CALCULATING PARAMETERS FOR INFILTRATION EQUATIONS FROM SOIL HYDRAULICFUNCTIONS

Simple equations for predicting infiltration of water into soil are va luable both for hydrological application and for investigating soil hy draulic properties. Their value is greatly enhanced if they involve pa rameters that can be related to more basic soil hydraulic properties. In this paper we extend infiltration equations developed previously fo r positive surface heads to negative heads. The equations are then use d to calculate infiltration into a sand and a clay for a range of init ial and surface conditions. Results show errors of less than three per cent compared with accurate numerical solutions. Analytical approximat ions to parameters in the equations are developed for a Brooks and Cor ey power law hydraulic conductivity-water content relation combined wi th either a Brooks and Corey or a van Genuchten water retention functi on. These are compared with accurate numerical values for a range of h ydraulic parameters encompassing the majority of soil types and a rang e of initial and boundary conditions. The approximations are excellent for a wide range of soil parameters. An important attribute of the in filtration equations is their use of dimensionless parameters that can be calculated from normalised water retention and hydraulic conductiv ity functions. These normalised functions involve only parameters that it may be possible to estimate from surrogate data such as soil parti cle size distribution. Application of the equations for predicting inf iltration, or their use in inferring hydraulic properties, then involv es only simple scaling parameters.