Linking Flow and Load Duration Curves for the Wapello Gage Station-Iowa River, Iowa

The fate and transport of nutrients in agriculturally dominated watersheds are often closely linked with flow conditions. Understanding nutrient loads over the range of flow conditions are critical to modeling efforts and in predicting loads delivered to the Mississippi River with their potential impacts on the Gulf Hypoxia. To determine the concentrations and loads of selected nutrients and their variability in different flow conditions, water-quality and flow data at the Wapello Gage Station of the Iowa River, Iowa, were analyzed. The Wapello site integrates nearly 60% of all watersheds in Eastern Iowa before flowing into the Mississippi River. The relationship between the nitrogen loads (nitrate plus nitrite (NOx-N), ammonia, total nitrogen (TN) and total dissolved nitrogen (TDN)) and their concentrations were described. In particular, nitrogen loads during high flows were compared with those in mid-range flows and low flows. Using daily stream flow data at the Wapello Gage Station (05465500) from Aug.1 1999 to Aug. 1 2009, produced a flow duration curve with high flows at the 10 percentile exceeded of 23, 400 cubic feet per second (cfs), and the mid-range flows in the 40 to 80th percentiles of flow with 7,200 and 4,390 cfs respectively. Based on the monthly water quality data collected by the US Geological Survey at the Wapello Gage Station, and using the contaminant load equation, load (kg per day) = streamflow (cfs)times concentration of contaminants (mg per litter) times 2.056, the average monthly nitrogen (nitrate plus nitrite, NOx-N) load and the nitrate standard MCL (maximum contaminant level) load (at the value of 10.0 mg/L) were calculated. Similarly, ammonia, total nitrogen (TN) and total dissolved nitrogen (TDN) loads were calculated. In general, the monthly NOx-N loads were lower than the nitrate standard MCL loads during the period of Aug.1999 to July 2008. However, approximately 31 percent of the instantaneous loads using daily water quality concentrations and daily flows exceeded the nitrate 10 mg/L standard load. Results showed that instantaneous NOx-N loads during dry condition of flow (60 to 80th percentiles on the flow duration curve) are usually greater than the nitrate standard MCL load curve. Ammonia loads were significantly lower than that of the total nitrogen. And this difference is less during high flows (days flow exceeded at 0-10% range). Additionally, during the mid-range and dry flow conditions (at the range of 50-80%), the different values between TN and TDN were much larger than at other times, which indicates the dissolved nitrogen percentile to total nitrogen in water is comparatively less when the stream flow is lower.