Présentations Colloques

The first step in the development of sustainable water management strategies is the accurate quantification of all components of the hydrological cycle. Among them, the natural percolation rate through the unsaturated soil zone is of great importance but often difficult to determine due to soil heterogeneity and the influence of vegetation. Different methods to estimate the spatial and temporal distribution of natural groundwater recharge are available including field measurements and hydrologic modeling. Field investigations (mostly tracer tests) are time-consuming and site-specific but allow a detailed assessment of processes in the unsaturated soil zone. Modeling tools cover different scales (point, catchment) and complexities (empirical, numerical) but need a variety of climatic and soil parameters as input. A combination of these two methods shall lead to the best understanding of water flow dynamics and provide the most reliable estimates of flow parameters. **Therefore, a tracer test was conducted for the estimation of water percolation rate through the unsaturated soil zone at Pirna, Germany. Ten liters of 10 g L KBr solution was applied once on a one mC surface area. The electrical conductivity, soil water content and temperature were measured by Hydra probes installed in a trench at different depths below surface (0.25 – 0.8 m). Additionally soil water probes were taken to determine Br concentrations at specific times and depths using suction cups (ceramic head). Grain size distribution was determined using soil probes. An unsaturated flow and transport model (HYDRUS-1D) was set up using the soil parameters and field boundary conditions and calibrated using the water content measurements of the Hydra probes. **Results indicate that due to high soil heterogeneity and preferential flow paths, a tracer test is not suitable to quantify the percolation rate at the chosen field site. Nevertheless, water content measurements and soil probes are valuable to calibrate an unsaturated flow model, which was used to estimate the groundwater recharge rate without taking into account soil heterogeneity.