基于氢氧同位素技术的喀什噶尔绿洲水汽再循环对局地降水的贡献评估

Recycled moisture from surface evaporation and transpiration is an important source of terrestrial precipitation. The proportional contribution assessment of recycled moisture to local precipitation using stable hydrogen and oxygen isotopes is considered as one of hot topics in hydrometeorological studies. In arid central Asia, especially in southern Xinjiang, the local moisture in oases may greatly originate from surface evaporation (open water and soil) and transpiration, but the quantitative assessment of proportional contribution is still limited. The Kaxgar Oasis (Kaxgar River Delta Oasis), one of the three largest natural oases and the second largest artificial oasis in southern Xinjiang, is selected as study region in this program. The isotopic three-component mixing model will be applied with an assumption that precipitating moisture is an intensive mixture of moisture from advection, surface evaporation and transpiration. The measured isotopic ratios in precipitation samples and corresponding meteorological parameters will be input into the model, and isotopic values in surface water, shallow soil water and plant xylem water will also be involved. The three-component mixing model can be improved and assessed for uncertainty in this program, and can provide proportional contributions of surface evaporation and transpiration as well as their errors. The program aims to investigate the influence of moisture recycling on local precipitation in a typical oasis of southern Xinjiang, and to contribute to a better understanding of the eco-hydrological processes and land-atmosphere interactions in arid central Asia. The results will be also useful to a wider application of isotopic mixing model in an arid condition.

地表蒸发和植物蒸腾形成的再循环水汽是陆地降水的重要来源，利用稳定氢氧同位素技术评估水汽再循环对降水的贡献率是水文气象研究的热点之一。在亚洲中部干旱区，特别是南疆地区，绿洲地表水体和土壤蒸发以及植物蒸腾贡献了大量水汽，但其对降水的贡献率定量评估却比较有限。喀什噶尔绿洲（即喀什噶尔河三角洲绿洲）是南疆三大天然绿洲之一和第二大人工绿洲，本项目拟以该绿洲为研究区，利用同位素三元混合模型，即假设局地降水水汽由外来水汽、地表蒸发水汽和植物蒸腾水汽三部分混合形成，以实测的降水同位素和气象数据作为主要输入参数，辅以实测的地表水、浅层土壤水和植物水同位素数据，改进和优化三元混合模型，计算绿洲地表蒸发和蒸腾对降水的贡献率及其误差范围，并评估模型的不确定性。研究旨在明确南疆典型绿洲水汽再循环对局地降水的影响，为认识亚洲中部干旱区水文生态过程和陆—气相互作用做出贡献，同时有助于同位素混合模型在干旱区的推广。

在亚洲中部干旱区，绿洲的地表蒸发与蒸腾为局地降水提供了大量水汽，氢氧稳定同位素是水循环的天然示踪剂，可以用于定量评估水汽再循环过程对局地降水的贡献。本项目以南疆西部的喀什噶尔河三角洲绿洲（简称喀什噶尔绿洲）为重点，建立了大气降水氢氧稳定同位素监测网络，明确了南疆西部大气降水氢氧稳定同位素的基本特征以及影响因素，系统分析了典型绿洲地表蒸发和植物蒸腾对局地降水的贡献率以及同位素三元混合模型的不确定性。研究得到如下主要结论：①通过实测资料揭示了喀什噶尔绿洲降水同位素的时空格局，利用区域聚类全子集回归模型建立了高空间分辨率的逐月降水同位素景观图谱，对比了经向环流和纬向环流与降水同位素联系的差异，模拟了云下二次蒸发对降水同位素不同尺度下的影响。②全面梳理了以往文献中有关同位素三元混合模型模拟水汽再循环贡献率的算法异同，利用多种算法评估了再循环水汽对喀什噶尔绿洲局地降水的贡献率及其不确定性，发现各算法得到夏季水汽再循环贡献率平均在10%左右，其中蒸腾的贡献率大于蒸发的贡献率，但是不同的算法得到的结果存在较大差异。③在利用降水同位素估算水汽同位素时，平衡分馏假设会低估水汽同位素值，平衡分馏假设和非平衡分馏假设得到的氧同位素比率平均相差3.9‰；评估了主要输入参数变化下水汽同位素模拟值的敏感性，非平衡分馏假设下气温升高或相对湿度降低会导致水汽同位素升高，而气温降低或相对湿度升高对应着水汽同位素降低。项目得到的结论有助于认识亚洲中部干旱区水文生态过程和陆—气相互作用，并且对稳定同位素方法在干旱区水文生态研究中的应用有参考价值。