SIMULATION OF THE MIGRATION BEHAVIOR OF HERBICIDES IN UNSATURATED SOILS WITH A MODIFIED LEACHP-VERSION

The incorporation of the hysteresis effect into the model LEACHP has l ed to a remarkable improvement in the prediction of herbicide leaching behavior. This modified version uses non-singular and non-linear isot herms for the description of ad- and desorption processes in contrast to the original version that assumes linear adsorption. In most cases of the laboratory studies the hysteresis version calculated less herbi cide discharge from the soil profile than the original version. At the same time the modified model showed a much better agreement with the measured data regarding both the herbicide's first appearance in the s oil percolate and the location of its maximum concentration. The hyste resis version also calculated less chemical loss under field condition s. The movement of the herbicidal solutes within the soil profile coul d be identified as highly dependable on large rainfall events.