Development of unstable flow and reduced hydraulic conductivity due to water repellence and restricted drainage

The effect of water repellence and antecedent soil moisture on wetting front stability and infiltration rate are reported for a seasonally water repellent topsoil. The effect of water repellence on infiltration was determined by comparing the in situ infiltration of water to that of a 7M ethanol solution. Wetting front stability was measured during infiltration of water into repacked, wettable and water repellent soils, within a Hele-Shaw chamber. Water repellence restricted in situ movement of water through large macropores (>500 µm), which decreased intrinsic permeability by 1 to 2 orders of magnitude. In repacked soils, water repellence caused the development of unstable wetting fronts and reduced infiltration from 240 mm h-1 to 101.7 mm h-1. Infiltration into wettable soils at moisture contents near field capacity was expected to result in rapid infiltration and stable wetting fronts. However in repacked soils, wetting front instability developed, and infiltration rates were 190% lower when air and/or water movement through the base of the chamber was restricted. Infiltration into in situ soil was also slower at high antecedent soil moisture. The hydraulic conductivity of the 7M ethanol solution decreased significantly from 112.3 mm h-1 in dry water repellent conditions, to 35.6 mm h-1 in wettable soils at high antecedent moisture contents. Consequently the previously reported development of wetting front instability and reduced infiltration into in situ wettable soils at high moisture contents were confirmed and attributed to difficulty displacing existing soil water during infiltration of new water.