Divergent Responses of Water Isotopes to Summer Hot‐Dry Extremes and Their Implications for Evaporation: A Case Study of the Jinyun Mountain Area, Southwest China

ABSTRACT Compound hot‐dry events are increasingly frequent, threatening water resources in the humid subtropical mountainous regions of Southwest China. This study investigated stable isotopic responses and evaporation dynamics of spring water, lake water and reservoir water in the Jinyun Mountain area of Chongqing during the extreme hot‐dry summer in 2024. Water samples were collected for δ 18 O and δD analysis and evaporation losses were quantified using an isotope mass balance approach based on the revised Craig–Gordon model. Marked isotopic contrasts were observed among water bodies. Spring water exhibited high and stable d‐excess values and a δD‐δ 18 O regression line closest to the Local Meteoric Water Line (LMWL), indicating dominant groundwater recharge and weak evaporative effects. Lake water showed slight spatiotemporal isotopic variations, reflecting continuous and steady recharge that buffered against evaporative enrichment. In contrast, reservoir water underwent progressive isotopic enrichment and a decrease in d‐excess, suggesting amplified evaporation under prolonged drought and long residence time, with d‐excess decreasing from north to south. Notably, model‐derived evaporation‐to‐inflow ratios ( E / I ) remained low and stable in lakes, but rose markedly in reservoirs over time. These E / I ratios remained far below that of the global lake average of approximately 20%, underscoring the critical role of sustained groundwater inputs in mitigating evaporative impacts during the extreme hot‐dry event. The evaporative loss fraction of spring water ( f ) decreased from ~14.8% to ~3.8%, consistent with the groundwater storage‐controlled discharge rather than progressive evaporative losses. Isotopic evidence highlights the strong hydrological resilience in groundwater‐fed systems under extreme hot‐dry conditions, whereas the impounded waters exhibited rapid evaporative intensification. These insights inform water resource management under compound hot‐dry extremes in humid mountain regions.