Three-dimensional eutrophication model for Lake Biwa and its application to the framework design of transferable discharge permits

The main goal of this study is to evaluate the eutrophication status under different management approaches in a basin, by combining the Biwa3D model (three‐dimensional eutrophication analysis model) with several nutrient‐reduction cases to calculate their effects on the water quality in Lake Biwa, Japan, and by applying the model to evaluate the cost‐effectiveness between different scenarios designed to control eutrophication. A non‐hydrostatic hydrodynamic model featuring very large eddy simulation (VLES) concepts, combined with ecological components that consider three types of phytoplankton species, one zooplankton, and nutrient cycling in each grid, is developed as the basic component of the research. Nitrogen and phosphorus loads from 111 sub‐basins that supply Lake Biwa are set as the boundary condition for the numerical simulation. The chlorophyll a concentration calculated near the water intake, weighted in proportion to the intake discharge, is set as the water quality index. Transferable discharge permits are introduced for estimating the total reduction cost for achieving the objective water quality. Two types of scenario are tested: one considers the difference in location of nutrient reduction in the north and south basins, and the other ignores this difference. Initially, the effect of nutrient loads on the water quality of Lake Biwa was calculated using the Biwa3D model, using estimated nitrogen and phosphorus loads from the basin as the boundary condition. Transferable discharge permits were then designed according to the impact factor on each basin calculated by the model in order to compare the cost‐effectiveness of the reduction scenarios. It is concluded that consideration of the characteristics of each basin and the distribution of effluent in designing transferable discharge permits reduces the total costs by around 4–25% of that required for achieving the target concentration of chlorophyll a. Copyright © 2003 John Wiley & Sons, Ltd.