Spatio-temporal characteristics and factors influencing groundwater recharge in a large agricultural drainage lake basin, northwest China

The Ulansuhai Lake basin is a key agricultural region in northwest China. However, the impact of climate change and human activities on groundwater recharge patterns is not understood well. Herein, water samples during different seasons were analysed for δD and δ¹⁸O. Based on HYSPLIT and statistical methods, the results show that groundwater recharge primarily comes from agricultural irrigation return flow and precipitation. During monsoon seasons, precipitation infiltrates and recharges shallow groundwater, while in the autumn irrigation period, evaporated water recharges shallow groundwater via soil infiltration. Spatially, the eastern basin experiences more rainfall recharge, leading to heavy isotope depletion, while irrigation activities in the western basin result in isotopic enrichment. Factors including elevation, evapotranspiration, salinity, and irrigation influence the isotopic composition of groundwater, demonstrating significant regional and seasonal variations. Recharge from high-altitude areas usually leads to isotope depletion, while an increase in potential evapotranspiration enhances the enrichment of δ¹⁸O and δD. Additionally, the rise in salinity caused by evaporation is closely associated with isotope enrichment, and the infiltration of high-phosphorus concentration irrigation water further promotes isotope enrichment. This study provides new insights into the groundwater recharge characteristics in the Ulansuhai Lake basin and encourages the management of groundwater resources.