Rainfall Excess Model from Soil Water Flow Theory

The complex computer-dependent solution of the partial differential equation for unsaturated soil moisture flow (often called Richard) equation has been used to develop a relatively simple parametric description of the performance of a variety of soils under various rainfall input rates and patterns, and initial moisture contents. This equivalent parameterized description is expressed in terms of an infiltration model, termed SEQM, that is dependent on accumulated soil water. The SEQM model is incorporated in a kinematic model of 6 x 12 ft. plot rainfull-runoff response, and its predictions are compared with those of an empirical model of infiltration (INFIL) developed by the USDA Hydrograph Laboratory. The results emphasize: the practical utility of the model and its potential advantages over comparable empirical models; and the inadequate sensitivity of equipment currently used to measure rainfull and runoff from small plots.