The use of riparian land buffers is widely promoted as a method of mitigating the effects of sediment and nutrient runoff from intensive land use in New Zealand. Farmers receive advice and financial assistance from Regional Councils for activities such as establishment and planting of riparian buffers, but funding is limited. The effect of buffers on water quality goals varies across land types so the optimum size of riparian buffer width varies across farms. We build a stylised model to determine the optimum buffer width and apply it to the Karapiro catchment. The model can easily be extended to model salinity removal, conservation reserve programmes, establishing wetlands and carbon sequestration.