Development of a water quality index model for lakes and reservoirs

Lake water quality and trophic state are evaluated using various parameters which may have different interpretations. Therefore, it is useful to adopt a proxy index that shows normalized values of parameters having different units and distribution characteristics. In this study, a model for integrated water quality index was developed for lakes and reservoirs in Korea. Water quality and phytoplankton were examined in 36 lakes, two natural lakes and 34 artificial lakes. The study lakes were selected to represent the range geographic regions and lake morphology in Korea. After investigating the interrelationships among water quality parameters, four parameters (total organic carbon, chlorophyll-a, total phosphorus, and turbidity) were selected as surrogate indicators of overall water quality. A relative evaluation system was developed by adopting a logistic function index that describes a cumulative distribution function and reflects the relative position of each parameter among the study lakes. The cumulative distribution probability ranging from 0 to 1 was multiplied by 100 and then transformed into the Korean Lake Water Quality Index (LQI) ranging from 0 to 100. A score of 50 was assigned to the median value of the dataset, 0 to the highest concentration value and 100 to the lowest concentration value. Thus, the LQI is an integrated easy-to-understand index that provides information about the relative status of each lake. The results of this study can represent a model to provide a relative evaluation system for lake and reservoir water quality, which can be useful for ecosystem management within an ecoregion or a jurisdictional district.