Evaluation of a dual-porosity model to predict field-scale solute transport in a macroporous soil

A one-year field-scale leaching experiment was conducted on a structured clay soil for the purpose of evaluating the dual-porosity/dual-permeability model MACRO. The model was first calibrated against measurements of water contents, drainflow, and bromide contents in the soil profile and concentrations in drain discharge. Bentazone transport was then simulated without any further model calibration. The model gave acceptable predictions of the water balance, providing the significant water inflow into the plot from the surrounding areas was accounted for. Simulated bromide contents in the soil were, for the most part, within one standard deviation of the measured values. Bromide was measured in tile drainage 26 days after spraying at concentrations > 3 mg l(-1) (after 43 mm of precipitation), while groundwater concentrations at 2 m depth were as large as 10 mg l(-1) only 42 days after spraying. This is a strong indication of macropore flow. The agreement between model predictions and bentazone measurements was on the whole good, especially for the depth profiles and the initial breakthrough in the drainflow, whereas short-term fluctuations in drainage water concentrations were poorly captured. This was probably caused by the model description of first-order mass-exchange between micro- and macropores, which neglects lateral concentration gradients. Judging from statistical criteria, the model accurately predicted bentazone amounts in the soil profile (model efficiency 0.87), while 66% and 89% of the simulated bentazone concentrations in tile drainage were within a factor of 2 and 5 of measured values respectively. Simulations run without macropore flow overestimated bentazone leaching by ca. 20%. In other words, macropore flow reduced leaching in this clay soil, because much of the bentazone was 'protected' against bypass flow in macropores, being stored in micropore water moving at a 'reduced' convective transport velocity. (C) 1999 Elsevier Science B.V. All rights reserved.