Reinforce lake water balance components estimations by integrating water isotope compositions with a hydrological model
Authors: Mahmoodi Nariman , Struck Ulrich , Schneider Michael , Merz Christoph 
Published: September 2024

Abstract. Accurate estimation of water balance components of groundwater-fed lakes, including subsurface inflow, as well as actual evaporation from lakes, poses a complex task for hydrologists employing hydrological models. Hence, an alternative approach was used to capture the dynamic behavior of the hydrological groundwater/surface water system, which can be used for integration with the hydrological model and serves as a validation for the hydrological model estimates of the water balance components. The approach, based on measurements of the stable isotopes (δ18O and δD) enables the quantitative estimation of the individual water flux and evapotranspiration rates. An isotope-mass-balance model was used to quantify lake water balances over a one-year sampling period. The approach is based on the global relationship between the δ18O and δD values of 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 evapotranspiration rate considering the groundwater inflow compensating 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 (GG), southwest of Berlin in the Havel catchment), over the period from 2015 to 2023 with the integrated hydrological model HydroGeoSphere. Utilizing the isotopic mass balance model, HydroCalculator, under steady-state hydrologic regime conditions, the evaporation-to-inflow (E/I) ratio is determined for the period of one year spanning 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/I ratios based on the isotopic measurement of the lake water. Isotopic signatures of surface water, groundwater, and rainwater (δ18O and δD) confirm a flow-through type for the lake. The calculated E/I ratio for GG Lake is around 40 %. The calculated evaporation for the years 2022 and 2023, within the isotopic mass balance model framework (Eiso22 = 601 mm, Eiso23 = 553 mm), aligns well with the actual evaporation from the lakes calculated by the HGS model (EHGS22 = 688 mm, EHGS23 = 659 mm). The change in the ratio of evaporation to inflow (E/I) leads 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 estimates show a high degree of agreement with the evaporation rates predicted by the HydroGeoSphere (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 to validate the hydrological model calculations of the water balance components.