Appropriate Modelling Complexity: An Application to Mass Balance Modelling of Lake Vänern, Sweden

This work is about finding an appropriate modelling complexity for a mass-balance model for phosphorus in Lake Vänern, Sweden. A statistical analysis of 30 years of water quality data shows that epilimnion and hypolimnion have different water quality and should be treated separately in a model. Further vertical division is not motivated. Horizontally, the lake should be divided into the two main basins Värmlandssjön and Dalbosjön. Shallow near shore ares, bays and areas close to point sources have to be considered as specific sub-basins if they are to be modelled correctly. These results leads to the use of a model based on ordinary differential equations. The model applied is named LEEDS (Lake Eutrophication Effect Dose Sensitivity) and considers phosphorus and suspended particles. Several modifications were made for the application of the model to Lake Vänern. The two major ones are a revision of the equations governing the outflow of phosphorus and suspended particle through the outflow river, and the inclusion of chemical oxygen demand (COD) into the model, in order to model emissions from pulp and paper mills. The model has also been modified to handle several sub-basins. The LEEDS model has been compared to three other eutrophication models applied to Lake Vänern. Two were simple models developed as parts of catchment area models and the third was a lake model with higher resolution than the LEEDS model. The models showed a good fit to calibration and validation data, and were compared in two nutrient emission scenarios and a scenario with increased temperature, corresponding to the green house effect