Scale effects of Hortonian overland flow and rainfall-runoff dynamics : laboratory validation of a process-based model

Hortonian runoff was measured in the laboratory from uniform slopes of lengths of 1?5, 3?0, and 6?0 m for steady, high-intensity rainstorms with durations of 1?0 to 7?5 min. A clear reduction in runoff per unit slope length was found as slope lengths were increased. This effect becomes more pronounced with decreasing storm duration. The runoff data were used to validate a simple process-based model that combines the Philip-two-term infiltration equation with the kinematic wave overland flow principle. The predicted and experimental results agreed well. Laboratory findings were extrapolated with the aid of the model to slopes and rainfall durations similar to those found under West African conditions. The calculated reduction of runoff per unit length is similar to reported observations. Thus, this process-based model can largely explain the phenomenon of runoff reduction with increasing slope length.