Using Van Genuchten for Soil Water Calculations in the Fast All-Season Soil Strength (FASST) Model.

The Fast All-season Soil STrength (FASST) land surface model was developed to predict soil strength as a quantitative method of determining the ease of vehicle mobility in areas with undeveloped road networks. FASST was also developed to quantify heat and moisture movement in the soil and the exchange of heat and moisture at different interfaces. As in other land surface models, FASST utilizes pedotransfer functions to determine soil hydraulic parameters (SHP's) based on soil texture classes defined by the Unified Soil Classification System (USCS) or the United States Department of Agriculture (USDA). However, unlike the Community Noah Land Surface Model and other similar land surface models, which typically use the Richard's equation, FASST utilizes analytical expressions of the Mualem-van Genuchten model for infiltration between nodes (e.g., layers) and for relating hydraulic conductivity (K), pressure head (Ψ), and volumetric water content (θ). Preliminary results from two sites under bare soil in the Midwestern United States showed that infiltration between nodes during heavy rain events was accurate in the upper soil profile but lower than observed in the deeper part of the soil profile. Additional research into runoff algorithms of FASST is needed to determine if deep soil infiltration was reduced because of too much runoff or because of inaccuracies in the parameters used to drive the Mualem-van Genuchten soil water model.

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