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素水文过程研究


青藏高原中部那曲河流域稳定同位素水文过程研究

摘要: 摘要:气候变暖对全球许多地区的自然生态系统、能量水分循环过程以及人类的社会经济活动等均产 生了深刻的影响。青藏高原作为全球气候变化的敏感区域,已经成为全球关注的焦点地区。受全球变暖的 影响,青藏高原现代环境与地表过程相互作用,引起当地气候、水循环和生态系统等发生一系列变化。但 是,由于高原广袤复杂的地表状

况和极端的气候环境,气象水文台站仍然十分稀少,利用传统的水文观测 方法很难长时间系统地获取有效的水文数据对高原地区流域水文过程进行研究,然而,利用稳定同位素方 法却可以有效地解决这一问题。随着对稳定同位素分馏机制认识的深入以及同位素分析技术的发展,同位 素已经广泛应用于水文和气候研究,其中,氢(D)和氧(18O)作为水分子的组成部分,由于在其水循环 不同阶段(主要是蒸发和凝结)发生同位素分馏而表现出系统的时空变化,从而成为最理想的天然示踪剂 广泛应用于区域和全球规模的水文气候过程研究。 为了洞察变化环境下的水文循环过程和重建高原的气候变化,本研究首先利用模型建立了中国降水 δ O 与纬度和海拔的定量关系,并结合纬度、海拔和水汽来源影响等产生了较高分辩率的中国降水 δ18O 空 间分布图,为中国,尤其是青藏高原地区的气候和水文过程研究提供了参考信息,接着又以高原中部的那 曲河流域作为研究对象,通过在流域内开展降水、河水和湖水中稳定同位素监测研究,揭示了该流域稳定 同位素水文循环过程。主要的研究结果体现在以下几个方面: (1)确定了我国降水中 δ18O 与纬度和海拔的定量关系模型为:δ18Oppt= -0.0073LAT2 + 0.3261LAT 0.0015ALT - 9.7776。利用地理信息系统分析软件 ArcGIS,综合纬度、海拔、水汽源地以及循环过程等影响 因素,产生了较高分辨率的中国降水中 δ18O 空间分布图,揭示了我国降水中 δ18O 空间分布规律。 (2)建立了那曲河流域大气降水线、蒸发线以及河水中 δD 和 δ18O 的关系 大气降水线(LMWL) δD = 7.93δ18O + 14.04 δD = 8.05δ18O + 16.25 蒸发线(LEL): 河水 δD-δ18O 关系线: (R2 = 0.95, n = 545)(降水事件) (R2 = 0.97, n = 32) (月加权平均值)
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δD = 5.2 δ18O – 32.3 (R2 = 0.98, n = 18) δD = 5.70δ18O -30.12 (R2 = 0.93, n = 91)

(3)那曲河流域降水稳定同位素主要受不同季节环流模式控制,夏季受印度季风控制时,流域降水 同位素值比较低,且波动比较频繁,而在其它季节,尤其是春季受大陆气团控制时,降水同位素值一般比 较高;在夏季,流域内降水同位素的空间变化受到地理要素的影响,尤以地形为主,其和地理要素的关系 为:δ18Oppt= 6.4344LAT + 2.8595LONG - 0.0020ALT - 474.8110 (R2 = 0.90, n =6)。在夏季季风降水期间,降 水同位素的频繁波动和水汽输送过程密切相关,一般情况下,来源于印度洋的水汽,由于强烈的季风活动、 水汽的厚层输送以及翻越喜马拉雅山等都会导致降水中的稳定同位素产生低值现象,反之季风活动较弱、 水汽的薄层输送以及来源于西风带或当地蒸发的水汽,多使得降水中的同位素为高值。另外,在流域夏季 降水期间,降水量小于 4 mm 的降水事件会受到二次蒸发的影响,受其影响,大气降水线的斜率和截距均 会降低。 (4) 流域内降水稳定同位素展示了一定的降水量效应, 其中降水 δ18O 的降水量效应为: 18O = -0.54P δ - 14.00 (R = -0.21, P < 0.0001, n = 545),而当气温在-4℃以下时流域内降水同位素则展示了一定的温度效应, 其中降水 δ18O 的温度效应为:δ18O = 1.40T - 1.62
18

(R = 0.51, P < 0.005, n = 32) 。

(5)流域河水中 δ O 和 δD 平均值分别为-12.1‰和-99.3‰,显著高于同期的降水稳定同位素,但其 变化幅度则显著小于降水稳定同位素,其春高夏低的季节变化趋势及季风期间的波动均同降水比较一致, 反映了降水是流域河水重要的补给来源,通过对流域夏季流量过程线的划分,结果表明夏季径流量的 47% 来源于降水,而其它水体(主要为地下水和融水)大致贡献了 53%;在空间变化上,由于受流域海拔、集 水区规模和湖水等的影响,上游河水的同位素值和波动幅度均低于下游。 (6)错那湖水δ18O和δD的平均值分别为-9.6‰和-83.3‰,显著高于流域同期降水和河水的同位素值, 其变化幅度虽然小于后者但展示了明显的季节变化,这种变化同降水同位素的变化比较一致,表明降水是 湖水主要补给来源之一。在非结冰期,错那湖水受到蒸发的影响,基于稳定同位素质量平衡方法,并结合

相关的气象数据,计算出了湖面解冻期间输入错那湖的水量24%消耗于蒸发,其余水体则通过那曲河注入 怒江。 (7)蒸发是那曲河流域水体的主要支出部分,利用稳定同位素水量平衡方法,估算了流域内表面水 体蒸发通量和土壤及植被蒸散的通量,二者分别为 24 mm 和 395.4 mm。

Abstract: Global warming has exerted a profound influence on natural ecosystems, energy and water : cycle processes as well as economic-society lives of the human in many regions. The Tibetan Plateau has already become the focus of the global attention for its sensitivity to global change. Due to the impact of global warming, the interaction between modern environment and land surface processes on the plateau induces a series of variations in local climate, hydrological cycle and ecological system. However, the meteorological and hydrological stations on the Tibetan Plateau are still very sparse for its vast and complex land surface and extreme climate, which leads to enormous difficulties in well understanding the watershed hydrological processes using the traditional method of long and systematic hydrological observation. Alternatively, analysis of water isotope tracers is a practical alternative to assess the hydrological processes in such a remote environment. With detailed understanding of the stable isotopic fractionation mechanism in the condensation and evaporation processes and advancements in instrumentation, the stable isotopes oxygen-18 (δ18O) and deuterium (δD) are ideal tracers for investigation of hydrologic parameters since they are part of the water and can be applied naturally to the regional and global hydrologic system. In order to understand present hydrological process and reconstruct climatic change in the Tibetan Plateau, we first establish a quantitative relationship between precipitation δ18O and latitude and altitude over China, which is used to generate a higher-resolution map of mean annual δ18O for precipitation over China, integrating with latitude, altitude and the moisture cycle. This determination of the spatial distribution of δ18O in modern precipitation provides important information for ancient climate and stable isotope hydrology studies in China, especially on the Tibetan Plateau. And then, an isotopic investigation for precipitation, river water and lake water of the Nagqu River basin on the central Tibetan Plateau is carried out to reveal the stable isotope in the hydrological processes of the basin investigated. The followings are some main results of our study: (1) This study established a quantitative relationship between δ18O in precipitation and latitude and altitude using the existing δ18O data from 55 sites over China. The model describes as δ18Oppt = 0.0073LAT2 + 0.3261LAT - 0.0015ALT - 9.7776, and was used to generate a higher-resolution map of mean annual δ18O for precipitation over China using geographic information system software, integrating with latitude, altitude and the moisture cycle. This determination of the spatial distribution of δ18O in modern precipitation of China provides important information for ancient climate and stable isotope hydrology studies. (2) Presented the relationship plot of δD-δ18O for precipitation (LMWL), lake water (LEL) and

surface runoff in the Nagqu River basin of the central Tibetan Plateau. The regression equations were as follow: LMWL:δD = 7.93δ18O + 14.04 δD = 8.05δ18O + 16.25 (R2 = 0.95, n = 545)(based on individual events) (R2 = 0.97, n = 32) (based on monthly

amount-weighted average values) LEL: δD = 5.2 δ18O – 32.3 (R2 = 0.98, n = 18) δD = 5.70δ18O -30.12 (R2 = 0.93, n = 91)

δD-δ18O plot for surface runoff: (3)

The stable isotopes in precipitation of the basin investigated are mainly controlled by the

different seasonal circulation patterns. When controlled by the Indian Ocean monsoon in summer, the isotopic values is lower, with the frequent fluctuation, while in other seasons, especially in spring, the continental air masses dominated the basin, which usually leads to the higher values. The spatial variation of isotopic compositions in precipitation were influenced by the geographic factors, especially by the topography, and in summer the quantitative relationship between precipitation δ18O can be described as: δ18Oppt= 6.4344LAT + 2.8595LONG - 0.0020ALT - 474.8110 (R2 = 0.90, n =6). During monsoon precipitation, moisture transport from far distance and deep-layer as well as moisture crossing the Himalayas Mountains all lead to the lower δ18O values, on the contrary, close, thin-layer moisture transport and moisture from westerly or local evaporation all lead to the higher δ18O values. Additionally, during the summer precipitation, raindrops for the individual events with a precipitation amount below 4 mm are affected by secondary evaporation, and by which the slopes and intercepts of the local meteoric water lines will decrease. (4) The isotopic variations of precipitation in the basin showed an amount effect and the

quantitative relationship between precipitation δ18O and precipitation amount can be described as: δ18O = -0.54P - 14.00 (R = -0.21, P < 0.0001, n = 545). When the air temperature is below -4℃, However, The isotopic variations of precipitation showed a temperature effect: δ18O = 1.40T - 1.62 (R = 0.51, P < 0.005, n = 32). (5) Mean isotope values of river water in the Nagqu River basin are -12.1‰ for δ18O and -99.3‰ for δD, respectively. With smaller fluctuation amplitude, isotopic values of lake are all higher than those in precipitation of the corresponding period. However, the trend with high values in spring and low values in summer is the same as that of precipitation, which reflects precipitation is the important source of river recharge. We partition the runoff hydrograph based on an isotopic technique and the result show the discharge in summer comes from 47% precipitation and 53% groundwater and melted water. From spatial variation, the isotopic compositions and its fluctuation amplitude in the upstream are all lower than those in the downstream. (6) Mean isotope values of Cona lake water are -9.6‰ for δ18O and -83.3‰ for δD, respectively. Despite its higher values and smaller fluctuation amplitude, isotopic composition of lake display an obviously same seasonal trend, which indicates precipitation is the important source of lake recharge. During the ice-free season, due to arid environment and exposure to strong wind, intensive evaporation occurs to the surface water of the lake. Cona lake is estimated using an isotope-based method and the result shows approximate 24% of water flowing into Cona Lake subsequently undergoes evaporation and the remaining inflowing water outflows into Nujiang River by the Nagqu River. (7) Evapotranspiration is the main part of basin budget and an isotope-based method for water balance is used to evaluate the evapotranspiration from the basin investigated. Our analysis suggests the evaporation flux from surface water is about 24 mm and evapotranspiration flux from the soil and vegetation is about 395.4 mm.


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