Water Ponding Depths Affect Temporal Infiltration Rates in a Water‐Repellent Sand

Abstract

Water‐repellent soils exhibit a positive water entry pressure head, hp The effects of imposing differing water pressure head values, by using differing water ponding depths, h0, on infiltration into water‐repellent soils was investigated. A sand, with particulate size between 0.05 and 2.0 mm, was treated with two concentrations of octadecylamine to create a sand with hp values of 8.4 and 3.5 cm. The hydraulic conductivity, K, of the water‐repellent sands increased with increasing values of h0 The K of the treated sand was equal to K of untreated sand when the ratio h0/hp was ≈3.1 for each treated sand. The infiltration rate increased with increased time for lower h0 values, but decreased with increased time for higher h0 values. The transition from increasing to decreasing infiltration rates with time occurred when h0/hp was approximately equal to 2.6. The infiltration rate behavior of an aqueous ethanol solution was consistent with theoretical relationships based on liquid surface tension. A positive hydraulic head was created at the interface of an overlying wettable and underlying water‐repellent layer that affected the infiltration rate consistent with the effects of h0 on a nonlayered water‐repellent sand. The following mechanism is proposed to explain the increase in infiltration rate with time. In water‐repellent materials, positive hydraulic heads can be created within the profile during infiltration, which can increase as the depth to the wetting front increases. The higher hydraulic head induces an increase in hydraulic conductivity, which contributes to increased infiltration rate. Alternatively, if the depth of ponded water is sufficient to cause a hydraulic conductivity equal to that of the wettable material, the infiltration rate behavior is the same as traditionally observed for wettable soils.