Propagation of uncertainties in soil and pesticide properties

Pesticide properties contribute most to the uncertainty in pesticide leaching concentrations

In the new Dutch decision tree for the evaluation of pesticide leaching to groundwater, data on the spatial distribution of soils are used by the GeoPEARL model to calculate the 90th percentile of the leaching concentration in the area of potential usage (P90). So far it was not known to what extent the uncertainties in soil properties propagate to the predicted leaching concentrations. Therefore, a study was set up to quantify the uncertainties in the soil data and analyse their contribution to the uncertainty in the leaching concentrations.

The contribution from uncertain soil properties (e.g. soil horizon thickness, texture, organic matter content hydraulic conductivity and water retention characteristics) was compared to that caused by uncertainties in the most important pesticide properties, i.e. the coefficient of sorption on organic matter (Kom) and the half-life of transformation in soil (DT50). Firstly, the uncertainties in the soil and pesticide properties were quantified. Next, a regular grid sample of points covering the whole of the agricultural area in the Netherlands was randomly selected. At the grid nodes, realisations from the probability distributions of uncertain inputs were generated and used as input to a Monte Carlo uncertainty propagation analysis. Uncertainties in DT50 and Kom contributed most to the uncertainty in the leaching concentrations. Recommendations are made for further improvement of the model predictions.