The dynamics of soil water following single surface wettings

Summary Single rainfall events play an important agricultural and ecological role, especially in dry regions where precipitation is erratic. Infiltration, redistribution and evaporation of single quantities of water are important in this context and have been investigated in the laboratory. Three soils of differing texture were packed at two uniform initial water potentials (– 100 MPa and –1.5 MPa) into columns, after which 12.7, 25.4 and 50.8 mm of water were applied as a single irrigation. The columns were maintained in a controlled hot and dry atmosphere (evaporativity = 16.7 mm d‐1) for up to 30 days, during which water‐content profiles were measured at intervals. Infiltration was rapid to depths ranging between 35 and 250 mm. Thereafter redistribution was small. Evaporation caused the water profile to develop three zones: dry between the soil surface and the drying front, dry below the wetting front, and an intermediate wetter zone between the drying and wetting fronts. As evaporation continued, the drying front moved deeper into the soil and the water content in the intermediate zone decreased. During the first few hours evaporation was rapid and constant, at the evaporativity of the atmosphere. Subsequently, evaporation was slower. Total evaporation (E) increased with time (t) as E α t n for t > 1 d, where n = 0.24 for a loamy sand, 0.33 for a clay loam and 0.31 for a silty clay loam. Weighted‐mean soil‐water diffusivities, averaged over the profile above the wetting front, ranged between 1000–2000 mm 2 d −1 at the start of the falling‐rate stage and 200–400 mm 2 d −1 near air‐dryness, in reasonable agreement with the few results in the literature.