黄土丘陵区降水-土壤水-地下水转化实验研究

Experimental study on the precipitation-soil water-groundwater transformation in loess hilly region

通过对黄土丘陵区燕沟流域2005～2007年雨季的多次降水、0～400cm土层土壤水、沟道地表水、地下水（泉水、井水）水样中D和^18O采样分析，研究了该区降水、土壤水、地表水、地下水的转化关系。结果认为：燕沟流域的降水线与中国、世界的降水线有明显区别，斜率和截距偏小；降水、地表水、土壤水、地下水逐渐富集δD和δ^18O，且δ^18O富集速度高于δD，由D和^18O的蒸发分馏差异所致，可利用各类水体的δD和δ^18O变化情况甄别水体之间的水量转化；土壤水δD和δ^18O剖面在200cm深度处出现低值区，应是降水补给到达该深度且土壤蒸发影响逐渐衰减共同作用的结果，其在200cm以下逐渐升高则因为降水补给影响逐渐降低、土壤水本底同位素影响增强所致。由于380～400cm深层土壤水的δD和δ^18O对降水事件的响应存在，因此认为降水-地下水的转化存在，降水补给泉水的滞后期小于35d。而对井水的补给滞后时间以及土壤水对地下水的补给量还需进一步研究。

During the rainfall events in the rainy seasons of 2005-2007, a total of 87 water samples from rainfall events, soil water at 0-400cm layers, surface water in valleys, and ground water （spring water, well water） were collected, the D （deuterium） ＆ ^18O （oxygen-18） in these samples were analyzed to interpret the relationship among those water sources in Yangou watershed in the loess hilly region, and the locally meteoric water line （LMWL） was obtained. The result shows that the LMWL of Yangou is similar to that of Xi＇ an region but different significantly from the LMWL of China and the global meteoric water line （ GMWL）, both slope and intercept of the LMWL of Yangou are smaller than that of China LMWL and GMWL, the δD ＆ δ^18O content in different water sources are gradually richer in the order of rainwater, ditch water, soil water and groundwater due to isotopic fractionation, and the enrichment of δ^18O is faster than that of δD. The variation of δ^18O and δD values can be used to identify different water sources and to determine the transformation relationships among them. We have found that the minimum values of δ^18O and δD appear to be at the 180-200 cm depth, indicating that the precipitation can infiltrate into the subsurface soil at that depth and the influence of deep soil water on the surface evaporation vanishes at that depth too. The δD ＆ δ^18O values increase gradually below 200 cm depth, which suggests that the influence of precipitation infiltration is weakening and the groundwater component in soil water is increasing. In responding to rainfall the events, the δD ＆ δ^18O values are found to be fluctuated in deep soil at the 380-400 cm depth. This result indicates that the rainfall is able to recharge groundwater through either capillary micro-flow or/and priority macro-flow. The experimental data also show that the time lags between the rainfall events and recharging from the springs are less than 35 days. Further studies are warranted to investigate the time lags between rainfall events and recharging from wells, as well as the recharging amount of groundwater from soil water.