Few of the infiltration models in current use are suitable for the situation in which the rainfall intensity is initially less than the infiltration capacity of the soil. In this paper a simple two-stage model is developed for infiltration under a constant intensity rainfall into a homogeneous soil with uniform initial moisture content. The first stage predicts the volume of infiltration to the moment at which surface ponding begins. The second stage, which is the Green-Ampt model modified for the infiltration prior to surface saturation, describes the subsequent infiltration behavior. A method for estimating the mean suction of the wetting front is given. Comparison of the model predictions with experimental data and numerical solutions of the Richards equation for several soil types shows excellent agreement. There has been great interest during the past decade in mathematical modeling of the watershed rainfall-runoff process, and this interest is expected to continue. Many of the watershed models have been formulated by combining models that represent the actual components of the hydrologic cycle, such as infiltration, overland flow, and evapotranspiration [e.g., Crawford and Linsley, 1962; Huggins and Monke, 1966]. Of the many components, infiltration has the largest influence on the volume of watershed runoff. For the continental United States, )70% of the annual precipitation infiltr•tes into the soil [Chow, 1964], although this percentage varies widely for individual storms. Yet despite the importance of infiltration most models of the infiltration process have serious deficiencies in their representation of infiltration from rainfall.For many rainfall events there is an initial period during which all the rainfall infiltrates into the soil. During this time, as water infiltrates, the capacity of the soil to absorb water decreases until it becomes less than the rainfall intensity. At this point, water begins to accumulate on the soil surface, and runoff can begin. To properly represent a runoff event, the hydrologist must be able to predict this time and also the subsequent decline in infiltration capacity.

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[1] An investigation into landscape saturation has been carried out analytically using similarity parameters derived from topographic, soil, and climatic attributes. Three parameters (an input index (q/Z)(L/KS), convergence ratio (CR), and profile factor (B)) define the extent of hillslope saturation. From these, a single dimensionless parameter has been derived to describe the occurrence of saturation in planar, concave, and convex profile hillslope shapes, and with parallel, convergent, and divergent planforms. The relationship can be represented by a single dimensionless curve. The theory derived in this paper provides a basis for combining the individual parameters into a similarity criterion for saturation in landscapes. The theory and ensuing similarity hypothesis are applied to three natural catchments near Canberra, Australia, and are tested against observed rainfallrunoff data. The results support the validity of the similarity approach. There is a clear link between the distribution of hillslope similarity parameters and catchment saturation behavior.