Error propagation in groundwater pesticide vulnerability modelling

Paulette Posen 1, Andrew Lovett 1, Kevin Hiscock 1, Brian Reid 1, Sarah Evers 2 and Rob Ward 2
1 School of Environmental Sciences
University of East Anglia, Norwich NR4 7TJ, UK
Tel.: + 44 1603 592547; Fax: + 44 1603 591327
p.posen@uea.ac.uk
2 Environment Agency
Olton Court, 10 Warwick Road, Olton, Solihull B92 7HX, UK

Abstract
Although environmental modelling is increasingly performed within a GIS framework, analysis of the associated error is far from routine, and rarely presented with the results. An important benefit of performing error analysis is its value in determining which elements of a vulnerability assessment framework need improving. With this in mind, it was decided to examine the extent to which error might propagate through a model of groundwater vulnerability to pesticide contamination. A pesticide leaching model was developed and incorporated into an assessment of groundwater contamination risk from normal agricultural use of the herbicide isoproturon, in a 30 km x 37 km area of river catchment to the north-west of London, England. The model, which comprised two main components accounting for (i) degradation and (ii) attenuation of the pesticide, was based on conventional contaminant transport calculations, combined with existing  soil, rainfall, hydrogeological and depth to water table data. The results of an error analysis on the model were used to assign confidence limits to the resulting risk maps. In this instance, correlation of model variables led to a reduction of error in the final output. However, the results  of the analysis showed how inclusion of low quality input data can lead to  a large increase in output uncertainty. It is suggested that error propagation analysis should be routinely included in groundwater vulnerability assessment.

Keywords: error propagation, GIS, groundwater, pesticide, leaching model

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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