SENSITIVITY ANALYSIS OF A MATHEMATICAL-MODEL FOR PESTICIDE LEACHING TO GROUNDWATER

The sensitivity of pesticide leaching to pesticide/soil properties and to meteorological conditions was assessed by calculations with an existing convection-dispersion model. The model assumes equilibrium sorption (Freundlich equation), first-order transformation kinetics and passive plant uptake. The extent of pesticide leaching was characterized by the percentage of the dose leached below 1 m depth. The calculations were carried out for a humic sand soil cropped with maize and exposed to Dutch weather conditions. In general, the percentage leached was found to be very sensitive to the sorption coefficient, the Freundlich exponent (describing the curvature of the isotherm) and the transformation rate. The percentage leached was moderately sensitive to weather conditions (wet/dry years), long-term sorption equilibration and the relationship between transformation rate and temperature. Sensitivity to the extent of plant uptake was only significant for pesticides with low sorption coefficients. Sensitivity to soil hydraulic properties was small. The effect of application in autumn instead of in spring was found to be very large for non-sorbing pesticides with short half-lives. The sensitivity to spatial variability in sorption coefficient and transformation rate was found to be substantial at low percentages leached.