Quantifying surface water evaporation loss rates in the Manas River Basin using stable isotope tracers

ABSTRACT Calculating evaporation loss rates of surface water poses a significant challenge in remote regions, where meteorological data are scarce. To address this challenge, our study has adopted stable isotope tracers to determine the loss rates within the Manas River Basin. Utilizing the stable isotope composition in surface water and precipitation from April to October 2023 in the basin, this study aimed to estimate loss rates using the Craig–Gordon model and the Rayleigh fractionation model across mountainous and plain regions. The local meteoric water line has a smaller slope and intercept than the global meteoric water line, indicating that the precipitation is subjected to strong sub-cloud evaporation. The surface water evaporation line revealed further evaporation after runoff formation, with a decreasing slope from upstream to downstream, indicating increased evaporation loss. We compared the performance of three isotope tracer methods (δ18O, δD, and deuterium excess (d-excess)) for calculating evaporation. In the mountainous region of the watershed, the average evaporation rate per kilometer was 0.023% (δ18O), 0.028% (δD), and 0.030% (d-excess). In contrast, these values were notably elevated in the plain region, reaching 0.106% (δ18O), 0.141% (δD), and 0.117% (d-excess). The Rayleigh fractionation model using d-excess is the most effective among the three evaporation tracing methods.