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

Abstract

The main goal of a joint project undertaken by the geophysical and hydrological research units of Kiel University is to study preferential flow in a large open-air, full-scale model, looking in particular at near-surface penetration and flow of water through the unsaturated vadose zone. An artificial irrigation device is installed in place of natural rain, and a homogeneous sand body is used instead of natural soil. This provides a reference model for future field experiments. Inside the sand body there are a large number of geophysical and hydrological sensors to measure DC resistivity (using various electrode configurations), water content and water potential (using TDR and tensiometer instruments, respectively). A ground-penetrating radar (GPR) system is installed at the surface, whereas at the bottom several containers 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 controlled by the discharge measurements. These display some undulations and variations of the water ‘front’ and agree with the measurements of all other sensors. The redundancy achieved by the use of multiple methods was intended to enable an assessment of the reliability of the techniques used. The true values of electrical resistivity before and after irrigation reflect the distribution of water saturation within the sand body. A numerical 3-D inversion of the apparent resistivity provides information regarding future field experiments, in which it will be possible to install only some of the sensors in order to preserve the natural structure of the soil.