Measured and modeled unsaturated hydraulic conductivity of a Walla Walla silt loam

There are numerous methods of estimating unsaturated soil hydraulic conductivity [K(theta)], ranging from direct measurement in the laboratory or field, to models that use only basic soil data, e.g., texture or water release curves (WRC). We evaluated K(theta) for a Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll) using two field methods and the Mualem-van Genuchten model (MVG). Field methods were the internal drainage method (ID) and the Klaij and Vachaud method (MKV), which was modified to include hydraulic head (H), and used 20 yr of data from a dryland field experiment. Four approaches to estimate WRC were compared for the MKV, and two approaches to estimate parameters of the MVG. For water contents >0.20 m(3) m(-3), the MKV gave K values that were two orders of magnitude less than those obtained from internal drainage experiments conducted under wetter conditions. When MVG parameters were predicted by the Rosetta model, which uses pedotransfer functions, K values approached those of the MVK. However, when model parameters were estimated from internal drainage and saturated hydraulic conductivity (K,(at)) data, K(theta) values were closer to those of the internal drainage experiments. The study reaffirms the difficulty of reconciling K(theta) determined by different methodologies, and of extrapolating to values of theta outside those measured. It also demonstrates the sensitivity of the MVG and pedotransfer functions to different sources of input values. Advantages and disadvantages of the different approaches to determining K(theta) are discussed.