黄土高原岔巴沟流域降水氢氧同位素特征及水汽来源初探

Characteristics of Hydrogen and Oxygen Isotopes and Preliminary Analysis of Vapor Source for Precipitation in Chabagou Catchment of the Loess Plateau

降水是流域水循环中重要的输入因子，对其同位素组分的研究有助于确定流域水体间的水力联系，还可反映流域综合自然地理及气象气候信息；结合地表水、土壤水及地下水同位素组分变化，可以确定降水入渗及产汇流过程、地下水补给及更新能力，进而为完整的水循环机理研究提供依据。结合水文气象观测，本文分析了黄土高原典型丘陵沟壑区岔巴沟流域2004年-2005年月降水以及曹坪西沟实验流域次降水环境同位素组分雨量效应、高程效应、季节变化、δD与δ^18O间的关系等，并对不同时空尺度上大气降水线的变化规律进行分析，结果表明该地区不同年份及雨季前后，降水的水汽来源不同且经历蒸发过程差异较大；同时发现大气降水线中斜率与氘盈余之间存在良好的线性关系9．74×S—d=69．3，即虽然不同年份大气降水线存在差异，但两者存在相对稳定的线性关系，该关系可能与水汽来源之外的某种固定的综合自然地理特征有关，也为获得可靠的分析结果提供了参考。

Precipitation is the most important input to water cycle. Study of hydrogen and oxygen isotopes of precipitation can help identify relationships between different waters and reflect integrated geographical and climatic information of the study area, which is the foundation for integrated water cycle research. Combined with isotopic compositions of surface water and groundwater in the study area, systematic analysis for precipitation in different scales can quantify the runoff generating processes, as well as the recharge rate for precipitation reaching water table. By analyzing isotopic compositions of rains between 2004 and 2005 for δD and δ^18 O, the study discussed the amount effect, seasonal effect, spatial distribution of δ values not only for monthly precipitation on the upper, middle and lower reaches of the Chabagou Catchment, also for precipitations in Caoping Xigou Experimental Watershed by event scale. In the whole watershed, the isotopic results for monthly precipitation shows obvious spatial and temporal variation, and an altitude effect only prominent from May to August, following which a further decline of values for rain in September reflecting that the rains are mainly formed in the process of monsoon vapor withdrawing from the northwest. In the Chabagou Catchment, the notable spatial variation of δ values indicates that spatial variation of δ values should be fully considered when getting a confidential local meteoric water line, which can well reflect the local geographical and climatic characteristics, especially for isotopic ungauged basins with short observing series. Meteoric water lines, i.e. linear relationships between δD and δ180, differs from each other both for stations and years, even for different periods of a year, indicating a dissimilar water vapor and variant processes water vapor having experienced before reaching the ground, which can be a good tool for study of water vapor origin if hydrological and climatic observations be involved. And then, relationships between slope （S） and deuterium excess （d） for meteoric water lines at each rain gauge stations or seasons are well linearized. The formula is 9.74 × S - d = 69.3, namely, though meteoric water line may differ seasonally and annually and relationship between S and d is relatively solid. This formula is also applicable for many other places of China, even for many regions in the world. It gives possible promise for studies from spatial and temporal variation of meteoric line, transportation of water vapor to main geographical and climatic factors affecting the study area, and may make some difference in theoretical and practical system of isotope hydrology. Based on all the analysis above, a further study on relationship between precipitation, surface water, soil water and groundwater will be identified and water cycle mechanism of gully and hilly regions of the Loess Plateau can be studied in detail.