Modelling water flow through undisturbed soil cores using a transfer function model derived from 3 HOH and Cl transport

SUMMARY Large undisturbed soil cores (20 cm diam. × 25–30 cm long) were irrigated at rates of 0.5–4 cm h −1 with 0.005 M CaCl 2 solution labelled with 3 HOH. The cores were used at varying initial water contents and flow in all cases was unsaturated. Breakthrough curves for Cl and 3 HOH were markedly asymmetric and unlike those reported for columns of packed aggregates. The data could be satisfactorily described using a density distribution function of the logarithm of cumulative drainage D. The mean and standard deviation of In D were estimated by a curve‐fitting procedure from Cl and 3 HOH effluent concentrations in each core. The mean pore water velocity and fraction of the soil water that participated in solute transport (the mobile volume) were also calculated. The apparent velocity of Cl movement was always greater than that of 3 HOH which suggested that the mobile volume involved in convective and diffusive transport of Cl was less than that for 3 HOH. We suggest that Cl and 3 HOH diffused at different rates out of flowing water films in a relatively few large conducting channels into essentially immobile water within the surrounding soil matrix. The difference in mobile volume for Cl and 3 HOH was used to calculate the perimeter of voids in any horizontal cross‐section of the soil through which water flowed, assuming a planar interface between the mobile and immobile water.