Water application frequency effects on steady-state solute transport parameters

In many studies, data from transient or quasi steady-state solute transport experiments are modeled using steady-state models. Few investigations, how ever, have been carried out to verify the validity of this approach. In thi s paper, we study the effects of water application frequency on the paramet ers derived using steady-state models. Bromide transport was studied in an undisturbed soil column of loamy sand exhibiting fingered flow. Four solute displacement experiments were carried out under quasi steady-state flow co nditions. The mean water flux in all experiments was 1.42 cm d(-1), but wat er was applied at different time intervals for each; that is one, two, thre e, or six times daily. Water content and solute concentration were measured following an in situ calibration, using time domain reflectometry (TDR) pr obes at nine different depths. The TDR probes were placed in two alternate directions, with every second probe being orthogonal to the first. The conv ective-dispersive (CDE) and the convective log-normal transfer function (CL T)models were fitted to the solute transport data. The CDE model best descr ibed the solute transport. The dispersivity decreased when water was applie d more often. The decrease can largely be attributed to lesser variability of the water flux velocity, especially in one of the probe directions. The mass recovery was about 40% for all depths and did not change with the diff ering intervals between water applications. Solute transport heterogeneity was also constant during the experiments. These observations lead to the co nclusion that preferential flow paths in this soil were consistent and inde pendent of application frequency. (C) 1999 Elsevier Science B.V. All rights reserved.