Spatial and temporal nitrate-nitrogen patterns in rivers across Iowa

Excess nutrients from agricultural land are often transported to streams and rivers. High levels of nutrients in rivers such as the Mississippi River are the primary cause of hypoxic conditions in regional water bodies, including the Gulf of Mexico. Nitrate concentration and discharge data were evaluated for 62 water monitoring sites across Iowa. Twenty-three of the 62 sites were considered main watershed areas that capture drainage to the Mississippi or Missouri river basins. Monthly data was obtained through the Iowa Department of Natural Resources (DNR) ambient water quality database. The monthly samples were then linearly interpolated to obtain daily nitrate concentration estimates. Daily nitrate loads were calculated at co-located stations and then summed to estimate annual nitrate loads. The annual nitrate loads for the main 23 sites were used to calculate an annual statewide nitrate load. Linear regression modeling was used to analyze data at a spatial scale to understand what variables impact the differences in average nutrient yields between sites. Percent of land cultivated as row crop and annual streamflow water yield were the main predictors of spatial nitrate yields. Regression modeling was also used to fit data to identify what variables could be used to predict temporal annual nitrate loads. Streamflow water yield was the main predictor of temporal nitrate loads. Regional equations were developed to predict temporal equation parameters (slope and intercept). These regional equations could be used to estimate annual or long-term nitrate loads and yields. DNR ambient nitrate loads were also compared to loads from a high-resolution water quality sensor network. Nitrate loads between the two networks can vary significantly during the summer. Nutrient load reduction could be more confidently determined with improved statewide load export estimations.