THE PRACTICABILITY OF USING RICHARDS EQUATION FOR GENERAL-PURPOSE SOIL-WATER DYNAMICS MODELS

Use of finite difference solutions of Richards' equation is generally considered to be impracticable for general purpose models of soil-water dynamics, because numerical performance is not predictable with unfamiliar parameter values, and because of excessive computer execution times. The Broadbridge-White model of soil hydraulic properties yields finite soil-water diffusivity, so that solutions of the finite difference equations always exist. Further, it permits the flow equation to be scaled to realistic soils in terms of three independent variables. Searching a practical three-dimensional parameter space yields criteria for numerical stability and for guaranteed convergence of the iterative procedure for obtaining the solution at each time step. An efficient numerical scheme yields soil-water models with practical execution times. Comparison shows higher computational speed and greater model simplicity, relative to an alternative, less rigorous, model based on generalising the sharp wetting front infiltration model of Green and Ampt.