Improved Prediction of Unsaturated Hydraulic Conductivity with the Mualem-van Genuchten Model

variables as input (e.g., Rawls and Brakensiek, 1985; Ahuja et al., 1989; Vereecken et al., 1989; Schaap et In many vadose zone hydrological studies, it is imperative that the al., 1998). Far fewer alternatives exist for unsaturated soil’s unsaturated hydraulic conductivity is known. Frequently, the Mualem–van Genuchten model (MVG) is used for this purpose behydraulic conductivity. Although some pedotransfer cause it allows prediction of unsaturated hydraulic conductivity from functions are available (Saxton et al., 1986; Schuh and water retention parameters. For this and similar equations, it is often Bauder, 1986; Vereecken et al., 1990), pore-size distriassumed that a measured saturated hydraulic conductivity (Ks) can bution models by Burdine (1953) and Mualem (1976), be used as a matching point (Ko) while a factor S e is used to account among others, are more popular. for pore connectivity and tortuosity (where Se is the relative saturation Generally speaking, the Burdine and Mualem models and L 5 0.5). We used a data set of 235 soil samples with retention infer the pore-size distribution of a soil from its water and unsaturated hydraulic conductivity data to test and improve preretention characteristic. By making assumptions about dictions with the MVG equation. The standard practice of using Ko 5 continuity and connectivity of pores, integral expresKs and L 5 0.5 resulted in a root mean square error for log(K ) sions can be derived that describe unsaturated conduc(RMSEK) of 1.31. Optimization of the matching point (Ko) and L to the hydraulic conductivity data yielded a RMSEK of 0.41. The fitted tivity in terms of water content or pressure head. A Ko were, on average, about one order of magnitude smaller than general expression can be given as (after Hoffmannmeasured Ks. Furthermore, L was predominantly negative, casting Riem et al., 1999): doubt that the MVG can be interpreted in a physical way. Spearman rank correlations showed that both Ko and L were related to van K(Se) 5 KoS e 1# 0 hd Se/# 1