Quantifying evaporation from a large reservoir in the Yellow River Basin: Based on stable isotope methods

Study region Liujiaxia Reservoir, a large and highly regulated reservoir in the upper Yellow River Basin, northwestern China. Study focus Stable water isotopes in precipitation and reservoir surface water were monitored from August 2021 to July 2023. Local evaporation lines (LELs) were used to characterize evaporative enrichment, and the Craig-Gordon isotope mass-balance framework implemented in Hydrocalculator was used to quantify evaporation over inflow ratios (E/I) and their seasonal variability. New hydrological insights for the region Reservoir water became progressively enriched in δ¹⁸O and δ²H from the inlet toward the outlet and open water sections, indicating cumulative evaporative fractionation. The LEL slope decreased from 8.79 in winter to 3.46 in summer, while isotope derived E/I ranged from 0.41% to 7.82%, with the largest relative evaporation losses occurring in summer. Variability in inlet and outlet water isotopic composition dominated the propagated uncertainty in E/I, emphasizing the importance of representative isotope sampling for constraining evaporation estimates. Stable isotope mass balance therefore provides a useful means of identifying periods of relatively high evaporation loss in large regulated reservoirs and can complement conventional water balance monitoring, although E/I should be interpreted as a relative evaporation loss indicator rather than an absolute evaporation flux.