Simulation of the movement of bentazon in soils using the CALF and PRZM models

Abstract
Concentrations of bentazon in leachate draining from lysimeters containing intact monoliths of Nåntuna sand were simulated using a model for leaching and degradation of pesticides in field soils (CALF) and the Pesticide Root Zone Model (PRZM). The CALF model empirically simulates the movement of water and the hydrodynamic dispersion of solutes by dividing the soil water into mobile and immobile categories; it Calculates Flow, hence the name, CALF. Sorption was calculated, using a linear isotherm, as a function of depth in soil. Bentazon degradation rates were calculated as functions of temperature, water content and depth in soil. Neither model simulated the early appearance of bentazon in leachate 100 days after application but both models closely estimated the cumulative amount of bentazon which had leached by day 325. Neither model made correct simulations when preferential flow of herbicide was thought to occur. The results of simulations by CALF and PRZM were similar.