Effect of rainfall-induced soil seals on soil water regime: wetting processes

The effects of a rainfall-induced soil seal on wetting processes are studied in the cases of two different soils: Hamra (sandy loam) and loess (loam). The soil seal is identified as the disturbed upper layer where the average hydraulic properties have been changed during rainfall from the initial properties that were identical to those of the undisturbed soil bed. These properties are derived by applying the seal model of Mualem and Assouline (1989), calibrated for the two soils. Determination of the seal thickness, bulk density, retention curve, and hydraulic conductivity function made it possible to solve systematically the flow equation for different boundary conditions, allowing the simulation of the wetting processes within the seal layer as well as the undisturbed profile underneath. The infiltration curves and the water content profiles during and after rainfall are derived for three rainfall intensities. The water content and the capillary head profiles, as well as the variation of these variables with time, at different depths under the soil surface, were also calculated. Smaller amounts of the rainfall infiltrate as the rainfall intensity increases. The infiltration rate diminishes faster as the rainfall intensity increases. The idea of a single infiltration curve as a function of the cumulative rainfall seems to be invalid. Each rainfall intensity yields a different infiltration curve. A rough validation test of the model yielded predicted infiltration curves closely bounded by the experimental results. The final infiltration rate after 50 mm of rainfall also seems to be a function of the rainfall intensity, with the higher rate obtained for the higher rainfall intensity as experimentally observed in other studies. Unlike the explanations found in literature, this study suggests that a shallower saturated zone within the seal layer is the reason for this phenomenon. This study points out that the disturbed layer which forms the surface seal is not saturated from the very beginning of rainfall. It has a significant water-holding capacity and, thus, takes time to reach saturation. Furthermore, under rainfall of high intensity, the seal layer reaches saturation only at its upper zone close to the soil surface. The lower part may remain unsaturated even after 50 mm rainfall and is larger for the soil of coarser texture.