黑河源区水汽内循环对当地降水贡献率的同位素研究

The stable oxygen and hydrogen isotopic compositions (δ18O and δD) in terrestrial water body and atmospheric vapor are the important parameters to qualitatively and quantitively assess moisture recycling in different regions of all over the world. Many results revealed that atmospheric moisture recycling plays an important role in the formation of precipitation. Therefore, using the isotopic technique to study the contribution of moisture recycling to local precipitation gradually becomes a hotspot in water cycle research. Our results showed that the contributions of soil evaporation moisture and plant transpiration moisture to local atmospheric vapor and precipitation are high in the upper reaches of the Heihe River Basin. However, there is little quantitive study about the contribution of soil evaporation moisture and plant transpiration moisture to local precipitation. In this study, by analyzing the spatial and temporal variations of δD and δ18O in precipitation, atmospheric water, surface water, plant xylem and shallow soil water of high mountainous and middle mountainous regions, and in surface water, soil water and plant xylem water in the whole upper reaches, as well as calculating spatial and temporal variations of δD and δ18O in advected air masses, surface evaporation and plant transpiration in the upper reaches of the Heihe River Basin, combining meteorologic data and three-end-member linear mixing model, the objectives in our project are as followed: 1) to quantitively assess vapor contributions of advection, evaporation and transpiration to local precipitation; and 2) to reveal spatial and temporal variations of moisture recycling of the upper reaches and its important role in studying the water cycle precesses in the Heihe River Basin. Our results will offer a theoretic basis in water resources management of the Heihe River Basin.

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)已成为定性和定量研究全球陆地水汽内循环的重要参数。研究表明陆地水汽内循环对当地降水有很重要的贡献，故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。已有研究表明我国西北干旱区黑河源区土壤蒸发水汽和植物蒸腾水汽对当地大气水汽和降雨均有很大贡献，然而上述要素对降雨贡献率的定量研究还很缺乏。本项目拟通过对黑河源区中山带和高山带夏季降水、大气水汽、浅层土壤水、地表水及植物木质部水的δ18O 和δD 的定点高频率测定和整个黑河源区面上测定，结合气象资料分析地表蒸发水汽、植物蒸腾水汽及上风向气团水汽中δ18O 和δD 的时空变化。并结合三端元混合模型，定量研究上风向水汽、地表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及其在黑河流域水循环研究中的作用，为黑河流域水资源的管理提供理论基础。

陆地蒸发和植物蒸腾形成的内循环水汽对干旱内陆河流域水文过程和大气水汽收支起着重要的调节作用。然而有关流域尺度上水汽内循环的研究比较匮乏。本项目通过对黑河流域降水、土壤水、植物水稳定氢氧同位素组成(δD和δ18O)的测定及对大气水汽、上风向输入水汽和土壤蒸发水汽δD和δ18O的模拟，基于三端元混合模型，研究了干旱半干旱区典型内流河流域-黑河流域的水汽内循环特征及其控制因子。结果表明：植物生长季节黑河上游的降水有近一半来源于当地植物蒸腾产生的水汽(fTr)。在黑河中游，人工绿洲和荒漠绿洲过渡带的降水有近一半是由内循环水汽贡献的，其中fTr和土壤蒸发水汽(fEv)对降水的贡献率在这两个区域有显著差异。在黑河下游，只有不到四分之一的降水由内循环水汽供给。中游戈壁和黑河下游河岸林的fTr都较低。fTr与降水和相对湿度呈正相关关系，而与水汽压亏缺负相关，说明降水量和相对湿度的增加会促进fTr，而水汽压亏缺的增加会降低fTr。fEv与温度和水汽压亏缺正相关，而与相对湿度负相关，说明温度和水汽压亏缺的增加会增加fEv，而相对湿度的增加会降低fEv。本研究结果说明，在我国干旱半干旱区，内循环水汽尤其是植物蒸腾水汽对当地降水的贡献对当地水资源分配有着极为重要的作用。