Deriving Transport Parameters from Transient Flow Leaching Experiments by Approximate Steady‐State Flow Convection–Dispersion Models

The applicability of two different steady‐state flow 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 flow 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, ζ, assuming that the flow 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 BTC when the water content fluctuates considerably during the leaching experiment. Alternatively, fitting this model to a BTC as a function of I results in an overestimation of the dispersion coefficient D and the dispersivityλ = D / vSince the second approximate model described the simulated BTCs well, good estimates of D and λ were obtained when this model was fitted to a BTC as a function of ζ. If λ is a function of the flow rate J w , the fitted λ could be related to an effective or flux‐weighted average flow rate so that the soil specific relation λ( J w ) could be defined.