Divergent Responses of Water Stable Isotopes to Hot-dry Extremes in Summer and the Implications for Evaporation: A Case Study in Jinyun Mountain Area, Southwest China

Compound hot-dry events are increasingly frequent, threatening water resources in subtropical mountainous regions. This study investigated stable isotopic responses and evaporation dynamics of spring water, lake water and reservoir water in the Jinyun Mountain area in Chongqing, Southwest China during the extreme hot-dry summer in 2024. Water samples were collected periodically for δ 18 O and δD analysis, and evaporation losses were quantified using the Craig–Gordon–based isotope mass balance approaches. Marked isotopic contrasts were observed among water bodies. Spring water maintained near-meteoric isotopic signatures with high and stable d-excess values, indicating dominant groundwater recharge and weak effects from evaporation. Lake water showed slight spatiotemporal isotope changes, reflecting continuous and steady recharge that buffered against evaporative enrichment. In contrast, reservoir water underwent progressive isotope enrichment and a decrease in d-excess, suggesting amplified evaporation under prolonged drought and longer residence time. Spatially, lake d-excess decreased along altitudinal gradients, while reservoir d-excess decreased from north to south. Notably, model-derived evaporation-to-inflow ratios ( E/I ) remained low and stable in lakes (~5.9–7.6%), but rose markedly in reservoirs over time (~13.1% to ~17.5%). These E/I values remained far below global lake norms, underscoring the critical role of sustained groundwater inputs in mitigating evaporative impacts during the extreme hot-dry event. Spring water evaporation loss fraction ( 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.