Sensitivity Analysis of a Model for Pesticide Leaching and Accumulation

The sensitivity of pesticide leaching and accumulation to variations in pesticide properties, soil temperatures, soil water fluxes, and transport parameters was investigated with the general solute transport model, SOTRAS. Pesticide interactions include non-linear Freundlich equilibrium sorption, temperature and pressure head dependent first-order transformation kinetics, and plant uptake. For a number of pesticides with different mobility and half-lives, Monte Carlo simulations were carried out with Latin Hypercube Samples in a preset range of the input parameter domain. The sensitivity of model inputs to model outputs was quantified by statistics of linear regression. The time evolution of model sensitivity and the contribution of various model inputs to the total sensitivity were quantified as well. The standardized analysis gives rapid quantitative information about model behaviour. The results from the analysis are used to determine which parameters should be measured in greater detail and which need further calibration. Results are also used to set up sampling strategies. In general, the accumulation of pesticides in the plough layer was very sensitive to model inputs influencing the transformation rate of the pesticide (soil temperature and half-life) and almost insensitive to sorption characteristics and soil water fluxes. Only in the case of very persistent and mobile pesticides, accumulation was most sensitive to soil water fluxes. The concentration of pesticide in ground water was most sensitive to the Freundlich concentration exponent, and, to a lesser extent, to the Freundlich coefficient, except for some pesticides which are hardly sorbed. The leaching of these pesticides was most sensitive to half-life and soil temperature. The linear regression model could not be used for pesticides with high sorption coefficients, even if variation of the input was kept as low as 1%, but good results were obtained after logarithmic data transformation.