A GIS based approach identifying phosphorus sources within the lake Flaten catchment in Salem, Sweden

Abstract: The purpose of this study has been to test if GIS based methods calibrated with limited local data can be used to determine the origin and quantity of phosphorus transport to and within a small eutrophic lake, and to assess the effects of this transport on the lake water quality. The lake studied is Lake Flaten in Salem municipality, Sweden. The objectives have been achieved by delineating catchment areas from digital elevation data, classifying land cover by manual editing and based on spectral characteristics, and identifying leach from point sources. The geographical data has been complemented with attributes, measured and interpolated, on monthly and annual water flow, lake water phosphorus content, assessed mass transport and estimation on internal load from sediments. Phosphorus load per catchment and subcatchment was quantified by assessing export coefficients per land cover class. Finally future scenarios like changed water flow and urban development as well as some measures of mitigation were simulated. A comparison of different land cover data, both vector and raster based, show that manually editing data is not necessary, available land cover data is sufficient for estimating phosphorus transport even from small research areas. Moreover a couple of raster-based methods presented in this study might be useful for fast overall estimations on nutrient transport and sources. Results on TP transport and lake water quality show that Lake Flaten is in a constant eutrophic state with high in-water phosphorus levels. Phosphorus load to the lake mainly origin from urban areas, and the subcatchments with the highest total and area specific contribution have been identified. The substantial internal load from lake sediments must cease, and actions be taken to reduce the external load, if the lake is to meet guidelines stipulated by the Swedish Environmental Protection Agency (the Swedish EPA). While the GIS based approach is a feasible methodology for overall estimations on phosphorus transport and identifying the origin of phosphorus load, further measurement data than available for this study would make it possible to improve the accuracy of the predictions.