Modeling water flow and nutrient losses (nitrogen, phosphorus) at a nested meso scale catchment, Germany

Eutrophication of stream water due to nutrient inputs has become a worldwide aquatic problem. Understanding nutrient transport processes and the relationship with hydrologic processes is important for water resource management and prediction of stream water quality under climate and land use change. In this study we used the integrated process-based hydrologic and water quality model HYPE (Hydrological Predictions for the Environment) to simulate water flow and stream water nutrient concentrations (nitrogen and phosphorus) at a meso scale nested catchment (Selke) in Germany. The hydrograph was represented well with NSCE of 0.84 and was characterized by high flow during winter due to snow melting and low flow in summer. The nitrogen seasonal dynamics was caught by the model which presented a proportional relationship to water flow. The simulated phosphorus concentrations were at the similar level as the measured values which were more constant compared to nitrogen across the year. The calculated nutrient loads were 8.92 kg IN/yr/ha and 0.162 kg TP/yr/ha at the Silberhuette subbasin, which are consistent with former findings at catchments with similar hydrological pattern and land use type. However, it has to be pointed out that sparse nutrient measurements are not sufficient to fully identify the model due to inability to evaluate model performance on representation of more detailed internal variability. In future work, available high resolution multiple hydrochemical data sets will be integrated for further model identification.