Assessment of pore space changes during drying and wetting cycles in hysteresis of soil water retention curve in Russia using X-ray computed tomography

Abstract

Hysteresis of soil water retention curve (SWRC hysteresis) is a unique character of SWRC. It can be attributed to changes in distribution, form, geometry, and connectivity of pores, as well as entrapping air. X-ray computed tomography (X-CT) is widely used for estimating the number, volume, shape, size distribution, and structure of pores. The aim of the work was to monitor visually and quantitatively the pore space changes during the drying and wetting cycles using X-CT. Moreover, the dynamics of the occurrence of SWRC hysteresis was visually observed as a function of pore space structure changes using visual images of pore space. The hysteresis of SWRC was measured including drying and wetting curves using capilaremeter. Furthermore, the visual images of soil samples were implemented for the drying and wetting processes at different values of soil water potential. Also, the number, volume and the percentages of open, closed and total porosity were estimated using X-CT software during the drying and wetting processes. Moreover, the effective degree of saturation was calculated for the drying and wetting curves. The results observed that the visual images of pore space and the percentage of total porosity were increased during the drying process, whereas the pore space and the percentage of total porosity were decreased during the wetting process. However, the values of open, closed and total porosity for the wetting curve were usually larger than their values for the drying curve due to inadequate shrinkage and swelling processes. On the other hand, the effective degree of saturation was decreased during the drying process, but it was increased during the wetting process. The reason of the difference of soil moisture and an effective degree of saturation between drying and wetting curves is due to the difference of pore space structure for drying and wetting curves at the same soil water potential.