WATER-FLOW THROUGH INTACT SOIL COLUMNS - MEASUREMENT AND SIMULATION USING LEACHM

An evaluation of the water Bow submodel, LEACHW, of the LEACHM solute transport model was conducted using profiles of volumetric water content, theta(z), and hydraulic head, H(z), measured under steady sated and near-saturated flow conditions in two large, intact, soil columns. The hydraulic conductivity (K)-water content (theta)-pressure head (psi) relationships, K-theta-psi, required by LEACHW were obtained by fitting the Van Genuchten functions to measurements of sated hydraulic conductivity (K-s) and theta vs. psi obtained: (i) using in-situ TDR and tensiometer measurements, with the theta-psi data being collected during the initial column wetting; and (ii) using the traditional saturated Bow-desorption technique on intact soil core subsamples collected from the columns at the end of the experiment. The LEACHW model provided reasonably accurate predictions of the theta(z) and H(z) profiles, regardless of whether the in-situ or soil core-based K-theta-psi relationships were used, and regardless of whether sated or near-saturated Bow occurred. The LEACHW predictions were substantially more accurate, however, when the K-theta-psi relationships derived from the in-situ measurements were used. It was concluded that the LEACHW model, coupled with independently measured K-theta-psi functions, could provide reasonably accurate predictions of steady theta(z) and H(z) profiles in large, intact soil columns, regardless of whether the soil macropores are largely water-conducting (sated flow) or largely nonwater-conducting (near-saturated flow). It also appears that in-situ K-s and theta-psi measurements, with the theta-psi data collected during initial wetting, may provide more appropriate K-theta-psi relationships for use in water and solute transport models than the traditional saturated Bow-desorption method using intact soil core subsamples.