Comparative simulations of past and future water balances with the models ArcEGMO, BROOK90 and Raven in managed catchments – Does the model selection have an influence?

The KliWES 3.0 project investigated the historical and future development of the water balance in Saxony under changing climatic conditions. The simulations covered the reanalysis period 1961–2020 as well as the future up to the year 2100 using 21 climate projections. A comprehensive hydrological modeling was carried out for the entire Free State of Saxony (≈22,200 km²), including the opencast mining areas of Lusatia. The water balance of this region is characterized by decades of lignite mining and the associated intensive groundwater management, which led to a cumulative regional deficit of approximately 4 billion m³ by 2020. By flooding of the former opencast mines and the gradual reduction of groundwater drainage a large-scale recovery of groundwater levels shell be achieved within the coming decades, despite the expected reduction in water availability due to climate change.

The state-wide water balance calculations for Saxony are based on the ArcEGMO model, which was improved by the integration of a floodplain approach. Furthermore, ERA5 land data were used to determine the potential grass reference evaporation. To evaluate the robustness of the model, a comparison between ArcEGMO, BROOK90, Raven 3, and Raven 4 was carried out in the catchment area of the Spree River in Saxony.

BROOK90 is a 1d site model, which was used as validation for actual evaporation due to its detailed description of vertical soil-plant-atmosphere processes. Raven is a modular, object-oriented open-source model that allows flexible combinations of different hydrological process modules – from precipitation-runoff concepts and various snowmelt models to a wide range of evapotranspiration and soil moisture approaches. It can replicate both conceptual and physically based model structures. The current version, Raven 4.1, offers significant enhancements over Raven 3, in particular an integrated optimization module based on a linear solver for water management. This allows operating rules, reservoir target specifications, and water management discharge rules to be embedded directly into the hydrological system model - a functionality that is highly relevant in the context of the Lusatian opencast mining landscape.

The results of the model comparison are examined with regard to structural differences, the sensitivity of the models to climatic changes, and the extent to which the choice of model influences the assessment of future water availability and runoff development. Particular focus is placed on the representation of anthropogenically modified systems in which management measures play a central role. The analysis demonstrates how model structure and process understanding shape the interpretation of future hydrological scenarios, and the resulting uncertainties for water management.