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Revisiting Nitrate Concentrations in the Des Moines River

1945 and 1976–2001

^a Department of Natural Resources and Environmental Sciences, University of Illinois, W-503 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL 61801
^b Iowa Department of Natural Resources, Geological Survey Division, Iowa City, IA 52242

View larger version (29K): [in a new window]   Fig. 1. The Des Moines River drainage basin and water quality and quantity monitoring locations used in this study.

View larger version (16K): [in a new window]   Fig. 2. Composite NO3–N concentration as a function of average daily discharge during composite sample collection period in 1945 for the Des Moines River at Des Moines below the confluence with the Raccoon River (r = model residual from previous sample).

View larger version (19K): [in a new window]   Fig. 3. Measured NO3–N concentration in noncomposited samples as a function of concentrations estimated from the concentration–discharge relationship developed from composited samples.

View larger version (20K): [in a new window]   Fig. 4. Annual water yield (water year basis) from the Des Moines River basin below the confluence with the Raccoon River (United States Geological Survey Station 5485500).

View larger version (26K): [in a new window]   Fig. 5. Water year arithmetic average NO3–N concentrations as a function of the natural logarithm of April through September water yield (mm yr-1). The open circles are the 1982–2001 observed annual values, with error bars representing the 95% confidence limits for individual observations based on regression analysis that incorporates autocorrelation of model residuals (r = model residual from previous year); the curved line is the expected mean value estimated from the regression analysis; the solid triangle is the expected mean at the 1945 water yield with 95% confidence limits; and the solid square is the observed 1945 value with an error bar representing the maximum estimated influence (+19%) of compositing samples over approximately 10 d.

View larger version (26K): [in a new window]   Fig. 6. Flow-weighted average annual NO3–N concentration as a function of the natural logarithm of the April through September water yield (mm yr-1). The open circles are the observed values for the 1982–2001 water years, with error bars representing the 95% confidence limits for individual observations based on regression analysis that incorporates autocorrelation of model residuals (r = model residual from previous year); the curved line is the expected mean value estimated from the regression analysis; the solid triangle is the expected mean value at the 1945 water yield with 95% confidence limits; and the solid square is the 1945 value with an error bar representing the maximum estimated effect (+19%) of compositing samples over approximately 10 d.