SUMMARYA sigmoidal, non‐hysteretic two‐part retentivity function having only two constants in addition to porosity was developed for use in soil‐water flow simulation models. Values of the constants, shapes of the retentivity curves, and soil textural properties were related by fitting the retentivity function to retentivity data generated using regression equations. Two sets of regression equations, which relate water content at specific matric potentials to soil texture and bulk density, were used, one for British soils and one for South African soils. Hydrologically inhomogenous soils may be modelled by varying the values of the retentivity constants through the profile to reflect changing soil prope