Relations between triazine flux, catchment topography and distance between maize fields and the drainage network

This paper puts forward a methodology permitting the identification of farming plots contributing to the pollution of surface water in order to define the zones most at risk from pesticide pollution. We worked at the scale of the small agricultural catchment (0.2-7.5 km(2)) as it represents the appropriate level of organisation for agricultural land. The hypothesis tested was: the farther a field undergoing a pesticide treatment is from a channel network, the lower its impact on pollution at the catchment outlet. The study area, the Sousson catchment (120 km(2), Gers, France), has a "herring bone" structure: 50 independent tributaries supply the main drain. Pesticide sales show that atrazine is the most frequently used compound although it is only used for treating maize plots and that its application rate is constant. In two winter inter-storm measurement exercises, triazine flux values were collected at about 30 independent sub-basin outlets. The contributory areas are defined, with the aid of a GIS, as different strips around the channel network. The correlation between plots under maize in contributory zones and triazine flux at related sub-basin outlets is studied by using non-parametric and linear correlation coefficients. Finally, the most pertinent contributory zone is associated with the best correlation level. A catchment typology, based on a slope criterion, allows us to conclude that in steep slope catchments, the contributory area is best defined as a 50 m wide strip around the channel network. In flat zones, the agricultural drainage network is particularly well developed: artificial drains extend the channel network extracted from the 1/25.000 scale topographic map, and the total surface area of the catchment must be taken to account. (C) 2000 Elsevier Science B.V. All rights reserved.