Simplified Model to Predict Runoff Generation Time for Well-Drained and Vegetated Soils

The study of generation process of subsurface stormflow, typical of well-drained and high permeable soils, can be theoretically carried out by applying the continuity and the motion equations with the appropriate boundary conditions. However, difficulties and uncertainness on determining soil hydraulic properties and soil physics heterogeneities let this way not always feasible. In a different way, processes dynamic can be derived by the local scale through a coarse graining procedure, allowing to preserve medium motion character, while hydraulic fluctuation of the motion are lost. Following an approach as this, in this paper a simplified model to predict the runoff generation time, the so-called delay time, is developed. Under the assumption of sloped hillslope and Dunnian mechanism of runoff generation, which implies a time-varying active length of the hillslope where runoff occurs, a relationship of the active length versus time, is derived. Finally, the effect induced by the interception process on the delay time is modelled and discussed, and an application is carried out. Generally, these simplified solutions could be applied for any condition in which high permeable soil surface horizons laid on much lower permeable layers, and it could be of some interest for the simplified modelling of the surface/subsurface hillslope hydrologic response and in planning agricultural subsurface drainage systems.