Examining a model and assessing its performance in describing nutrient and sediment transport dynamics in a catchment in southwestern Finland

The Eurajoki basin, including Pyhajarvi, was chosen as the Finnish test catchment in an EU project on benchmarking models for the Water Framework Directive due to the elevated nutrient concentrations and algal biomass production of the lake. One aim of the project was to test the suitability of models for the assessment of management options proposed to meet the surface water quality targets. Additionally, the benchmarking protocol developed to facilitate the dialogue between a modeller and a water manager in a model selection situation was tested. The catchment scale model SWAT was assessed for its applicability to analyse water and nutrient transport in Finnish environmental conditions. The results indicated that SWAT can be calibrated against measured data, especially for discharge, using a short list of key parameters, but further calibration is needed, especially for water quality variables. This result was supported by the attempt to validate using other monitoring points within the catchment since it revealed that the model, in the present setup, cannot reproduce observed catchment dynamics correctly. The model benchmarking guideline proved to give the process of model selection a clear structure and aided communication in a situation where the vocabulary and needs of the different parties were not established.