An analytical framework for the water budget of the unsaturated zone

In this contribution an analytical framework is put forth for the water budget of the unsaturated zone. The most important assumptions made are (1) an instantaneous redistribution of moisture and (2) a linear relationship between evapotranspiration and relative soil saturation. For storm periods, infiltration is described using the well-known solution of Philip [1957] in conjunction with the time compression approximation. For interstorm periods a new solution is derived for the moisture depletion of a soil water reservoir that experiences simultaneous evapotranspiration and percolation. The framework compares favorably with numerical solutions of Richards' equation for a wide variety of climate conditions and soil hydraulic properties. Its simplicity and computational efficiency make the framework well suited for the analysis of the impact of spatial soil and rainfall heterogeneity; on the water budget of the unsaturated zone.