Soil water content variability at the hillslope scale: Impact of surface sealing

Spatial and temporal variability of water content in the upper soil layer, close to the surface, affects the intensity of hydrological processes. The impact of accounting for soil surface sealing on the spatial and temporal variability of the water content at the hillslope scale was studied in a semiarid environment. Relevant physical properties of the experimental site were derived by means of extensive field surveys, resulting in a detailed database used to characterize the 8240 cells used to represent the hillslope domain. The simulated spatial and temporal water content variability was achieved by aggregating a numerical 1-D simulation at each of these cells. Accounting for surface sealing improved water content predictions, more efficiently during the drying regime. Furthermore, extensive synthetic simulations show the sealed layer to be a highly efficient mechanism reducing temporal water content variability, compared to an unsealed system. It was also found that reduced evaporation in the sealed domain compensates for the loss in infiltrated water due to runoff enhancement. Depending on rainfall intensity and soil depth, a transition could occur from a positive feedback mechanism where the seal layer suppresses evaporation and conserves water stored in the profile to a negative one where the seal layer mainly reduces infiltration and increases water losses through runoff. Thus, the sealing process was found to substantially affect water budgets at all observed scales in the experimental site.