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 motion equations with the appropriate boundary conditions. However, difficulties and uncertainness on determining soil hydraulic properties and soil physics heterogeneities mean this way is not always feasible. In a different way, process dynamic can be derived by the local scale through a coarse graining procedure, allowing preservation of medium motion character, while hydraulic fluctuation of the motion is lost. Following an approach like this, a simplified model to predict the runoff generation time, the so-called delay time, is developed in this paper. 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 modeled 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 lay on much lower permeable layers, and it could be of some interest for the simplified modeling of the surface-subsurface hillslope hydrologic response and in planning agricultural subsurface drainage systems. (C) 2016 American Society of Civil Engineers.