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ARS Home » Pacific West Area » Corvallis, Oregon » Forage Seed and Cereal Research Unit » Research » Publications at this Location » Publication #260887
Title: Application of index number theory to the construction of a water quality index: aggregated nutrient loadings related to the areal extent of hypoxia in the northern Gulf of Mexico

Author
item Whittaker, Gerald
item Barnhart, Bradley
item FARE, ROLF - Oregon State University
item GROSSKOPF, SHAWNA - Oregon State University

Submitted to: Ecological Indicators
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2014
Publication Date: 2/28/2015
Citation: Whittaker, G.W., Barnhart, B.L., Fare, R., Grosskopf, S. 2015. Application of index number theory to the construction of a water quality index: aggregated nutrient loadings related to the areal extent of hypoxia in the northern Gulf of Mexico. Ecological Indicators. 49:162-168.

Interpretive Summary: The USGS identifies and measures six nutrients transported to the Gulf of Mexico by the Mississippi River. These nutrients are associated with growth of hypoxia, which produces a "dead zone". In the discussion of policy to remediate Gulf hypoxia, it is important to stake-holders and policy makers to have a single measure that characterizes the information from water pollutants. An index of water quality is commonly constructed by weighting each variable (defined by expert opinion or statistical characteristics), then summing the variables to form a single number. In this study, we apply and validate a method that derives the weight for each constituent from the observed data. To validate the Malmquist water quality index proposed here, we compared the index with simulated values of the area of Gulf hypoxia resulting from nutrients transported by the Mississippi River. The Malmquist water quality index indicated good water quality when the hypoxia area was small, and poor water quality with increasing hypoxia area. This result supports the hypothesis that the Malmquist index represents the fundamental relationship among the constituents of water quality.

Technical Abstract: The development of an index for description and monitoring of surface water quality has received significant attention in the water resources literature in recent years, primarily because of the increasing need for assessing water quality and the complex, multidimensional data collected from water quality monitoring. The study of index numbers is relatively mature, and properties that are required for construction of a useful index number are well known. The USGS identifies and measures six nutrients transported to the Gulf of Mexico by the Mississippi River. These nutrients are associated with growth of hypoxia, which produces a "dead zone". In this study, we apply and validate a method that derives the weight for each constituent from the observed data. A water quality index with endogenous weights requires no expertise to apply, and is reproducible. Because it is based on a Malmquist index, it also has the theoretical properties necessary for construction of a useful index. To validate the Malmquist water quality index proposed here, we compared the index with simulated values of the area of Gulf hypoxia resulting from nutrients transported by the Mississippi River. The Malmquist water quality index indicated good water quality when the hypoxia area was small, and poor water quality with increasing hypoxia area. This result supports the hypothesis that the Malmquist index endogenously assigns weights to the variables used in the index in a way that captures the fundamental structure of the relationships among the constituents of water quality.