Scenario Modelling of Future Water Balance in a Transboundary Basin: Potential and Limitations from the Tisa Pilot Basin within the Danube Water Balance Project

The Danube Water Balance project was launched with the objective to improve harmonized data management, to develop a joint water balance calculation methodology for the entire Danube River Basin and to foster the common acceptance of the elaborated modelling framework.In this study, we demonstrate the advantages of the CWatM model by presenting its water balance calculation capabilities for the Tisa River Basin (TRB), the largest tributary catchment of the Danube. In addition to the standard model setup steps, a scenario evaluation will be presented for future climate change scenarios. The TRB offers several modelling challenges as it is a transboundary catchment with mountainous and lowland areas, strong groundwater influence and complex water management solutions in the alluvial plains.Data availability posed a limitation for the study site. While global OA data on elevation, land use, soil, and meteorology was adequate, local information on water management was scattered thematically and regionally. Calibration-validation resulted in acceptable performance for river discharge (validation KGE ranging from 0.18 to 0.88 with a median of 0.64). Gridded precipitation data from seven databases were used to check model sensitivity on the biases of meteorological forcing.Scenario analyses were performed with a two-fold focus: (i) a comprehensive climate impact assessment involving 11 climate models (ensemble) and three SSP pathways and (ii) demonstration-oriented scenarios were simulated to verify the CWatM model's capabilities to represent surface-subsurface water balance components under future climate and changing water demands. Simulations were evaluated with respect to changes in discharge characteristics as well as storage changes.We also present the future steps planned for the project to develop the modelling framework as a suitable tool to answer water balance-related questions across the Danube Region.This work was supported as part of DANUBE WATER BALANCE, an Interreg Danube Region Programme project co-funded by the European Union.