Cary A. Talbot

A Moisture Content-Discretized Infiltration Method

In an effort to improve the computational efficiency and robustness of one-dimensional vadose zone flow calculations, alternatives to the Richards’ Equation (RE) are sought as part of on-going System-Wide Water Resources Program (SWWRP) research by the US Army Engineer Research & Development Center (ERDC) in the development of coupled surface and subsurface flow interaction codes. This paper will describe a depth- continuous, moisture content-discretized infiltration method under development at ERDC. The range of moisture content within a given soil is discretized into vertically continuous, interactive bins. The entry and vertical movement of wetting fronts in each bin are simulated by means of explicit infiltration approximations based on capillary and gravitational driving forces. Advancement of wetting fronts within a bin create pore- water deficits that are satisfied by capillary-driven inter-bin flow. The method inherently provides numerical stability and robustness because it precludes the need to include the explicitly non-linear constitutive models estimated by other vadose zone flow codes. Comparisons of method results with RE solutions and computational efficiency will be presented.

Discrete Water Content Solution of the Infiltration Problem

Efficient and accurate simulation of the exchange between atmospheric, surface, and ground waters is essential in large-scale hydro-climatic models where high-resolution computational grids are not practical. This objective is met with a new and innovative method for simulating the entry and redistribution of water in the vadose zone using a discrete water content solution. Unlike infiltration predictors such as the solution of Richards’ equation or approximations thereof, our method employs a novel discretization in terms of water content. An explicit relationship is used to describe water movement in each discrete water-content region of the pore space. Water is advanced according to capillary drive, and is re-distributed according to the capillarity of each region of discrete water content. Evapotranspiration removes water from the profile according to a root distribution. The linear-explicit nature of the computations provides for computational efficiency in both speed and the required resolution for numerical stability and accuracy. The finite volume solution methodology guarantees conservation of mass for all soil types and wetting front conditions.