LOCAL CONTRIBUTIONS TO INFILTRATION EXCESS RUNOFF FOR A CONCEPTUAL CATCHMENT SCALE-MODEL

The response of a conceptual soil water balance model to storm events is compared to a detailed finite element solution of the one-dimension al Richards equation in order to test the capabilities of the former i n calculating the local contributions to infiltration-excess runoff in a distributed catchment scale model. Local infiltration excess runoff is computed from ground level precipitation using the time compressio n approximation and a Philip infiltration capacity curve with Brooks-C orey constitutive equations. The validity of applying the conceptual m odel for local runoff and soil water balance calculations is investiga ted by performing numerical experiments over a range of soil types, co ntrol volume depths, and initial soil moisture conditions. We find tha t a good agreement between the conceptual and detailed models is obtai ned when the gravitational infiltration rate in Philip's formula is se t to the saturated hydraulic conductivity, and when percolation from t he control volume is updated as a function of the soil moisture conten t in a stepwise fashion. The comparison between these two models sugge sts that the simpler (and much less computer-intensive) conceptual wat er balance technique could be incorporated into distributed models for large scale complex terrains as an efficient means of retaining consi deration of spatial variability effects in catchment scale hydrologic simulations. This is illustrated in an application to the Rio Missiaga catchment in the eastern Italian Alps, where the local contributions to surface and subsurface runoff are routed onto a digital elevation m odel-based conceptual transport network via a simple numerical scheme based on the Muskingum-Cunge method.