Author(s): Marta Altissimo; Marco Marani; Andrea Rinaldo; Mario Putti
Keywords: Hillslope response; 3D Richards equation model; Subsurface flow
Abstract: One of the crucial themes in hydrological processes is how much of the observed hillslopescale complexity and heterogeneity needs to be included in any predictive hydrological model at the watershed scale. This research field has been investigated by use of analytical and theoretical approaches, numerical models with various degrees of simplification, and field experiments. In this paper we present a modelling experiment based on a three-dimensional Richards equation numerical model of variably saturated flow in porous media, coupling surface and subsurface water flow in a real hillslope. We use the model to test whether Richards-like unsaturated flow dynamics can explain the observed hillslope response. To this end, we calibrated the model on the basis of literature data for an artificial irrigation experiment conducted on a well-known experimental hillslope (CB1 site in Oregon, USA). Simulations show the model ability to capture surface saturation patterns and some of the main characters of the discharge hydrograph (e. g. peak timing and volumes). Shortcomings also emerge as the model could not accurately reproduce the initial rapid increase of the discharge and the shape of the recession limb. Such discrepancies are interpreted as partly due to ambiguities in the information regarding initial conditions, soil characteristics, and downstream boundary conditions. However, the numerical experiments suggest that the fast actual hillslope response with respect to the model point to the presence of flow processes which may likely not be represented using Richards approach.