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By adopting two extreme assumptions concerning the behavior of unsaturated soil hydraulic conductivity K near saturation, we derived a two-branched model for pending time and infiltration rate decay for arbitrary rainfall rates. One assumption was that K varies slowly near saturation and leads to an expression for pending time and infiltration decay. For initially ponded conditions, pending time is zero, and with rainfall rate r -• •o, the familiar Green and Ampt (191 l) expression results. The other, rather opposite assumption was that K varies rapidly, e.g., exponentially, near saturation. This model also holds for both rainfall and ponded surface conditions, and for ponded conditions the expression is identical to that of Parlange (1971). Each model uses only two parameters, saturated soil conductivity K8 and a parameter that is roughly related to sorptivity and responds nearly linearly to variations in initial saturation. Both parameters are physically related to measurable soil properties. Methods are presented to estimate parameters of either model from infiltrometer tests. The two models are compared with a precise numerical solution of the unsaturated soil water diffusion equations for three soils that represent a range of soil behaviors near saturation. Our results show that either assumption would be an excellent model for most hydrologic purposes, and the relative goodness of fit of each model is generally consistent with the appropriate behavior of K(0 -• 08). K -D(OO/Oz) = r(t)This equation may be integrated and combined with an expression of conservation of mass [Parlange and Smith, 1976] to yield fot f•' O(O) dO (5) r dt = , (0-Or) r-K(O-'-----'•where subscript 1 refers to conditions at the surface and i refers to initial conditions. Pending time occurs when 0• = Os (saturated water content), and therefore the left integral limit is t = t•o.Paper number 8W0017.

No abstract

We present a relatively simple analytical/conceptual model for rainfall infiltration during complex storms. It is an approximate but physically based model which can treat intervals of either no rain, low rain, or evaporation. The infiltration model is based on the very general three‐parameter analytic model of Parlange et al. (1982), extended to treat soils with very high initial water content. The redistribution model is based on profile extension with shape similarity. A wide range of soil types can be simulated. The model is tested by comparison with numerical solutions of Richards's equation carried out for a variety of events upon four selected soils. The model simulates the solution to Richards's equation quite accurately, provided basic soil retention relations are parametrically represented. It simulates redistribution particularly well for redistribution intervals up to 20 hours. The model usefulness in comparison with the common and simple approach which disregards soil water redistribution is also shown.