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

This paper puts forward a methodology permitting the identification of farm ing 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 appr opriate 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 "her ring bone" structure: 50 independent tributaries supply the main drain. Pes ticide sales show that atrazine is the most frequently used compound althou gh it is only used for treating maize plots and that its application rate i s 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 str ips around the channel network. The correlation between plots under maize i n contributory zones and triazine flux at related sub-basin outlets is stud ied by using non-parametric and linear correlation coefficients. Finally, t he most pertinent contributory zone is associated with the best correlation level. A catchment typology, based on a slope criterion, allows us to conclude tha t 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 dr ainage 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 Els evier Science B.V. All rights reserved.