Similarities and differences between measured and predicted concentrations of pesticides in Dutch surface waters

In order to have a thorough evaluation of the progress and effectiveness of Dutch crop protection policy, both model predictions and measured pesticide concentrations in surface waters are considered. To this purpose, monitoring data obtained by various water boards and other monitoring institutes were processed. Data were aggregated over a two year time period and over space (at 1x1 km-grid). A geographic view is given in the Dutch Pesticides Atlas (www.pesticidesatlas.nl). The model used for the predictions was the Dutch National Environmental Indicator NMI version 2 (www.nmi.alterra.nl) that has input data regarding spray drift data, crop interception, soil and climate and many more. Information on aggregation steps over time and space, grid sizes, information on crop areas was geared to one another for both instruments. Results on measured pesticide concentrations in surface waters and model predictions were compared to each other at the national scale. For this study, 10 different cases were selected covering a large range of pesticides' characteristics and pesticides' use. In 60% of the cases, the results were largely in agreement with each other when expressed as absolute numbers of measurements exceeding the environmental quality standard. This is very accurate and useful for policy purposes. Based on concentrations and on the order of magnitude, no significant agreement between measurements and model predictions was found. Differences were explained by various factors, and an overview of predominant systematic differences between the measurements and the model predictions was presented. Using both measurements and model predictions in supporting environmental policy evaluations is warranted, because of higher Weight-of-Evidence. Combining both can assist in optimizing the knowledge on pesticides behaviour, fate and ecological problems and therefore this is the preferred evaluation method.