Initial Soil Water Content as Input to Field-Scale Infiltration and Surface Runoff Models

Evidence is given of the role of initial soil moisture content, θi, in determining the surface runoff hydrograph at field scale, that is a crucial element when distributed models for the estimate of basin response to rainfall have to be formulated. This analysis relies upon simulations performed by a model that, because of the necessity of representing the infiltration of surface water running downslope into pervious saturated or unsaturated areas, uses a coupled solution of a semi-analytical/conceptual approach for local infiltration and a nonlinear kinematic wave equation for overland flow. The model was applied to actual spatial distributions of θi, earlier observed over different fields, as well as to a uniform value of θi assumed equal to the average value or to the value observed in a site characterized by temporal stability. Our results indicate that the surface runoff hydrograph at a slope outlet is characterized by a low sensitivity to the horizontal heterogeneity of θi, at least in the cases of practical hydrological interest. In fact, in these cases the correct hydrograph can be simulated with considerable accuracy replacing the actual distribution of θi by the corresponding average value. Moreover, the surface hydrograph is sufficiently well reproduced even though a single value of θi, observed at a site anyhow selected in the field of interest, is used. In particular, this extreme simplification leads to errors in magnitude on peak runoff and total volume of surface water with values typically within 10% and 15%, respectively.