Simulation models of soil drying require relationships between the water content of the soil and the energy status (water activity or potential) of the soil water. A simple, one-parameter model has been found to be adequate for many such simulations. The model has the form:h= exp {−(Mw/RT)exp[(1 −w/w1)ln(−ψo)]} wherehis equilibrium relative humidity or water activity; MwR, and T are molecular mass of water, gas constant, and Kelvin temperature;wis gravimetric water content; and ψois the water potential of oven dry soil. The parameter, w1, is soil dependent, but can be predicted from measurements of clay fraction or air dry water content. Regression fits for the one-parameter model, using data from nine soils with different clay content, were comparable to those obtained with the three-parameter Fink-Jackson model.