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 conte nt, theta(z), and hydraulic head, H(z), measured under steady sated an d near-saturated flow conditions in two large, intact, soil columns. T he hydraulic conductivity (K)-water content (theta)-pressure head (psi ) relationships, K-theta-psi, required by LEACHW were obtained by fitt ing the Van Genuchten functions to measurements of sated hydraulic con ductivity (K-s) and theta vs. psi obtained: (i) using in-situ TDR and tensiometer measurements, with the theta-psi data being collected duri ng the initial column wetting; and (ii) using the traditional saturate d Bow-desorption technique on intact soil core subsamples collected fr om the columns at the end of the experiment. The LEACHW model provided reasonably accurate predictions of the theta(z) and H(z) profiles, re gardless of whether the in-situ or soil core-based K-theta-psi relatio nships were used, and regardless of whether sated or near-saturated Bo w occurred. The LEACHW predictions were substantially more accurate, h owever, when the K-theta-psi relationships derived from the in-situ me asurements were used. It was concluded that the LEACHW model, coupled with independently measured K-theta-psi functions, could provide reaso nably accurate predictions of steady theta(z) and H(z) profiles in lar ge, 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 wa ter and solute transport models than the traditional saturated Bow-des orption method using intact soil core subsamples.