Numerical weather prediction model output may be used in simulating the movement of water borne pollutants with little significant loss of accuracy in comparison to the use of observed wind data of equivalent temporal resolution. Spatial variations in the instantaneous wind field over the region of influence of a spill mere found to be less significant than the uncertainties in the wind at a single point. However, temporal variations in the wind must be included in the spill simulations, with a minimum recommended resolution of 3 h,