A distributed parameter method was developed to calculate diffuse phosphorus emissions at the scale of small watersheds based on relative pollution potential and transmission coefficients. Utilizing digital base maps covering the whole territory of Hungary as well as equations and parameters found in the literature, pollution potential raster maps of the main diffuse pathways were determined. These indicate the capability of each cell to contribute to the diffuse emissions. Besides this, for each delineated sub-basin a surface transmission coefficient was set based on catchment properties. Absolute emission values of the cells were assigned by an inverse way: large watershed scale emissions calculated by an empirical watershed model were distributed to grid cell scale by a weighing function according to pollution potential and transmission coefficients. Finally, single cell values were summed in the delineated small sub-catchments. To check the accuracy of the distribution procedure the results for several sub-catchments were compared to measured river loads including point sources and river retention. The results suggest that the method is capable to determine the spatial distribution of the main diffuse sources and to assess the risk of eutrophication of small water bodies in absence of local loading data.