PARAMETER CONSTRAINTS ON CLOSED-FORM SOILWATER RELATIONSHIPS

The constraints on the different fitting parameters used in the water retention equation, h(theta), and hydraulic conductivity, K(theta), are analyzed using the infiltration equation as a testing tool. The following characteristic equations are considered: those of Gardner; Brooks and Corey; Brutsaert; Van Genuchten subject to both Mualem's and Burdine's condition; Van Genuchten combined with Brooks and Corey; and Fujita. It is shown that most combinations of h(theta) and K(theta) or K(h) break down, when tested over the large range of soil types encountered in field situations. For clay soils, especially, the best-fit parameter values often become inconsistent with the infiltration theory. The best combination is the Van Genuchten equation for h(theta) with the Burdine condition m = 1 - 2/n and the Brooks and Corey equation for K(theta). This combination satisfies the infiltration condition for all soil types, even when applied to the two extreme cases used by Green and Ampt and Talsma and Parlange. The interdependence of h(theta) and K(theta) parameters is discussed.