A simple infiltration model is proposed for the idealized case of a constant intensity rainfall infiltrating into a homogeneous soil at uniform initial moisture content. The infiltration volume prior to the beginning of runoff is predicted from a modified form of Darcy's Law. The succeeding infiltration capacity 'decay' curve is described by the Green and Ampt equation with a correction included to allow for the volume of infiltration prior to surface saturation. All parameters in the model can be obtained from the rainfall intensity and measurable properties of the soil; no fitting is required. The model equations were tested against numerous solutions of the diffusion equation for soil moisture flow. Comparisons were made for five different soil types (sand, sandy loam, loam, silt loam, light clay) and for a variety of rainfall intensities and initial soil moisture levels. A further independent test of the prediction of the water uptake before runoff was made against published experimental observations. Good to excellent agreement was obtained for all tests except in the case of the silt loam, for which agreement was fair. (Author)