Impact of soil heterogeneity on the water budget of the unsaturated zone

The impact of soil heterogeneity on the spatially average water budget of the unsaturated zone is investigated using a framework of analytical solutions. This framework was previously shown to adequately represent the long-term average water budget as well as its transient evolution. Nine combinations of three climate and three soil types are considered, covering a wide range of different conditions. In highly conductive soils, where evapotranspiration is limited by percolation through the lower boundary, heterogeneity increases the spatially average evapotranspiration relative to the uniform (scaled average) soil. In contrast, for less conductive soils the decreasing infiltration rates due to soil heterogeneity cause evapotranspiration to become smaller. Formally derived "equivalent" parameters, which are intended to capture the effect of soil heterogeneity by means of one set of soil hydraulic parameters, are shown to depend not only on the spatially heterogeneous parameters, but on climate as well. Moreover, the "equivalent" parameters derived for the long-term average water budgets are not valid for transient behavior