Elevation-dependent hydrological vulnerability of lakes and reservoirs to evaporation loss in the Yarlung Zangbo River Basin, Tibetan Plateau

The Tibetan Plateau’s Yarlung Zangbo River Basin is undergoing rapid hydroclimatic transitions compounded by extensive reservoir development. Accurately quantifying evaporative water losses and water balance and interpreting their associated isotopic signatures is critical for improving hydrological models and regional water security assessment. In this study, we tested the application of an isotope-based Bayesian framework for quantifying evaporation and water balance dynamics. Utilizing steady-state models for the long-term water balance of natural lakes and transient models for the short-term water balance of artificial reservoirs, this framework calculated a unified mass-balance metric, the evaporative loss index (E/(E+Q)), where E represents evaporative water loss and Q denotes water outflow. Results reveal that despite operating on different temporal scales, the inflow-dependent evaporative loss indices exhibit diametrically opposed altitudinal dependencies between lakes and reservoirs, which can substantially mask the natural riverine δ 18 O altitude effect. Natural lakes, for which evaporative loss index is functionally equivalent to the evaporation-to-inflow ratio under steady-state conditions, are found to have average values of 0.27 ± 0.19 and show a strong positive correlation with elevation. In high-altitude basins (> 4500 m a.s.l.), limited natural inflows cause substantial proportional evaporative losses relative to the water balance (mean E/(E+Q) = 0.51), highlighting severe hydrological vulnerability to evaporative loss despite low absolute evaporation rates. This intense fractionation can substantially modify the pristine cryospheric baseflow, acting as a contributing mechanism to the reversal of the steep negative gradient in localized surface waters.. Conversely, artifically-constructed reservoirs exhibit a negative altitudinal evaporative gradient, with evaporation accounting for 0.47 ± 0.16 of total water losses in low-elevation shallow basins during the dry season. Rapid downstream transport of water from high-altitude reservoirs disrupts and homogenizes natural isotopic gradients in rivers. This systematic disruption of natural isotopic baselines by both climate-driven evaporation and human water retention explicitly warns against utilizing uncorrected surface water isotopes for hydrological tracing or paleoaltimetry reconstructions. These findings highlight extreme vulnerabilities in regional water security, demanding altitude-specific management adaptations.