Preferential flow: first results of a full-scale flow model

The main goal of a joint project undertaken by the geophysical and hydrolog ical research units of Kiel University is to study preferential flow in a l arge open-air, full-scale model, looking in particular at near-surface pene tration and flow of water through the unsaturated vadose zone. An artificia l irrigation device is installed in place of natural rain, and a homogeneou s sand body is used instead of natural soil. This provides a reference mode l for future field experiments. Inside the sand body there are a large numb er of geophysical and hydrological sensors to measure DC resistivity (using various electrode configurations), water content and water potential (usin g TDR and tensiometer instruments, respectively). A ground-penetrating rada r (GPR) system is installed at the surface, whereas at the bottom several c ontainers and a thin gravel layer are embedded to measure the flow arrival and the discharge of water. Irrigation is varied in intensity, time, area, and salt content (tracer). Results of the first six experiments show that the percolation of intruding water can be followed by all techniques and percolation is finally control led by the discharge measurements. These display some undulations and varia tions of the water 'front' and agree with the measurements of all other sen sors. The redundancy achieved by the use of multiple methods was intended t o enable an assessment of the reliability of the techniques used. The true values of electrical resistivity before and after irrigation reflect the di stribution of water saturation within the sand body. A numerical 3-D invers ion of the apparent resistivity provides information regarding future field experiments, in which it will be possible to install only some of the sens ors in order to preserve the natural structure of the soil.