Simplified method to estimate the Green-Ampt wetting front suction and soil sorptivity with the Philip-Dunne falling-head permeameter

Wetting front suction and soil sorptivity (S) are relevant parameters to water movement in the vadose zone. Both may be estimated with the Philip - Dunne falling-head permeameter, given the moisture increment (Delta theta) and measured times (t(med) and t(max)) during an infiltration event. Previous studies have shown that the Philip - Dunne falling head permeameter can be used for estimating saturated hydraulic conductivity (K-s), but its potential to estimate the soil's sorptivity has received little attention. We investigate the ability of the Philip - Dunne method estimate S and the Green - Ampt's suction at the wetting front, Psi, by performing a parameter sensitivity analysis, focusing on the boundary conditions that limit the search space of physically sound solutions, and studying the shape factors used in Philip's analysis to reduce the three-dimensional flux of water in the soil to one dimension. Finally, a useful approximate solution is provided that allows computing both K-s and the Green - Ampt's suction at the wetting front, Psi, ( and hence the macroscopic capillary length parameter, alpha*) from only two infiltration times, t(med) and t(max), without having to resort to such a costly measurement as the soil moisture increment, Delta theta, required by the original method.