The Role of the Unsaturated Zone for Rainwater Retention and Runoff at a Drained Wetland Site

Drained wetland sites with shallow water tables cover large parts of Central and Western European lowlands. Their hydrological behaviour is complex and depends on their specific characteristics. In this paper, we analysed how the water budget components of such areas behaved when undergoing rainfall events with amounts greater than 10 mm. All the water budget components were determined using a weighable groundwater lysimeter that was installed in the Spreewald wetland, Germany. On average, 69% of the rainfall was stored in the wetland, while only 8% was discharged and 23% was dissipated by evapotranspiration during the time of the runoff process during and after the rainfall event. More than half of the water that was stored could be attributed to storage within the unsaturated zone, while only a minor part was due to the water storage change under quasi-equilibrium conditions. Hence, the soil moisture depletion in the unsaturated zone in the period before the rainfall had a big influence on the site’s available water storage capacity. The findings show that models and approaches assuming hydrostatic conditions might strongly underestimate the water storage capacity of shallow water table sites and, consequently, overestimate the runoff. Hence, the hydrostatic assumption does not describe the process dynamics of these sites in an appropriate manner.