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 (Δθ) and measured times (tmed and tmax) during an infiltration event. Previous studies have shown that the Philip–Dunne falling head permeameter can be used for estimating saturated hydraulic conductivity (Ks), but its potential to estimate the soil's sorptivity has received little attention. We investigate the ability of the Philip–Dunne method to estimate S and the Green–Ampt's suction at the wetting front, Ψ, 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 Ks and the Green–Ampt's suction at the wetting front, Ψ, (and hence the macroscopic capillary length parameter, α*) from only two infiltration times, tmed and tmax, without having to resort to such a costly measurement as the soil moisture increment, Δθ, required by the original method.