MODEL COMPARISON OF UNSATURATED STEADY-STATE SOLUTE TRANSPORT IN A FIELD PLOT

Models of solute movement in soil are seldom compared using independently generated data. The objectives of this study were to leach Br- through a Hecla fine sandy loam (sandy, mixed Aquic Haploboroll) and to compare the results with four previously published solute transport models. Solute was applied over the surface of four 9-m(2) subplots within a 65.5-m(2) plot and leached at constant rates: 1.28, 2.53, 4.28, and 5.94 mu m s(-1). Soil solution samples were taken from 0.3-, 0.6-, 1.0-, 1.5-, 2.0-, 2.3-, and 2.5-m depths. At the same depths, water content and soil suction were measured using a neutron probe and tensiometers. Data from this experiment were used to evaluate four solute transport models. Model performance was evaluated using the observed to expected sum-of-squares ratio, the normalized objective function, and the Kolmogorov-Smirnov statistic. A numerical finite-difference solution of the convective-dispersive equation (LEACHM) was more successful in predicting solute movement under unsaturated steady-state conditions than the analytical solution for the convective-dispersive equation (CDE), the continuous-time Markov process (CTMP), or the transfer function model (TFM). The CTMP model predicts observed solute concentration better than the TFM. The TFM needs to be calibrated by a solute transport experiment while CTMP requires water flow and soil moisture as input data.