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

Highlights

• Electrical resistivity method (ERM) was proposed for characterizing soil evaporation process.

• An exponential relationship between soil electrical resistance and water content was established.

• The performance of electrical resistivity method was validated by TDR measurement and numerical simulation approaches.

• The spatial and temporal variations of soil water content during evaporation process can be well characterized by ERM.

• ERM presents potential in laboratorial test and feasibility in field monitoring of soil hydraulic responses to drought.

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.

Introduction

Because of global climate change, droughts happened many times in North America, West Africa, East Asia, and other regions in the past decades (Dai, 2011), attracting more and more attention from researchers on its detrimental impacts on the environment and people's livelihoods. Many geotechnical and geo-environmental problems resulting from drought climate have also been reported, including soil subsidence of embankments/buildings (Silvestri et al., 1992; Cui and Zornberg, 2008; Corti et al., 2011; Al Qadad et al., 2012; Wang et al., 2018), landslides and contaminant transport in landfill cover and clay barrier layers (Yanful et al., 2003), land salinization/degradation and desertification (Shimojima et al., 1996; Baram et al., 2013), etc. During the droughts process, a significant amount of water is evaporated from soil, leading to soil shrinkage, progressive desiccation and formation and development of cracks (Tang et al., 2011a; Song et al., 2016; Wang et al., 2016; Li et al., 2019; Cheng et al., 2020). The presence of cracks can significantly affect soil hydro-mechanical behavior, and therefore weakens soil engineering properties, resulting in severe damages to buildings and infrastructures (Silvestri et al., 1992; Corti et al., 2011; Tang et al., 2019; Liu et al., 2020). From a fundamental point of view, the moisture content gradients close to the ground surface are the main precursors of soil cracking (Tang et al., 2011b; Zeng et al., 2019). Therefore, it is of great importance to map the spatial distribution of soil moisture content close to the ground surface, for the future analysis of desiccation cracking mechanism.

Various experimental/numerical approaches were proposed to map and quantify the soil hydraulic response to evaporation. Different sensors, such as neutron moisture meters, frequency domain reflectometers (FDR) and time domain reflectometers (TDR), have been frequently employed for soil moisture monitoring in both laboratory and field scales (Aluwihare and Watanabe, 2003; Lal and Shukla, 2005; Cui et al., 2010; Toll et al., 2011; Smethurst et al., 2012; Song et al., 2013). However, those sensors mentioned above are point-based. Their installations may disturb soil structure (Rothe et al., 1997), leading to some inevitable measurement errors in small-scale tests. Especially, in near soil surface zone, it is a challenge to install these sensors due to their large sizes (Zhou et al., 2001). In clayey soil, the measurement may produce some errors when the soil water content is low (Bittelli, 2011). Besides, various numerical models have been developed (Wilson et al., 1994; Aluwihare and Watanabe, 2003; Cui et al., 2005, 2010; An et al., 2018) to study soil hydraulic/thermal/mechanical response to the drying/wetting process. These numerical approaches performed well in the estimation of temporal and spatial variations of water content when the relevant soil parameters, boundary conditions, and the information of the water table were provided properly.

As a well-known geophysical technique, electrical resistivity/resistance method (ERM) is sensitive to soil moisture variations. One of the most popular techniques is electrical resistivity tomography (ERT). It has been applied in various fields, including the regional evaluation of water shortage (Brunet et al., 2010), estimation of soil hydraulic parameters (Cosentini et al., 2012), water contaminant detection (De Lima et al., 1995; Benson et al., 1997; Martinez-Pagan et al., 2010), locating buried artifacts or structures in Archaeological surveys (Negri et al., 2008) as well as providing the information of subsurface desiccation cracks (Sentenac and Zielinski, 2009; Sentenac et al., 2013; Ackerson et al., 2014; Jones et al., 2014; Gunn et al., 2015; Tang et al., 2018). In terms of desiccation cracking, the vertical and horizontal crack propagation and fissuring network can be well captured through ERM (Sentenac and Zielinski, 2009; An et al., 2020). ERM also presents the potential to map soil cracks' positions even before possible visualization. In order to detail the process of desiccation cracking, the temporal and spatial distribution of water content close to the surface should be studied in depth, as mentioned earlier. Theoretically, this would be realized with closely-spaced electrodes in ERT mode. However, to the authors' best knowledge, comprehensive investigation addressing the application of this technique as a non-destructive approach to estimate the near-surface soil moisture dynamics, especially to characterize the distribution of soil moisture in horizontal and vertical directions with high spatial resolution, is still absent.

The objective of this paper is to develop an ERM with a high spatial resolution based on mini-arrays (closely-spaced electrodes), and study its performance in the characterization of soil water content dynamics from a quantitative point of view in small-scale evaporation tests, aiming to support the future study of desiccation cracking in field investigations. In this study, an experimental chamber was designed to monitor the temporal and spatial variations of soil water content during evaporation through ERM. With considering temperature effect, a calibration relationship between the measured soil electrical resistance and water content was proposed for the studied sandy soil. Besides, TDR measurement and numerical estimation of soil moisture were conducted, respectively, to validate the effectiveness of the developed ERM. The details of the evaporation process with the movement of the evaporation front in the studied soil sample were further discussed.