Accurate estimation of water balance components of groundwater-fed lakes, including subsurface inflow and actual evaporation from lakes, is a complex task for hydrologists employing hydrological models. In this study, an approach that integrates isotope analysis and hydrological modeling is used to improve the representation of groundwater–surface water interactions. While based on 1 year of sampling, this method provides direct observational data to complement hydrological models and serves as a validation tool for water balance estimates. The approach, based on measurements of stable isotopes (oxygen-18: 18 O; deuterium: 2 H), enables quantitative estimation of the individual water flux and evapotranspiration rates. An isotope-mass-balance model was used to quantify lake water balances over a 1-year sampling period. The approach is based on the global relationship between the δ18O and δ2H values in the precipitation and kinetic isotopic fractionation in the lake water during evaporation. Assuming that the lake is hydraulically connected to the groundwater, the isotope mass-balance model accounts for the quantification of the evaporation rate considering the groundwater inflow compensating for the evaporation loss. The study addresses the model-based quantification of groundwater inflow and evaporation losses of a young glacial groundwater lake (Lake Groß Glienicke (GGS), southwest of Berlin in the Havel catchment) over the period from 2015 to 2023 with the integrated HydroGeoSphere (HGS) hydrological model. Utilizing the isotopic mass balance model, HydroCalculator, under steady-state hydrological regime conditions, the evaporation-to-inflow ( E/I) ratio is determined for the period of 1 year from August 2022 to September 2023. Employing the fully integrated hydrological model, calibrated and validated under monthly normal transient flow conditions from 2008 to 2023 for the lake catchment, subsurface and groundwater inflows to the lake are calculated and compared to the calculated E/Iratios based on the isotopic measurement of the lake water. Isotopic signatures confirm the lake's flow-through conditions. The calculated E/Iratio for GGS is around 40 %. The calculated evaporation for the years 2022 and 2023, within the isotopic mass balance model framework, aligns well with the evaporation from the lake calculated by the HGS model. The change in E/Ileads to a significantly improved estimation of evaporation rates after correction for temperature fluctuations and inflow data from previous years (2015–2021). With a correlation coefficient of 0.81, these revised values show a high degree of agreement with the evaporation rates predicted by the HGS model for the corresponding years. Despite the uncertainties associated with the analysis of the water isotope signature, its integration into the hydrological model serves as a validation of the hydrological model calculations of the water balance components.
