Atmospheric pollution due to agricultural pesticide is a major concern today, rearding both public health, sustainable agriculture and agrosystems quality monitoring. One aims to model pesticide atmospheric dispersion and to propose a useful predictive tool, using fluid mechanics equations and opent-source GIS programming capabilities. This paper focuses on pesticide spray drift modellin for viticulture applications at the watershed scale. Our model tends to provide spray georeferenced atmospheric transport simulation outputs at very low calculation cost using solution space reduction and reduced order modelling. This is necessary to make affordable assimilation/simulation and very helpful to quickly perform spatial based risk analysis. The coupling of fluid mechanic equations with GIS is first described, reagarding spatialization of inputs/outputs and scale variations. The GIS based predictive tool is the presented through explanations on coupling methods and technical ramblings about Quantum GIS plugin development.