Influence of Sub‐Cloud Evaporation on Precipitation Isotopes: Insights From Hourly‐Scale Meteorological Assessment in a Large Lake in the East Asian Monsoon Region

ABSTRACT Sub‐cloud evaporation is a critical aspect of the hydrological cycle, reducing surface precipitation totals and altering the stable isotopic composition as raindrops fall from the cloud base towards the surface. However, isotopic modelling of sub‐cloud evaporation in humid climates and its implications for hydrological processes remain poorly understood, posing challenges for regional water resource management and ecological conservation. In this study, a comprehensive assessment was conducted to understand the influence of sub‐cloud evaporation on precipitation isotopes in Poyang Lake, the largest freshwater lake in China. Using 4‐year hourly meteorological observations from 11 national meteorological stations, we found that there was significant sub‐cloud evaporation during the precipitation process in humid regions. The remaining fraction of evaporated raindrops varied between 81% and 95%, with the lowest values occurring in September and the highest in February. The monthly average ∆δ 2 H, ∆δ 18 O and ∆d‐excess values ranged from 2.9‰ to 7.0‰, 0.7‰ to 1.8‰, and −7.7‰ to −2.8‰, respectively, and the sub‐cloud evaporation effect during the rainy season was more intense than that during the dry season. By modifying the sub‐cloud evaporation effect, precipitation isotopes monitored at the surface and estimated at the cloud base were confirmed to exhibit consistent temporal patterns on both monthly and daily scales. Sensitivity analysis revealed that precipitation isotopic changes were more sensitive to fluctuations in relative humidity and precipitation intensity under varying meteorological scenarios. The underrepresentation of low‐intensity precipitation events was found to lead to a statistical underestimation of precipitation isotopic changes, and when the low‐intensity events (≤ 1.0 mm/h) were excluded, the average ∆δ 18 O and ∆d‐excess values shifted from 1.25‰ and −5.25‰ to 0.63‰ and −2.72‰, respectively. These findings contribute to a better understanding of hydrological cycle processes in Poyang Lake and other regions with similar humid climate characteristics, especially for the interpretation of regional paleohydrological records and ecohydrological mechanisms using stable isotopes.