Eachyear, millions of tonnes of faecal waste are applied to agricultural land in the UK. Due to their low density and resistance to shear, they may easily be detached and transported to surface waters (Schofield 1988). The potentially harmful microorganisms they contain together with other chemical components and soil particles may seriously endanger the water quality. These elements are mainly transported by overland flow, that is the water running off fields during storms. An event-based model has been developed to simulate the behaviour of surface water and transported particles during a storm. The objective is to apply this model to fields with a surface of up to one square kilometre. As in most fluid applications, the Navier–Stokes equations would provide a valid model for overland flow. However, they are notoriously complex to solve and do not exploit the thin geometry of the film. Our model is based on lubrication theory, an asymptotic simplification of traditional equations (Ockendon & Ockendon 1995, Myers 1998). This results in a single transient non-linear equation for the film thickness, driven by gravity. However, fields are roughsurfaces and this may significantly affect the flow. The equation is then modified to include the soil roughness, through two parameters inherent to the catchment that may be evaluated from the flux and velocity (Myers 2002). Results were compared withsmall-scale experiments. This model of overland flow is then coupled with a transport equation to study the behaviour of soil particles or microorganisms during a storm. The particle detachment is based on a model developed for the EUROSEM code (Morgan et al. 1999). Simulations based on the lubricationEUROSEM transport model are presented for test cases and some real field data. Experiments to evaluate the model are currently underway on a specially designed flume.