Comparative study of two watershed scale models to calculate diffuse phosphorus pollution.

The aim of this study was to compare and assess models having different principles to calculate diffuse phosphorus emissions on a selected watershed. The empirical MONERIS model and the physically based SWAT model were evaluated for comparative purposes. The approaches were applied for a sub-basin of the Hungarian Zala River watershed for five years. The calculated river loads were checked by the measured values at the catchment outlet. Due to the dissimilar results of water balance and erosion calculations, a highly different phosphorus emission was computed. It was also concluded that in the case of transport-limited watersheds, the SWAT model calculates phosphorus river loads slightly inaccurately, since it does not include the description of fate of inorganic phosphorus interacting with sediment during the channel transport. When these processes are taken into account, modeling results fit better the measured loads. The MONERIS model calculates acceptable river load by assuming very intensive in-stream retention. Additionally, the empirical method can be useful for long-term investigations as a decisions support tool for preliminary design. However, for detailed emission assessment and scenario development the physically based approach seems to be more appropriate.