THE DYNAMICS OF SOIL-WATER FOLLOWING SINGLE SURFACE WETTINGS

Single rainfall events play an important agricultural and ecological r ole, especially in dry regions where precipitation is erratic. Infiltr ation, redistribution and evaporation of single quantities of water ar e important in this context and have been investigated in the laborato ry. Three soils of differing texture were packed at two uniform initia l water potentials (-100 MPa and -1.5 MPa) into columns, after which 1 2.7, 25.4 and 50.8 mm of water were applied as a single irrigation. Th e columns were maintained in a controlled hot and dry atmosphere (evap orativity = 16.7 mm d(-1)) for up to 30 days, during which water-conte nt profiles were measured at intervals. Infiltration was rapid to dept hs ranging between 35 and 250 mm. Thereafter redistribution was small. Evaporation caused the water profile to develop three zones: dry betw een the soil surface and the drying front, dry below the wetting front , and an intermediate wetter zone between the drying and wetting front s. As evaporation continued, the drying front moved deeper into the so il and the water content in the intermediate zone decreased. During th e first few hours evaporation was rapid and constant, at the evaporati vity of the atmosphere. Subsequently, evaporation was slower. Total ev aporation (E) increased with time (t) as E proportional to t(n) for t greater than or equal to 1 d, where n = 0.24 for a loamy sand, 0.33 fo r a clay loam and 0.31 for a silty clay loam. Weighted-mean soil-water diffusivities, averaged over the profile above the wetting front, ran ged between 1000-2000 mm(2) d(-1) at the start of the falling-rate sta ge and 200-400 mm(2) d(-1) near air-dryness, in reasonable agreement w ith the few results in the literature.