Monitoring soil water content and deficit using Electrical Resistivity Tomography (ERT) – A case study in the Cevennes area, France

SummaryAt the hillslope scale, surface runoff depends to a great extent on the water deficit of the soils, i.e. the difference between the effective porosity and the water content, all through the soil profile. The aim of this study was to assess water deficits using Electrical Resistivity Tomography (ERT). ERT offers several advantages: (i) it is not intrusive, (ii) it exhibits spatial patterns at a decametric or hectometric scale, and (iii) it provides information on both water content and soil depth. The study was conducted on a Mediterranean shaley sandy soil, in the Cevennes (South of France). First, the factors (nature of the pores and organization of the material, water content, soil solution and temperature) on which electrical resistivities depend were analyzed in the laboratory. Soil samples were used to derive the ‘m’ and ‘n’ parameters of Archie’s relation, changes in the soil solution over time, and the effect of temperature on resistivities. These results were then used to interpret ERT in situ at different times of the year. The water content and water deficit obtained from ERT were then compared with local measurements made using Time Domain Reflectometry (TDR) and the comparison was satisfactory. The results of this study show the potential of measuring soil water content and water deficit using electrical resistivities, and describe how uncertainties in temperature, porosity, and fluid conductivity impact ERT-obtained estimates of soil water content and deficit.