Ecosystem-specific water quality indices
The water quality index (WQI) has emerged as a central tool for analysing and reporting quality trends since 1965. It provides a better overview of water quality variability in a catchment than conventional monitoring programmes that use individual variables. Since water quality is not static, due to point and non-point pollution sources, water managers require tools that are easily adaptable to changing trends. For aquatic environments, different WQIs have been developed to classify specific areas and to determine the fitness of various water resources for specific uses such as drinking. The development of indices poses the challenge of standardising methods for selecting input variables, data transformation and aggregation. Inappropriate input variables may lead to a wrong evaluation of the overall water quality status, possibly resulting in the use of polluted water. This paper reviews methods and aspects to consider when developing ecosystem-specific WQIs – their strengths, limitations and the suitability of the methodologies. These could be applied when developing a water quality index for the uMngeni Basin, KwaZulu-Natal, South Africa, where ecosystem-specific modelling is being done to enhance basin management.
Keywords: aggregation, fitness-for-use, pollution, water quality index, weight factor