–2004. More specifically, the sediment load declined steadily since the early 1990s so that in 2004 it was about one-third of the mean level of pre-90s. However, annual water discharge was not affected by reservoirs/dams construction, which showed a slightly increasing trend from 283.0 × 109 m3 averaged for the period 1957–1995 to 294.5 × 109 m3 for the period 1996–2004. Sediment load decline of the Zhujiang was due to sediment concentration decline in the three main rivers of the Zhujiang, the Xijiang, Beijiang, and Dongjiang, which can be clearly observed from the relationships of sediment load and water discharge in these three rivers. Based on monthly records of sediment load and water discharge at Gaoyao (No. 5), Shijiao (No. 8), and Boluo (No. 9), the change in the rating relationship between sediment load and water discharge in the three main rivers were examined before and after their respective

Fig. 6. The relationships between monthly sediment load and water discharge in the three main rivers and the whole Zhujiang.

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break points (as identified in the double mass plot of cumulative sediment load versus cumulative water discharge in Fig. 6). Smaller regression slopes can be observed for the relationships after the breaking points at all three stations, which may reflect the dominant sediment-trapping impacts of reservoirs/dams in the three main rivers of the Zhujiang. Coinciding with the decline of sediment load in the Zhujiang main rivers, some geomorphologic changes have happened in the downstream river channels and delta region. Although there would be other factors contributing to these changes, such as in-channel sediment mining and unreasonable coastal constructions, the decline of sediment load is also an important factor. Dramatic channel incisions (up to 8 m) have been observed in the lower reaches of the Xijiang and Beijiang during the period of 1980–2003 (Xie et al., 2005). Most of eight distributaries in the Pearl River Delta have been eroded in the 1990s rather than silt up like before, such as Modaomen in the period 1990–1998, Humen in the period 1989–1997, and Hutiaomen in the period 1989–1995 (Chinese sediment bulletin, published online http://www.hydroinfo.gov.cn/ gb/hlnsgb.asp). The slow-down of delta acceleration rate has been reported in the Pearl River Delta although the general trend is still prograding seaward (Li et al., 2004). The similar delta responses to the declines in sediment load have been reported in other large rivers in China, such as the Changjiang (Yang et al., 2002, 2003) and the Huanghe (Wang et al., 2003). It has been predicted that delta recession in the Changjiang is inevitable to occur after the Three Gorges Dam completion in 2003 (Yang et al., 2003). However, it is too early to predict the future fate of the Pearl River Delta at the moment, although the anthropogenic impacts on water discharge and sediment load will be definitely more evident in the Zhujiang with the completion of the Datengxia hydroelectric project and the practice of the afforestation policy. 6. Conclusions A detailed examination of water and sediment discharge time series has revealed a number of interesting changes that appear to correspond with phases of land degradation and reservoir construction. In the Zhujiang Basin, long-term changes of annual water discharge are not significant at the significance level of 0.05, and are mainly controlled by precipitation variation. The construction of reservoirs/dams in the Zhujiang Basin has made little influence on water discharge. However, sediment load at all main channel stations shows decreasing trends during the study period of the 1950s–2004. Not all changes detected are significant at the significant level of

0.05 but more stations have shown significant decreasing trends from the 1990s to the present. The decreasing trends of sediment load in the main channels are mainly caused by the construction of reservoirs and dams. In contrast, the Liujiang, the second largest tributary of the Xijiang, has seen increasing sediment load due to exacerbated rock desertification in its drainage basin. The total sediment load of the Zhujiang (excluding the delta region) also shows a significantly decreasing trend from the late 1990s due to the dominant impacts of constructions of reservoirs/dams in the three main rivers. This study demonstrates that while the impacts of anthropogenic activities on large river basins are rarely obvious and unequivocal, a detailed analysis of time series can begin to pick out subtle variations within basins and provide a means of establishing the relative importance of climate variability and human interventions on water and sediment discharge. The evidence suggests that the impact of land degradation on increasing sediment supply in the 1980s has been more than compensated by the construction of reservoirs/dams which have reduced sediment conveyance. Together with other impacting factors, such as inchannel sediment mining, the decline of sediment load in the Zhujiang has exerted substantial influences on downstream river channels and delta environments. Further reservoir developments in the Zhujiang Basin, especially the completion of the Datengxia hydroelectric project in the Xijiang, and an intensification of the afforestation policy in the karst regions will likely cause more significant anthropogenic impacts on the water discharge and sediment load of the Zhujiang in the future, which will further impact the river channel evolution and delta development. Acknowledgements This research was funded by SARCS (Project Number 92/01/Carbon) and the National University of Singapore (Grant Number R-109-000-054-112). The authors are grateful to Prof. John D. Milliman and an anonymous reviewer for their constructive comments in improving the manuscript. References
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