Higher temperatures and increasing droughts are likely to lead to a reduction in available water resources in the Berlin-Brandenburg region. Concurrently, there is an increasing demand for water due to the population and economic development in the region. This results in serious water management challenges. To mitigate this water scarcity, in 2021, work began on setting up a coordinated joint groundwater management system based on a transboundary groundwater flow model for the capital region (approx. 2800 km², from which Berlin covers 892 km²). The central component is the modelling of the reciprocal effects of the groundwater abstraction by the water suppliers in the region with regard to sustainable management of the common groundwater body. In the first step, the hydrogeological structural model (HSM) for the region is created. In doing so, up-to-date geological sections from the Berlin and Brandenburg Geological Surveys as well as stratigraphical settings from existing calibrated groundwater models are brought together. The groundwater models of the Berliner Wasserbetriebe are also incorporated in this process. These models have been developed since approximately 20 years and are regularly updated. They are used both in approval processes and in daily operation. In the second step, the HSM is converted into a flow model. This flow model is calibrated and compared with the existing models. In the end, a model for the entire capital region will be available for the first time. At the conference, the current status of the project will be presented and the challenges will be highlighted.

In vielen Regionen zeigt die Auswertung der Grundwasserstandsentwicklung, dass ein Rückgang der Grundwasserstände zu verzeichnen ist (z.B. NLWKN, 2020). Jedoch steigen gleichzeitig der Wasserbedarf und die Anzahl der Wasserrechtsanträge, vor allem in landwirtschaftlich geprägten Regionen. Vor diesem Hintergrund, wird auch in der Forderung des Deutschen Vereins des Gas- und Wasserfaches e.V. (DVGW Wasser-Impuls: Zukunftsbilder 2030 bis 2100) beschrieben, dass bei der Entwicklung und Umsetzung langfristiger Zukunftskonzepte im Bereich Wassermanagement digitale Lösungen eine erhebliche Unterstützung sein sollen. Dazu werden u.a. Prognose- und Managementmodelle für ganze Einzugsgebiete vorgeschlagen.

In diesem Sinne wurde für den Landkreis Vechta ein Grundwassermodell in FEFLOW erstellt, um bei der Bearbeitung von Wasserrechtsanträgen zu unterstützen (Flechtner et al., 2022). Grundwassermodelle sind jedoch oft teuer und nur von Experten nutzbar. Das bedeutet, dass die Modellierung von weiteren Szenarien (z.B. erhöhte Pumpraten oder zusätzliche Brunnenstandorte), an denen der Kunde oder die Behörde interessiert ist, sprich die Weiterverwendung des Modells, meist von externen Experten durchgeführt werden muss. Um der Behörde ein wie vom DVGW beschriebenes Prognose- und Managementmodell bereitzustellen, welches von dieser selbstständig im täglichen Geschäft verwendet werden kann, wurde von DHI WASY das Grundwassermodell zu einem interaktiven Modell erweitert. Dieses kann die Behörde in Ihrem täglichen Geschäft über eine Benutzeroberfläche selbst, mit wenigen, einfachen Arbeitsschritten, bedienen, ohne auf externe Experten angewiesen zu sein. Die Modellergebnisse der von der Behörde erstellten Szenarien werden automatisch exportiert und in einem interaktiven HTML-Format angezeigt, so dass der Kunde alle relevanten Daten und Ergebnisse schnell und auf einen Blick für weitere Entscheidungen zur Hand hat.

Climate projections indicate that extreme weather events have increased in frequency and intensity in the past and will affect sensitive Germany in the near future. Our study site, the catchment of the lower Spree, is located in Brandenburg. The region is affected by water stress showing a water deficit of 80 Mio. m³ on an area of 3500 km² with many competing water users. Our goal is to investigate the impact of shifts in climate, land use and vegetation on recharge and available groundwater resources. In four different catchments with different characteristics we monitor groundwater und lake levels, stream discharge, and depth dependent soil moisture. Isotopic composition of precipitation, groundwater, and surface water is investigated to calculate water fluxes between different compartments. Electrical conductivity down to a depth of 200m below surface is recorded. Measured NaCl concentrations reach up to 10 – 15 g/kg. Employing the hydrological model SWAT evapotranspiration, discharge and soil moisture are calibrated to calculate long-term groundwater ranges pattern, which ranges in average at 77 mm/a. The M-K test shows decreasing recharge from May to August, an increase from December to February, and almost no change from September to November leading to an overall decreasing trend over the last 20 years. We are planning to derive concepts for areas with highest economic or ecological value derived from socio-economic analyses for the implementation of artificial recharge using greywater, treated waste water or storm water to replenish the groundwater deficit and to retain uplift of the saline groundwater water.

We explore the effects of a changing climate on groundwater dynamics based on thermo-hydraulic simulations to reconstruct the temperature and pressure below the State of Brandenburg between 1950 and 2010. In this time period, observations point to ~1°C surface temperature warming, large annual fluctuations in groundwater recharge, and periods of high groundwater abstraction volume — all leading to water stress conditions. Our input structural model integrates Permian to Cenozoic sedimentary units with essential geological features controlling the regional groundwater flow, including salt structures, permeable glacial valleys, and aquitard discontinuities. We use a grid-based hydrologic model to derive inflow and outflow rates across the top boundary of the subsurface model. Simulation outputs are verified against data from available observation wells.

The simulation results demonstrate that the regional flow pattern in the deep aquifers (>1 km deep) is mainly controlled by the basin geometry, while shallow groundwater dynamics is heavily influenced by high-frequency climate forcing. Seasonal fluctuations in groundwater level are observed in areas of shallow (<10 m) water table, with the highest levels corresponding to months of greater recharge rates. Where the water table is deeper, it responds to precipitation pulses with a delay of several months. Seasonal groundwater heating and cooling is limited to the first 10–30 meters, except within glacial valleys where high hydraulic gradients and permeabilities lead to a deepening of the advective heat transport. In addition, we identified periods and regions of significant groundwater abstraction and sustained groundwater warming over the entire simulation period within urban areas.

Water stress is increasing in Northeast Germany due to climate change. New approaches for water management are needed to mitigate the impacts on the water system and water users. Therefore, our study deals with the development of a web-based toolbox for subsurface water storage with focus on the lower catchment of the river Spree in the federal state of Brandenburg.

Our approach is based on a systematic combination of site selection criteria and spatial data on land use, soil, groundwater and potential water sources. This will provide relevant information for the preliminary planning of managed groundwater recharge measures by authorities and water suppliers. Considering surpluses from runoff and surface waters, also caused by extreme weather events, suitable locations for surface-induced recharge will be identified.

Supported by additional modelling-based indications for implementation, efficiency and costs, as well as simplified site selection through a query system, the toolbox will offer an initial knowledge for such planning considerations.