An experimental application of electrical resistivity/resistance method (ERM) to characterize the evaporation process of sandy soil

Abstract

Because of global climate change, drought occurs in increasing intensity and frequency across the world. Drought-induced evaporation can reduce soil water content and cause progressive desiccation cracking, which is responsible for various geotechnical and geo-environmental problems. This study aims to develop an ERM with a high spatial resolution and study the ERM performance in the characterization of soil water content dynamics from a quantitative point of view in small-scale evaporation tests, in order to support the future study of desiccation cracking in field investigations. An environmental chamber filled with initially saturated sand was designed for the evaporation test. As the chamber was subjected to drying, the evolutions of soil electrical resistance and water content at different depths were monitored continuously by the installed sixty electrodes and six time domain reflectometers (TDR) sensors, respectively. Experimental results indicate the good correlation between the measured soil electrical resistance and the water content. As evaporation continues, soil electrical resistance increases exponentially with decreasing water content. The variations of soil electrical resistance present an evidentially delayed effect along with the depth. A calibration relationship between the recorded soil electrical resistance by ERM and the water content measured by the oven drying method was established. Afterwards, the variations of soil water content at different depths were estimated based on the developed calibration relationship and compared with the TDR results. Besides, a numerical approach combining a soil-atmosphere interaction model and a coupled hydro-thermal model was employed to study the evaporation-induced variations of soil water content. The estimated soil water contents by ERM were further compared with the results obtained using the numerical method. The evaporation process with the movement of the evaporation front in the studied soil sample was also discussed in depth. This study presents that the developed ERM is effective to record soil moisture dynamics, especially in the near-surface zone, in small-scale evaporation tests. It also provides insights on the potential of ERM in field applications to characterize soil hydraulic responses to drought climate.