Overland Flow on a Pervious Surface

This article presents the analytical solutions for overland runoff hydrographs produced by a uniform rainfall with decay soil infiltration rates. It was found that the kinematic wave travel time through a catchment under such a nonuniform rainfall excess is not a constant, but varies between the time of concentration and the time of equilibrium according to the soil moisture condition. The analytical solutions reveal that kinematic wave travel times are part of the hydrograph convolution process and can hardly be measured from observed hydrographs. The findings of this article suggest that the time of concentration of a small catchment shall be estimated by velocity-based methods rather than those empirical formulas developed for and calibrated by the time difference between the center of mass of the rainfall excess and the inflection point on recession of the observed runoff hydrograph.