Estimating the van Genuchten retention curve parameters of undisturbed soil from a single upward infiltration measurement

Estimation of the soil-water retention curve, theta(h), on undisturbed soil samples is of paramount importance to characterise the hydraulic behaviour of soils. Although a method of determining parameters of the water retention curve (alpha, a scale parameter inversely proportional to mean pore diameter and n, a measure of pore size distribution) from saturated hydraulic conductivity (K-s), sorptivity (S) and the beta parameter, using S and beta calculated from the inverse analysis of upward infiltration (UI) has been satisfactorily applied to sieved soil samples, its applicability to undisturbed soils has not been tested. The aim of the present study was to show that the method can be applied to undisturbed soil cores representing a range of textures and structures. Undisturbed soil cores were collected using stainless steel cylinders (5 cm internal diameter x 5 cm high) from structured soils located in two different places: (1) an agricultural loam soil under conventional, reduced and no tillage systems; and (2) a loam soil under grazed and ungrazed natural shrubland. The alpha and n values estimated for the different soils using the UI method were compared with those calculated using time domain reflectometry (TDR) pressure cells (PC) for pressure heads of -0.5, -1.5, -3, -5, -10 and -50 kPa. To compare the two methods, alpha values measured with UI were calculated to the drying branch of theta(h). For each treatment, three replicates of UI and PC calculations were performed. The results showed that the 5-cm high cylinders used in all experiments provided accurate estimates of S and beta. Overall, the alpha and n values estimated with UI were larger than those measured with PC. These differences could be attributed, in part, to limitations of the PC method. On average, the n values calculated from the optimised S and beta data were 5% larger than those obtained with PC. A relationship with a slope close to 1 fitted the n values estimated using both methods (n(PC) = 0.73 n(UI) + 0.49; R-2 = 0.78, P < 0.05). The results show that the UI method is a promising technique to estimate the hydraulic properties of undisturbed soil samples.