A process based method to estimate the hydrograph from small agricultural watersheds

Abstract

The kinematic wave model for overland and channel flow is used to develop a method to calculate the runoff hydrograph from a watershed consisting of a cascade of planes and channels. First, a general, semi-analytic solution based on the method of characteristics and incorporating flow on infiltrating surfaces is derived. Unrealistic properties of previous solutions based on the method of characteristics are avoided by defining lateral inflow in relation to the presence or absence of flowing water on the flow surface. It is shown that the new definition of lateral inflow results in both a more physically realistic representation of the overland and channel flow process during the recession stage of the hydrograph and a more accurate estimation of runoff volume than obtained by previous solutions. Second, the concept of the storage of water at kinematic equilibrium is used to calculate a depth-discharge coefficient for a single plane system and a two plane, one channel system, each of which is hydraulically equivalent to a complex cascade of planes and channels. The method is validated on two rangeland watersheds in Arizona. The results show that hydrographs generated from the two simplified watershed geometries are essentially the same as those from complex geometries when the duration of the runoff event is equal to or greater than the time to kinematic equilibrium. At durations less than the time to kinematic equilibrium, the two plane, one channel system preserves the complex geometry properties better than the single plane system.