A multiple wetting front gravitational infiltration and redistribution model for water balance applications

A unit gradient multiple wetting front model is described, in which the infiltration and redistribution behavior of individual "square wave" fronts is simulated for unponded rainfall infiltration. Drainage recession behavior of this model is demonstrated to be equivalent to that of a capacitance (bucket) model for certain scenarios, suggesting a possible physical basis for capacitance model parameters with respect to drainage prediction. The ability of the wetting front model to account for the variable time lag between surface infiltration and drainage generation at depth, which cannot be achieved using a capacitance model, is demonstrated to improve predictions of the 25-year average drainage regime of a free-draining lysimeter in Colbitz, Germany, when compared to capacitance model predictions. Improvement in predictions compared to the capacitance model is most apparent in deeper and drier soils, where the infiltration-drainage time lag is significant. Comparison with a numerical Richards equation solution suggests that drainage behavior predicted by the model is reasonable for the conditions considered, despite neglecting capillary impacts. The ability of the model to differentiate between drainage response time and the age of draining water gives the approach general applicability for problems requiring simulation of the age of water involved in hydrological response to storms.