The Influence of Watershed Development on the Eutrophication of Shallow Lakes: A Modeling Approach Applied to Lake Balaton

A multiregional, multicriteria watershed development model is presented as a way of describing the eutrophication processes in shallow lakes on an annual basis. Its application to Lake Balaton is illustrated. The simulation and forecasting of eutrophication in the lake, which depend on different watershed development activities, is discussed for the period 1981-85. \n\nThe formulation of the model rests on the fact that a close connection exists between the eutrophication of a shallow lake and the development of its watersheds which generate nutrient loading. A multicriteria utility function is used to describe watershed development. The applications of the model to Lake Balaton are presented; they are based on the regionalization of the Balaton basin into eleven regional units (four waterbodies and seven watersheds). \n\nWatershed development needs are assessed for the various watersheds of the Balaton basin, for the period 1930-1980. The eutrophication indicated by a moving summer average of chlorophyll-a for each of the four basins of the lake depends on the level of watershed development in the corresponding or contributing watershed, their geometric character and the eutrophication in the preceding basin. The spatial distribution of the water quality within basins is not considered. The model parameters are elaborated by regression analysis. \n\nThe simulation of eutrophication is based on three development variations for each watershed for 1981-85, namely a strong, a medium and a zero growth variation. Using different combinations of the specific watershed variations, more than 30 spatial lake protection strategies and their effects on eutrophication were simulated on an annual time scale. The average uncertainty of the simulated eutrophication values for the whole lake is plus/minus 23%. With the help of this simulation, the spatial efficiency of the comprehensive control actions in the watershed has been measured. The forecast of eutrophication by 1985 is given by the most realistic watershed development strategy. \n\nOne of the more important recommendations is that the most efficient development management, i.e., control strategy from the viewpoint of the whole lake, is the very one which concentrates all of the limited, available control efforts in the next five years on the Zala watershed, which is the largest source of nutrient loading. This suggestion is based on the relative efficiency figures of the protection measures taken for the various watersheds and calculated by the model. To illustrate the use of the model for evaluating the effectiveness of different management actions and lake protection policies, the possible effects of the Kis-Balaton reservoir on eutrophication are identified. The model can be combined with other models dealing with the Lake Balaton problem and applied to the eutrophication problem of other shallow lakes and man-made storage reservoirs.