Spatial uncertainty analysis of pesticide leaching using a metamodel of PEARL

Spatially distributed modelling has recently been introduced in pesticide registration procedures and policy evaluations. The assessments are quite demanding with respect to computational efforts, which probably is the reason that little attention has been paid so far to uncertainty analyses of leaching concentrations at the regional scale. Recently, a metamodel of pesticide leaching was developed and calibrated with results of the spatially distributed model EuroPEARL (Tiktak, et al. 2004; Tiktak, et al. (acc.)). The metamodel is based on an analytical expression of the transport of solutes through porous media and calculates the fraction of the applied amounts which leach to a reference depth in the soil profile. Important inputs to the metamodel are the half-life and sorption constant of a pesticide, soil organic carbon content and soil moisture at field capacity, as well as long-term averages of temperature and precipitation surplus. The metamodel explained more than 90% of the variance in the simulation results and thus might be considered a good approximation of the original model. As opposed to the original model, the metamodel is very inexpensive with regard to computational efforts, which makes it suitable for calculations at high resolutions (i.e. small grid sizes) or uncertainty analyses using Monte Carlo techniques. Using the metamodel, this paper investigates the uncertainty in pesticide leaching assessments due to uncertainty in the half-life and sorption constant of pesticides and uncertainty and spatial variability in soil parameters and climatic conditions. The magnitude of uncertainty in the annual leached pesticide is computed for the temperate region of the EU and the relative contributions of individual uncertain inputs are compared. Possible implications for policy evaluations are discussed.