| Soil water is an important link connecting atmospheric precipitation,surface water and groundwater.It is the key of understanding this process of precipitation-soil water-groundwater to solve the shortage of water resources.The loess hilly area of Lanzhou is relatively scarce in water resources.In addition,Lanzhou has implemented the greening project of the north and south mountains in recent years.Therefore,it is urgent to carry out in-depth research on the hydrological process in this area,so as to realize the sustainable use of regional water resources and provide greening for the existing areas.The maintenance of achievements and the further promotion of greening work provide a scientific theoretical basis.At the same time,considering that the stable isotopes of hydrogen and oxygen(2H and 18O)are the components of water molecules,they are natural tracers to describe the hydrological process of"precipitation-soil water-groundwater".In view of this,this study took the loess hilly area of Lanzhou City as the research object,sampled precipitation,soil water and groundwater from 2020 to 2021,and determined the stable isotopic composition of hydrogen and oxygen.Mathematical statistics,Craig-Gordon model,lc-excess signal method,lc-excess equilibrium equation and Bayesian mixture model（Mix SAIR）were used to analyze the temporal and spatial variation characteristics of hydrogen and oxygen stable isotopes in different water bodies.On the one hand,the process of precipitation to soil water recharge was discussed,and two infiltration modes,including piston flow and preferential flow,were qualitatively analyzed.On the other hand,the relative contributions of these two modes to groundwater recharge were quantified.Based on this study,the results are concluded as following:（1）The isotope values of precipitation in the dry season is relatively depleted,and the rainy season is relatively riched.the LMWL（Local Meteoric Water Line,LMWL）isδ2H=7.16δ18O+5.16,and its slope is smaller than the GMWL（Global Meteoric Water Line,GMWL）,indicating that the local evaporation is strong.Soil water isotope values were gradually enriched from April to May,and gradually depleted from May to September,and the soil water stable isotopes with vegetation coverage profiles had less variation on the monthly scale.On the spatial scale,the soil water isotope values of the three profiles are depleted in the deep layer and enriched in the surface layer;in the profile covered by vegetation,the temporal and spatial changes of soil water isotopes are more stable and depleted,indicating that the vegetation cover has an effect on the production of soil water isotopes effect,while altitude had no significant effect.The variation of groundwater stable isotope values is small,and is close to the rainy season rainfall isotope,indicating that the groundwater may be mainly recharged by the rainy season rainfall.（2）The changes of soil water content in the three sampling points showed that the effect of altitude on soil water supply was not obvious,while vegetation coverage would significantly affect soil water supply.By comparing the isotope values of soil source water and precipitation,it was analyzed that rainfall with intensity of 10～20mm/day and 20～30 mm/day was the main source of soil water supply.The relative contribution of rainfall to soil water recharge at different soil depths was estimated based on the Bayesian mixture model.All types of rainfall had the highest contribution rate to the surface layer,and rainfall of 10-20 mm/day had a higher contribution to the middle and deep layers.（3）On the 0～200 cm soil profile in the study area,the infiltration of piston flow is dominant,and preferential flow signals appear at individual times.The relative contributions of piston flow and preferential flow to groundwater recharge were calculated by the lc-excess equilibrium equation and Bayesian mixture model（Mix SAIR）to be 10±2%and 90±2%,respectively.The preferential flow occurs when the rainfall in the profile covered by vegetation is above 30 mm,and the preferential flow signal appears only when the rainfall in the without profile covered by vegetation is above 40 mm,indicating that the existence of shrub vegetation can easily lead to the occurrence of preferential flow. |
