Variable Saturation Infiltration Model for Highly Vegetated Regions

Abstract. Observations have been made in studies that in watersheds with moist soils and lush vegetation, water does not pond on the soil surface, even during significant rainfall events. In these cases, all the water infiltrates into the soil. Furthermore, the water content in the pore space is below saturation.

The tools for modeling such situations are limited. A universal method for estimating groundwater in the unsaturated zone is the Richards' equation, a non linear system of 1D or 3D equations based on physical flow principles. While effective, the difficulties in solving Richards' equation can pose a significant problem in a complex system, which may require extensive amounts of calibration and numerical effort in procuring a solution. A second method is based on the idea of approximating the movement of moisture through the soil as a rectangular profile, e.g. assuming that the water moves through a soil column like a piston. Rectangular profile methods have been developed in the past for cases in which ponding occurs at the surface, and the water content inside the column is at saturation. More general rectangular profile methods allow the water in the column to move at sub-saturation. Both of these existing methods involve solving a non linear algebraic equation for the hydrologic system, involving constant flux at the boundary, due to a wetting event such as rainfall. The method proposed in this article may be used for situations in which the soil does not become saturated, and the soil and rainfall properties, specifically the rates of hydraulic conductance and rate of rainfall respectively, are allowed to vary in space or time. The method proposed is based on a simple principle in infiltration hydrology, that the rate at which water will infiltrate through the soil equilibrates with the rate of rainfall, when the rate of rainfall is smaller than the infiltration capacity of the soil. In application, this method does not require the solution of a non linear algebraic (or differential) system of equations, thus affording modelers computational economy.

Furthermore, this new method can be made to interact with the saturated profile methods in the event that the rainfall overwhelms the ability of soil to absorb moisture. If this happens, a portion of the field will come to saturation while other parts will be below saturation. The sub-saturation models can be made to interact with the redistribution and evapotranspiration processes by providing the initial and boundary condition for those events.