A simple model has been developed to simulate the relationships between stream flow and point and diffuse nutrient concentrations. The point source component is based on a mechanistic approach (including uncertainty), while the diffuse component relies on a statistical regression model. The model is explained and its application illustrated using flow and nutrient data from the South African Department of Water Affairs national monitoring database. The study sites represent a wide range of different combinations of point and diffuse source contributions to the total nutrient signature. The model has been successfully calibrated against the observed data and the study concludes that the point source component offers opportunities for use as a scenario assessment management tool, largely related to its mechanistic basis. However, further research is required to link the parameters of the diffuse source model to diffuse load causative factors so that the model can be used effectively in un-gauged situations or to assess management scenarios.