From Plot to Catchment: Multi-Scale Modeling of Overflow Weirs to Strengthen Regional Water Resilience in Northern Bavaria

Progressive climate change, historical drainage practices, and low precipitation levels in the district of Neustadt a. d. Aisch-Bad Windsheim (Northern Bavaria) necessitate innovative strategies to improve the regional landscape water balance. Within the GrüneGräben+ project, the Ansbach Water Management Authority integrated overflow weirs into existing drainage channels in the study areas Buchholzgraben, Langenwasengraben, and Bodenfeldgraben. These structures are designed to manage floodwaters in a controlled manner while simultaneously promoting infiltration of surface water into the soil. The infiltrated water can either be utilized as plant-available moisture or contribute to stabilizing groundwater levels by percolation.

Each location has been equipped with extensive measurement instrumentation, including rain gauges, surface water sensors, temperature sensors, and soil moisture sensors. In addition, comprehensive field surveys were carried out, where soil samples taken from the immediate vicinity of the channels were analyzed in the laboratory for their soil physical properties. Further measurements included soil moisture assessments via time domain reflectometry (TDR), infiltration tests using double-ring infiltrometers, and topographic data obtained from drone photogrammetry and GPS surveys. These data provide a detailed basis for characterizing runoff and infiltration processes, as well as microtopography, which are used to calibrate and validate hydrological model output.

To evaluate the effectiveness of the measures, hydrological models are employed across multiple spatial scales, primarily using the physically-based numerical models HydroGeoSphere (HGS) and SWAT+. Modeling first takes place at the plot scale (PE), where HGS simulates the interactions between surface water and the porous medium surrounding the trench while factoring macropores, surface crusting, and crop rotation. This complex water flow is represented by the three-dimensional Richards equation in the porous media domain, and the two-dimensional shallow water equation in the surface domain. By using the corresponding Van Genuchten parameters derived from laboratory experimentation, the impact and changes in borders of capillary fringe, field capacity, and wilting point are studied.

Moreover, HGS is also applied at the catchment scale to generate the boundary conditions required by the smaller plot-scale model. At the catchment level, scenarios such as using a series of weirs to improve the water balance on a broader scale are simulated. The SWAT+ model is likewise employed to investigate additional scenarios regarding the effectiveness of these measures across the catchment. The results provide a scalable foundation for transferring the effects of these interventions to larger landscape units, thereby enhancing the region’s resilience to water stress brought on by climate change.