Abstract
A two-layer model for 1D vertical unsaturated flow based on the hypothesis of a sharp wetting front is presented. The model further assumes a uniform pore pressure throughout the wetting zone at any given time during the infiltration and redistribution processes. These assumptions allow to reduce the theoretical Richards equation to an ordinary differential equation that can track average soil moisture in two soil layers behind the wetting front as the front moves downward. The model has been tested on many soil types by comparing results against numerical solutions of the Richards equation. It was shown that the conceptual model was able to represent local infiltration and average upper soil moisture accurately for complex rainfall sequences. Errors in soil moisture estimates never exceeded 0.04 cm³/cm³, which is the range of field measurement accuracies. Furthermore, as the conceptual model is numerically faster by an order of magnitude over the solution of the Richards equation, its use makes it possible to model spatially distributed infiltration and soil moisture at the watershed scale where soil layering is known to influence the water balance.
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