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Tuesday, January 26 • 4:20pm - 4:40pm
Development of a Water Quality Index For Lakes and Reservoirs in Iowa

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AUTHORS: Rebecca M. Krogman*, Iowa Department of Natural Resources; Michelle Balmer, Iowa Department of Natural Resources

ABSTRACT: A water quality index was developed for Iowa’s lakes and reservoirs using ten years of monitoring data from the Iowa Department of Natural Resources’ Lake Monitoring Program. The index ranged between 0 and 100 and was applied to over 130 significant, publicly-owned lakes and reservoirs to rank them by current water quality and to track historical trends in water quality at each location. The index was developed using stepwise logistic regression with water quality parameters from monitoring as predictor variables and water quality ladder score as the response variable. The water quality ladder is a commonly used tool which ties water quality perceptions to tangible aquatic uses; ladder scores had been collected during monitoring since 2004 and could be directly matched with chemical and physical water quality parameter observations. The resulting logistic model included mean depth, Secchi depth, turbidity, total phosphorus concentration, zooplankton density, and an indicator of stratification. Water quality index scores were normally distributed around a mean score of 58.6, with a range of 32.0 to 81.8. The index showed significant relationships with several water quality variables—including chlorophyll a concentration, total nitrogen concentration, and trophic state index—and with some morphometric variables—including mean depth along the longest fetch line. Surprisingly, index scores did not correlate with watershed area:surface area ratio or erosion variables. The index successfully indicated renovations and watershed improvements in several important public waters, but not in others for reasons which were explored. The sensitivity of the index to variance in predictor variables was investigated, and sample sizes for detecting change were modeled.

Tuesday January 26, 2016 4:20pm - 4:40pm EST