Short-term reestablishment of soil water repellency after wetting: Effect on capillary pressure-saturation relationship

Soil water repellency is known to be a dynamic property, which varies in the short term or between seasons. In the short term, its temporal nature is often studied based on soil water content, assuming that it is reestablished after the soil dries out. In this study, we examined the reestablishment of soil water repellency after wetting and subsequent drying. The reestablishment process was studied on: (i) natural water-repellent soils (WRS) subjected to different leaching rates; and (ii) wettable sand subjected to wetting-drying cycles with dissolved organic matter (DOM) solution. After air drying, the soils were packed in columns, rewetted, and from the maximum height of capillary rise for the target soil (H_eq1) and that of a "reference soil" (H_eq0), the equilibrium (static) contact angle (ω _eq1) was calculated, assuming that for the reference soil the contact angle ω_eq0 = 0. Increasing the leaching fraction of an initially WRS resulted in a decrease in ω_eq1 values. For an initially wettable soil, increasing wetting-drying cycles with DOM solutions resulted in an increase in ω_eq1. The variations in cosω_eq1 were reflected in a similar ratio for the capillary-pressure-saturation relations (CSR). The prediction from scaling the "reference soil" CSR curve by cosω_eq1 was found to be satisfactory, but more accurate where the effective saturation S <0.5. At S > 0.5, organic matter is more likely to detach and dissolve, changing the properties of the soil solution by altering its surface tension and the soil particles' surface, i.e., ω_eq1.