Impact of Overflow Weirs on Unsaturated Soil Water Dynamics in Agricultural Farmland

Agricultural farmland requires a delicate balance between providing sufficient water to crops and draining away excess moisture. This drainage is normally achieved through digging trenches that run through farming areas, allowing surplus water to flow away as surface runoff, contributing to river networks downstream. This drainage is soil-dependent and mostly uncontrolled, resulting in excessive water losses at critical points throughout the year, especially during dry periods.  This water might have contributed to plant growth otherwise.

To address this issue, the Wasserwirtschaftsamt Ansbach is leading a pilot project that aims to retain part of this excess water before it is lost as runoff. By installing overflow weirs along agricultural trenches, water can be temporarily stored and allowed to infiltrate back into the soil when moisture levels are low.

The project “Grüne Gräben” aims to investigate the effects of these weirs on both local and regional scales. Using numerical models, it is possible to quantify how much water is retained and subsequently re-infiltrated into the soil system. To achieve this, the project utilizes HydroGeoSphere (HGS), an integrated, physically based hydrological model that simulates interactions between surface water, unsaturated soil, and groundwater. Unlike simplified conceptual models, HGS numerically solves the Richards equation for variably saturated flow in the porous medium together with the diffusion wave equation for overland flow. This coupling allows for a detailed understanding of the interaction between the stored surface water and resultant infiltration into the unsaturated zone over space and time.

The meteorological, soil moisture, and soil textural data collected from field excursions are used to calibrate and validate the models. Parameters such as hydraulic conductivity, porosity, and soil-water retention characteristics allow for an assessment from a physically based approach. Additionally, vegetation and root growth provide a realistic representation of the evapotranspiration resulting from crop growth and harvesting. This, along with the infiltration resulting from the presence of the weir, helps determine the extent of evapotranspiration enhancement from the newly available soil moisture.

By modelling scenarios with and without overflow weirs, Hydrogeosphere provides data on the net benefit of installing such land management practices. The outcomes of these studies help in gauging whether this practice is worth scaling to other farms around Bavaria with similar soil characteristics.