The Green–Ampt limit with reference to infiltration coefficients

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Abstract
Recent progress with an analytic nonlinear model has provided the exact infiltration coefficients for realistic soil behaviors with nonsingular hydraulic functions, as well as their exact delta‐function diffusivity limits. After some correction and reinterpretation of the approximate analytical method, the exactly solvable model validates some previously obtained approximate infiltration functions. The Green–Ampt infiltration function follows from a delta‐function diffusivity limit with a hydraulic conductivity that may be, among other possibilities, a linear function of water content. Just as a linear conductivity function is an overestimate for a realistic soil, the second Philip infiltration coefficient S1 in the Green–Ampt infiltration function is too large due to conductivity being overestimated. Better agreement with experiment (halving the value of S1) is obtained from the analytic nonlinear model, with a limiting delta‐function diffusivity and a matching Gardner exponential hydraulic conductivity function. In general, infiltration behavior is determined by the limiting forms of the diffusivity and conductivity relative to one another at the saturated water content, or alternatively, the relationship between the conductivity and soil moisture potential. A new infiltration model demonstrates the possible range of S1 for physically valid limiting conductivity functions. We show that in the delta‐function diffusivity limit, the solution behaves as if the potential at the wet front were time dependent, decreasing in magnitude from an initial value at the traditional Green–Ampt level.

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