Abstract

Based on observations of constant wetting front velocities throughout a depth range of 1 to 2 m, we simplify infiltration as gravity-driven weak viscous flow. In theory, time-limited constant infiltration generates a sharp-crested water-content wave, WCW, at the surface that suddenly increases the soil water content as it arrives at a particular depth. This is in contrast to the more gradual increases produced by observed WCWs. The deviation between theoretical and observed WCWs is ascribed to water abstraction from the WCW to finer soil pores due to the gradient of capillary potential, ∇ψ. Water abstraction occurs across a finite time interval. The total amount of abstraction is the difference between the WCW's maximal water content and its antecedent water content, while its time interval lasts from the first increase to steady water content. The WCWs' maximal water contents are usually below complete saturation. The wave velocity multiplied by the time interval of abstraction yields the depth increment ZA of water abstraction during the passing of the wave. The frequency distribution of 215 ZA values was deduced from WCWs that were measured in situ under sprinkler irrigation. The ZA distribution covered a range from 0.2 to about 10 m, with its 50% midpoint at about 1 m. The limited depth range of increments indicates that abstraction is also a time-limited process that may end before the pores surrounding the WCW are completely water saturated

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