Characteristics of Water Infiltration in Layered Water-Repellent Soils

Water-repellent (WR) soil greatly influences infiltration behavior. This research determined the impacts of WR levels of silt loam soil layer during infiltration. Three column scenarios were utilized, including homogeneous wettable silt loam or sand, silt loam over sand (silt loam/sand), and sand over silt loam (sand/silt loam). A 5-cm thick silt loam soil layer was placed either at the soil surface or 5 cm below the soil surface. The silt loam soil used had been treated to produce different WR levels, wettable, slightly WR, strongly WR, and severely WR. As the WR level increased from wettable to severely WR, the cumulative infiltration decreased. Traditional wetting front-related equations did not adequately describe the infiltration rate and time relationships for layered WR soils. The Kostiakov equation provided a good fit for the first infiltration stage. Average infiltration rates for wettable, slightly WR, strongly WR, and severely WR during the 2nd infiltration stage were 0.126, 0.021, 0.002, and 0.001 mm min−1 for the silt loam/sand scenario, respectively, and 0.112, 0.003, 0.002, and 0.000 5 mm min−1 for the sand/silt loam scenario, respectively. Pseudo-saturation phenomena occurred when visually examining the wetting fronts and from the apparent changes in water content (ΔθAP) at the slightly WR, strongly WR, and severely WR levels for the silt loam/sand scenario. Much larger ΔθAP values indicated the possible existence of finger flow. Delayed water penetration into the surface soil for the strongly WR level in the silt loam/sand scenario suggested negative water heads with infiltration times longer than 10 min. The silt loam/sand soil layers produced sharp transition zones of water content. The WR level of the silt loam soil layer had greater effects on infiltration than the layer position in the column.