Estimation of diffuse phosphorus emissions at small catchment scale by GIS-based pollution potential analysis

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.

With the presented method, diffuse phosphorus emissions at small sub-catchment level can be quantified. The main advantages of the approach are the supportable data demand and the simple algorithm. Due to the distributed parameter hot spot analysis can be executed within a given sub-catchment. Based on the calculated emissions and measured runoff values, potential phosphorus concentrations of water bodies and their risk of eutrophication can be identified. Besides the implementation of WFD, this method can support strategic decisions in water management. However, due to the absence of refined process description the method is not capable to execute detailed analyses and dynamic simulations. Although the method requires large scale emission estimates or measurements to calculate the spatial distribution of diffuse loads, the data requirements of these are more feasible to provide than that of a detailed simulation with high spatial resolution in numerous sub-catchments. The distribution procedure was applied to 892 Hungarian sub-catchments for the period 1998–2000. Areas having remarkable erosion potential contribute at greatest amount to the N Cumulative phosphorus river loads [t P a−1] 0.001–0.1 0.1–1 1–10 10–50 50–100 100–200 200–500 500–1000 1000–2000 2000–3500 300 km Main rivers Main lakes Main outlets Fig. 4. Cumulative phosphorus river loads at the outlets of the Hungarian sub-catchments (1998–2000).