Simulating the water balance of a small lake in the subtropical monsoon region via the stable isotopic technique

The isotope tracing method provides a novel way to investigate the underlying mechanisms of water balance. This study focused on Taozi Lake in Changsha, a city in the subtropical monsoon region. We conducted continuous monitoring of meteorological factors and stable isotopes in precipitation and lake water samples and further employed the isotope mass balance model to estimate the evaporation to inflow ratio (E/I). The results revealed that the fitted slope of the lake water evaporation line (SLEL) was close to or even greater than the slope of the local meteoric water line (SLMWL) because of the inverse seasonal variations in precipitation isotopes and evaporation intensity. This led to significant depletion of stable isotopes in the water source. Conversely, the theoretical SLEL values derived from the Craig‒Gordon model provided a more accurate depiction of the evaporation enrichment processes of lake water and were more appropriate for identifying lake water sources. The E/I ratios exhibited seasonal variations, with the highest values occurring in autumn and the lowest values occurring in spring, reflecting the interplay between precipitation and evaporation. The interannual variability in the E/I ratios and lake water isotopes further underscored the lake's response to changing hydrometeorological conditions. The uncertainty in the E/I simulations was due primarily to the isotopic compositions of the inflow water and atmospheric vapor, followed by the relative humidity and surface water temperature. This study enhances our understanding of regional hydrological processes, particularly regarding lake water isotopes and simulations of lake water balance, while considering the uncertainties associated with the assumptions and input variables of the E/I ratio.