Journal of Hydrology ELSEVIER Journal of Hydrology 205 (1998) 232-247 Estimating transport parameters in an undisturbed soil column using time domain reflectometry and transfer function theory Magnus Persson, Ronny Berndtsson Department of Water Resources Engineering, Lund University, Box 118, SE-221 O0 Lund, Sweden Received 17 February 1997; revised 11 September 1997; accepted 27 November 1997 Abstract Modelling solute transport in the unsaturated zone is a complex problem given the inherent soil heterogeneity. Using time domain reflectometry (TDR) as described in this paper can improve data quality to be used in predictive models. The TDR technique requires appropriate calibration if it is to be used for solute concentration measurements under constant and varying water content. Two ways of determining the parameters in the relationship between bulk soil electrical conductivity, Oa, soil solution electrical conductivity, aw and volumetric water content, 0, were used. Among the two calibration methods tested here, the one which involved eluting the soil with several pore volumes of water of known electrical conductivity under transient flow conditions in an undisturbed soil column gave a linear Ow-aa-O relationship with depth dependent parameters. The other calibration method involved the use of disturbed soil under static conditions and resulted in a curvilinear aw-oa-O relationship. The former calibration was subsequently used to determine solute concentrations in breakthrough experiments. Two solute transport experiments were carried out in the same undisturbed soil column; one for steady-state and one for transient flow condition. Parameters of the convective log normal transfer function (CLT) model were determined using time- integral-normalized resident concentrations. It was shown that the stochastic-convective approach described the experimental data well. Horizontal heterogeneity was more important during the transient experiment, where the topsoil was periodically ponded during a short period, than in the steady-state experiment, where unsaturated conditions were maintained throughout the experiment. This increased heterogeneity during transient flow was also reflected by the fact that preferential solute flow was only observed in the column's outlet under transient conditions. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Soil water; Solute transport; Electrical conductivity; Transfer function; Time domain reflectometry I. Introduction Transport of solutes through unsaturated field soils is a complex process due to the inherent varia- bility of soil media. Based on the assumption of homogeneity and perfect mixing of solutes, the con- vective-dispersive approach to solute transport prob- lems was developed (e.g. Biggar and Nielsen, 1967). * Corresponding author. Fax: +46 46 2224435; e-mail: magnus.- persson@tvrl.lth.se 0022-1694/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0022-1694(97)00151-0 The approach has been shown to be able to accurately predict solute transport in uniform porous materials under uniform flow conditions. However, the enor- mous heterogeneity of natural soils limits the applic- ability of this approach for field conditions. An alternative way of describing solute transport is the stochastic-convective approach, in which the solute is assumed to move in isolated stream tubes at differ- ent velocities without mixing (Raats, 1978; Dagan and Bresler, 1979). This assumption may be more valid at the field scale during solute transport in