The role of antecedent soil water content in the runoff response of semiarid catchments: a simulation approach
The soil water content is recognized as one of many runoff controlling factors in semiarid environments. A simple, physically based distributed model has been developed to study the role of antecedent soil water content in runoff generation in three small catchments in semiarid southeast Spain. The catchments were set up in two different zones: a burnt area with scarce vegetation and unburnt area with a denser plant cover. The infiltration process is determined by the Green-Ampt equation and the initial soil water content is taken into account by means of maps obtained by Conditional Gaussian simulation. The model was run 2700 times with different soil moisture scenarios and design storms. Stochastic sensitivity analysis was used to examine the role of antecedent soil water content in the runoff response. The results showed that the hydrological response after high intensity, low frequency storms is independent of the initial soil water content. On the other hand, the antecedent soil water content is an important factor controlling runoff during medium and low intensity storms, a type of rainstorm that is relatively frequent in semiarid areas. The sensitivity of the runoff response to soil moisture depended on the predominant runoff mechanisms. When infiltration excess overland flow is predominant, as a result of high rain intensities or less permeable soils, the runoff response is more uniform and does not depend on initial soil moisture. Runoff from less intense storms on soils of higher permeability is controlled by the soil water content of the surface soil layers and is more dependent on initial conditions. The role of the initial soil water content is more important in catchments where the presence of plant cover produces a patchwork of runoff and runon sites, whose spatial pattern is highly dependent on the soil water status.