Many of the water management issues raised by the Water Framework Directive are complex. As such, water managers require robust and reliable models to assist them in their decision-making. Although the use of catchment models is increasing, the application of many catchment scale models remains difficult and requires proper evaluation to ensure that they reach acceptable levels of efficiency. This article investigates how simple rainfall-runoff models can be used to benchmark the hydrological modules of more complex diffuse pollution models. The complex SWAT model and the simple GR4J rainfall-runoff model used here as a baseline, were applied in two catchments, one in Scotland and one in Finland. Comparisons of model performance were made using the Nash-Sutcliffe criterion. Results show that a comparative assessment of models, using objective quantitative criteria, is very valuable and can be used to determine whether a model is suitable (or not) for a specific application. The level of acceptability being set by the baseline model, this quantitative methodology avoids subjective assessments being made without reference to a plausible model. It complements the preliminary qualitative assessment proposed by the BMW project, to determine model suitability for use.