Influence of soil hydrodynamic characteristics variability on surface and subsurface flows at a vegetative buffer strip scale

The objective of this study is to evaluate the influence of soil hydrodynamic characteristics variability on surface and subsurface flows at a vegetative buffer strip scale, using mechanist modeling. Cathy (CATchment HYdrology, Camporese et al. 2010) is a research partial-differential-equation-based model, solving Richards equation in 3 dimensions for water fluxes in the soil, and a simplified scheme of Navier-Stokes equation for surface runoff. Its particularity is to handle interactions between surface and subsurface, which is a key point concerning water but also solute transport in vegetative filter strips. Balance between runoff and infiltration, flow pathways, water content, are very sensitive to hydrodynamic characteristics, especially saturated hydraulic conductivity (Ksat). This soil property is very difficult to measure and to describe at a fine scale, since it is highly variable spatially in the 3 dimensions. Models described by PDE such as Richards equation need a value of Ksat at each soil layer and each node, though simpler conceptual modeling run with average values of larger cells or storages, using some 'representative Ksat' at a larger scale. This kind of models however can simulate with high quality the processes despite the simplifications they make on parametrization. Using a mechanist and physically-based modeling, we evaluate the influence of Ksat high spatial variability on fluxes, by comparison with observations from an experimental vegetative filter strip. It should allow to understand until which degree of simplification one can describe hydrodynamic characteristics in modeling for more conceptual models.