Hydrological Resilience and Baseflow Buffering Under Extreme Drought: Stable Isotopes Evidence from the Loess Plateau， China

Extreme droughts increasingly stress water systems in semi-arid regions， yet how surface water-groundwater interaction （SGI） responds when a basin shifts from its normal state into an extreme drought remains poorly constrained by field evidence. During an extreme summer drought in July 2024， we conducted basin-wide surface water and shallow groundwater sampling for stable isotopes (δD and δ¹⁸O) and chloride (Cl^-) across the Wuding River basin on the Chinese Loess Plateau， aiming to examine how SGI and hydrological processes respond to extreme drought. End-member mixing analysis revealed that SGI responses were dictated by contrasting hydrogeological units. In the wind-erosion sandy areas， groundwater sustained a stable baseflow， contributing on average 54.96% （up to 93.29%） of streamflow. In the loess areas， groundwater contributed only 8.54% of streamflow in the upstream reaches， whereas river leakage occurred mainly near the downstream basin outlet， reaching 32.96%. Craig-Gordon isotope modelling further indicated that evaporation accounted for approximately 18.86%-23.89% of the inferred inflow to the five major reservoirs， with a mean loss of 21.34%. The stable-isotope response to drought differed markedly between surface water and groundwater. Relative to previous dry- and normal-period conditions， surface water exhibited only marginal enrichment despite strong atmospheric evaporative demand. This was attributed to the continuous contribution of relatively isotopically depleted groundwater baseflow， which buffered the evaporative enrichment of surface water. In contrast， groundwater was more isotopically enriched than under previous hydrological conditions， which may reflect localized mixing between pre-existing groundwater and evaporatively enriched vadose-zone water. Together， these results reveal contrasting drought responses between the two hydrogeological units， with stronger groundwater buffering and sustained streamflow in the wind-erosion sandy areas， but weaker buffering and greater hydrological vulnerability in the loess areas. These findings demonstrate that drought responses are strongly controlled by hydrogeological setting and provide a basis for spatially differentiated water-resource management in semi-arid basins under increasing climate extremes.