SWAT developments and recommendations for modelling agricultural pesticide mitigation measures in river basins

Pesticides are useful for agriculture because of their ability to protect crops against pests. At the same time, excessive loading of pesticides in water bodies can produce toxic conditions that harm sensitive aquatic species, and render the water unfit for human consumption. Therefore, measures need to be designed, evaluated and undertaken in order to reduce pesticide pollution. In this study we focus on the Nil catchment, a small basin situated in the centre of Belgium. The necessary database and a watershed model (Soil and Water Assessment Tool-SWAT) were available to simulate different agricultural management scenarios. In order to make the model accurately predict pesticide loading to the river and instream transport, it was necessary to make several modifications to the source code. Special attention was given to implement an estimator for point losses (e.g. cleaning of spray equipment) and droplet drift, and improve the representation of physical processes in Filter strips. The closing of mass balances is also described. Once the model was modified and calibrated, it could be used to simulate the pesticide mitigation strategies and evaluate their effectiveness. The Simulation results revealed that strip-cropping seems to be more efficient than the sowing of cover crops, contour farming, the construction of filter strips, a 40% reduction of point losses and finally conservation agriculture. Several recommendations are given for further improvement of SWAT for management use.