Local contributions to infiltration excess runoff for a conceptual catchment scale model

The response of a conceptual soil water balance model to storm events iscompared to a detailed finite element solution of the one-dimensional Richards equationin order to test the capabilities of the former in calculating the local contributions toinfiltration excess runoff in a distributed catchment scale model. Local infiltration excessrunoff is computed from ground level precipitation using the time compressionapproximation and a Philip infiltration capacity curve with Brooks-Corey constitutiveequations. The validity of applying the conceptual model for local runoff and soil waterbalance calculations is investigated by performing numerical experiments over a range ofsoil types, control volume depths, and initial soil moisture conditions. We find that a goodagreement between the conceptual and detailed models is obtained when the gravitationalinfiltration rate in Philip’s formula is set to the saturated hydraulic conductivity, and whenpercolation from the control volume is updated as a function of the soil moisture contentin a stepwise fashion. The comparison between these two models suggests that the simpler(and much less computer-intensive) conceptual water balance technique could beincorporated into distributed models for large scale complex terrains as an efficient meansof retaining consideration of spatial variability effects in catchment scale hydrologicsimulations. This is illustrated in an application to the Rio Missiaga catchment in theeastern Italian Alps, where the local contributions to surface and subsurface runoff arerouted onto a digital elevation model–based conceptual transport network via a simplenumerical scheme based on the Muskingum-Cunge method.