Soil science society of america journal vol:64 issue:4 pages:1317-1327
The applicability of two different steady-state now approximations of the convection-dispersion equation (CDE) to derive transport parameters from time series of concentrations or breakthrough curves (BTCs) that are observed during transient Dow leaching experiments was evaluated. In the first often-used approximation, the time coordinate was transformed to a cumulative drainage coordinate, I, assuming that the water content remained constant during the leaching experiment. In the second approximation, the time coordinate was transformed to a solute penetration depth, zeta;, assuming that the Dow rate and water content remained constant with depth across the solute displacement front. Comparisons of numerical solutions of the CDE for transient flow conditions with analytical solutions of the approximate steady-state models revealed that the first approximate model underestimates the dispersion of the ETC when the water content fluctuates considerably during the leaching experiment. Alternatively, fitting this model to a ETC as a function of I results in an overestimation of the dispersion coefficient D and the dispersivity lambda = D/v. Since the second approximate model described the simulated BTCs well, good estimates of D and lambda were obtained when this model was fitted to a ETC as a function of zeta. If lambda is a function of the flow rate J(w), the fitted lambda could be related to an effective or flux-weighted average flow rate so that the soil specific relation lambda(J(w)) could be defined.