Analysis of the nonlinearity in the hillslope runoff response to precipitation through numerical modeling

SummaryCoupled, finite-element models are applied for hillslope runoff investigations. Subsurface flow is modeled by the 2-D Richards equation extended for the saturated zone. Surface runoff is described by the linear and also by the nonlinear kinematic wave equations coupled to the subsurface model through infiltration and/or saturation excess. It is concluded that the typical nonlinearity in the runoff response is the result of not only the surface runoff process and the antecedent soil moisture condition but also of the way precipitation intensity affects the subsurface process. Namely, higher intensity rains produce surface runoff sooner through infiltration excess and bring the soil/aquifer closer to saturation in a shorter time leading to taller unit hydrographs of total runoff and to shorter time-to-peak periods, even if surface runoff is kept linear in nature.