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A 3-D coupled surface-subsurface model to investigate the runoff dynamics on hillslopes

Esclaffer Thomas

The numerous field and laboratory studies that have been conducted on hillslopes have permitted the identification of various processes that explain hillslope runoff and the contribution of rapid subsurface flow during rainfall events. But, if some theories are now well accepted (i.e. the saturated contributing area theory in the temperate climate), no general and comprehensive view of hillslope response during rainfall events is available. We argue that detailed numerical models will be very helpful in building this comprehensive view. The possible coexistence, on hillslopes, of interacting surface and subsurface flows induces specific numerical constraints. We present a 3-D coupled numerical model where the Richards equations in the subsurface domain and the diffusive wave equations on the surface are solved simultaneously. Using this framework, numerical investigations are conducted on simplified hillslope transections to analyse the processes activation, and to identify the major control factors. The first results show that various processes described in the literature (water table ridging, saturated contributing areas, presure wave translatory flow) are simultaneously active even for a simplified and homogeneous hillslope. Moreover, exfiltration is often a major source of runoff in the saturated contributing areas. The results are compared to the ones obtained with the simplified hillslope storage Boussinesq model (HSB) proposed by Troch et al (WRR,2003). This comparison reveals the important role plaid by the unsaturated zones in the dynamics of hillslope response during floods.