Improved Method for Aggregation of Water Quality Subindices

Alternative methods to describe water quality using an aggregate index consisting of subindices for individual water quality variables are examined. Most aggregation methods suffer from three shortcomings: Ambiguity, eclipsing, and rigidity. Ambiguity problems exist when all the subindices indicate acceptable water quality for a given use, but the aggregated index does not. Eclipsing problems exist when the aggregated index fails to reflect poor water quality of one or more water quality variables. Rigidity problems exist when additional variables are included in the index to address specific water quality concerns, but the faulty aggregation function might artificially reduce the value of the water quality index such that it does not accurately reflect the true water quality. As the number of water quality variables increases, the magnitude of the aggregated index decreases raising the issue of ambiguity again. The writers developed a mathematical formulation for aggregate indices that avoids the problems of ambiguity, eclipsing, and rigidity with respect to the number of water quality variables required to be aggregated in a given index.