Modelling Climate Change Effects on Soil Water Dynamics and Groundwater Recharge in Temperate Forests in Germany

Studies on climate change effects reveal that the natural water cycle will continue to shift, often intensifying pressure on water resources. Understanding the interaction between forest ecosystems, soil water dynamics, and groundwater recharge under these changes is essential. This study investigates the forested areas of the Hessian Ried in Hesse, Germany—a low-lying, fertile region crucial for agriculture, forestry, nature conservation, and groundwater supply to the Frankfurt/Rhine-Main metropolitan region. Ensuring sustainable water management for this vital resource zone is necessary to meet future demands.

The LWFBrook90 model, a 1D Soil-Vegetation-Atmosphere-Transfer (SVAT) tool, was utilized to examine future soil water variability and its impact on groundwater resources in the study area. The model simulates the soil water balance using Richards’ equations and, with its latest version (LWFBrook90R 0.6.0), incorporates capillary rise. Historical daily climate data (1960 to present) and climate projections based on the RCP8.5 scenario (extending to 2100) were used. The forested area was discretized into 500 m x 500 m cells, each with representative vegetation, soil, and climate data. Vegetation data was obtained from forest inventories and used in combination with forest yield tables to derive leaf and stem area index using allometric functions. Calibrated model parameters for the main tree species—oak, beech, and pine—were obtained from Weis et al. (2013). A detailed discretization of soil profiles was performed and the corresponding soil physical properties were assigned layer-wise using pedotransfer functions (Wessolek et al., 2009).

The cell-based framework effectively captured spatial variability in tree species and soil properties, ensuring computational efficiency despite excluding lateral flow. Future projections (especially in 2081–2100) indicate a significant decline in soil water availability during summer months (July–September), increasing water stress and potentially impairing plant growth. In winter, soil moisture recovery may still occur but is less pronounced. Monthly transpiration ratios, averaged across periods 2021–2050, 2051–2080, and 2081–2100, revealed severe stress across all projections. Even scenarios with wetter conditions suggest that increased rainfall and infiltration may not sufficiently mitigate tree stress in vulnerable areas.

The model successfully simulated vertical water movement in the profile and groundwater recharge dynamics. Groundwater recharge during winter is projected to maintain current rates under moderate scenarios or slightly increase under wetter ones. However, under the driest scenario, the average rate was less than 50 mm per year for the period 2081–2100. These findings highlight significant stress on forest ecosystems under changing climate conditions, emphasizing the need for adaptive water management strategies. Ongoing research seeks to refine this prototype by examining vertical fluxes at sites with shallow groundwater depths and further investigate recharge rates under various forest conversion scenarios. These advancements will contribute to a more comprehensive understanding of forest ecosystem responses and guide water management efforts in the Hessian Ried and similar regions.