Phosphorus (P) is a limiting nutrient in many freshwater ecosystems and increases in its availability can lead to eutrophication. Effective management of P in freshwaters requires quantitative estimates of P supply from all significant sources. A simple GIS-based model, capable of predicting total diffuse source phosphorus export from catchments using readily available data, has been developed. The model is based on the idea of export coefficients but includes the effects of topography (slope and cumulative area), soil type (using the UK Hydrology of Soil Types (HOST) classification) and climate (hydrologically effective rainfall) as well as land use. Uncertainty in key model parameters is accounted for using Monte Carlo simulation which involves random sampling from probability density functions in a large number of iterations. This reduces the need for subjective optimisation of export coefficients. The model has been applied to the Greens Burn catchment, Scotland and predicts P exports within the confidence limits of the measured values.

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