黑河上游土壤蒸发和植物蒸腾对当地降水的贡献研究

Using stable oxygen and hydrogen isotopic technique and model, we partitioned evapotranspiration into soil evaporation and plant transpiration of forest ecosystem in the upper reaches of the Heihe River Basin. At the same time, the contribution of soil evaporation and plant transpiration to atmospheric vapor was also calculated. The results showed that the contribution of evapotranspiration to atmospheric vapor is very high. From this result we can concluded that the evapotranspiration may be play an important role in forming local precipitation. Until now, the stable oxygen and hydrogen isotopic composition (δD and δ18O) in different water body not only applied in the water cycle research, but also becomes a hotspot by combining meteorological factors and isotopic mixed model to study the contribution of evapotranspiration of the forest ecosystem to local precipitation. Therefore, in the present study, by analyzing the δD and δ18O in precipitation, atmospheric water, plant xylem water, and shallow soil water, as well as calculating δD and δ18O in advected moisture, soil evaporation moisture and plant transpiration moisture, the two objectives in our project are as followed: 1) to determine the contribution of soil evaporation and plant transpiration to local precipitation in the upper reaches of the Heihe River basin; and 2) to clarify seasonal variations of moisture recycling in the upper reaches of the Heihe River Basin. Our results will offer a theoretic basis in further study of the water cycle in the upper reaches of the Heihe River Basin.

应用同位素技术及模型模拟的方法，本研究组将黑河上游森林生态系统的蒸散发分解为土壤蒸发水汽和植物蒸腾水汽，并计算了二者对大气水汽的贡献率，结果表明生态系统的蒸散发对当地大气水汽的贡献率很高。据此可推断生态系统蒸散发对当地降雨也有很大的贡献。迄今为止，通过对陆地生态系统不同水体如降水、大气水汽、土壤水和植物木质部水的氢氧同位素组成(δD和δ18O)的测定，结合气象要素和同位素混合模型，对陆地生态系统水汽蒸散发对当地降雨的贡献率的研究已成为热点。因此，本项目拟通过对黑河上游中山带森林生态系统的降水、浅层土壤水、大气水汽及植物木质部水等不同水体δD和δ18O的测定及对上风向气团输送的水汽、土壤蒸发水汽和植物蒸腾水汽的δD和δ18O的模拟，结合模型模拟计算，阐述黑河上游中山带生态系统土壤蒸发水汽和植物蒸腾水汽对当地降水的贡献率，揭示黑河源区水汽内循环特征，为进一步深入研究黑河流域水循环提供理论依据。

陆地生态系统不同水体和大气水汽的稳定氢氧同位素组成(δ18O和δD)及氘过量参数(d-excess)已成为定性和定量研究全球陆地水汽内循环的重要参数。因此水汽内循环对当地降雨贡献率的同位素研究已成为近年来研究的热点。然而在我国尤其是西北干旱半干旱区该领域的研究相对薄弱，如对水汽再循环研究严重不足、水汽再循环量化方法存在较大缺陷及土壤蒸发和植物蒸腾对大气水汽的d-excess是否有调节作用等。为此，本项目在黑河上游中山带和中山带开展了为期4个和2个水文年降水的研究，分析了次降水的δ18O和δD，研究了降水氢氧同位素的季节变化及控制因子，阐明了黑河源区形成降水的水汽来源。通过对土壤水、植物木质部水和叶片水及大气水汽的d-excess，揭示了土壤蒸发和植物蒸腾对当地大气过程的调节作用。并结合气象资料和模型，建立了氘过量参数计算再循环比的定性与定量方法体系，探讨了本地水汽再循环过程对当地降水的贡献。结果表明：(1) 黑河源区降水的δ18O和δD与温度有显著的正相关关系，温度是降水δ18O和δD的主要控制因子。中山带和高山带月平均温度每增加1℃，降水δ18O、δD的雨量加权平均值分别增加0.87‰、5.82‰和0.63‰、4.71‰。且黑河源区降水δ18O和δD与我国西风控制区的乌鲁木齐有相似的季节变化规律，即冬季偏负而夏季偏负。结合冬季和夏季的风场与湿度场得出黑河源区夏季降水主要来源于西风输送，冬季除西风输送外还受极地气团的影响。(2) 相对湿度与植物叶片水的d-excess显著正相关，而与大气水汽d-excess显著负相关；温度与叶片水的d-excess显著负相关而与大气水汽d-excess显著正相关。且大气水汽的d-excess与叶片水和雨后第一个晴天浅层土壤水的d-excess呈相反的日变化趋势。即生态系统植物蒸腾水和雨后土壤蒸发水对当地大气水汽的d-excess有调节作用，揭示了生态系统蒸散发对当地大气过程的调节作用。(3) 黑河源区高山带野牛沟内循环对降水的年均贡献率为5.1%，年均降水量为533.4mm，再循环水汽形成降水量为22mm/年；葫芦沟内循环年均贡献率为8.6%，2009年7-12月降水量为370.9mm，再循环水汽形成降水量为32mm/年。中山带排露沟流域内循环年均贡献率为10.3%，年均降雨量为415mm，再循环水汽形成降水量为43mm。