MODELING OF SOIL-WATER RETENTION FROM SATURATION TO OVEN DRYNESS

Most analytical formulas used to model moisture retention in unsaturat ed porous media have been developed for the wet range and are unsuitab le for applications in which low water contents are important. We have developed two models that fit the entire range from saturation to ove n dryness in a practical and physically realistic way with smooth, con tinuous functions that have few parameters. Both models incorporate a power law and a logarithmic dependence of water content on suction, di ffering in how these two components are combined. In one model, functi ons are added together (model ''sum''); in the other they are joined s moothly together at a discrete point (model ''junction''). Both models also incorporate recent developments that assure a continuous derivat ive and force the function to reach zero water content at a finite val ue of suction that corresponds to oven dryness. The models have been t ested with seven sets of water retention data that each cover nearly t he entire range. The three-parameter sum model fits all data well and is useful for extrapolation into the dry range when data for it are un available. The two-parameter junction model fits most data sets almost as well as the sum model and has the advantage of being analytically integrable for convenient use with capillary-bundle models to obtain t he unsaturated hydraulic conductivity.