Iowa Statewide Stream Nitrate Loading: 2017-2018 Update

Abstract

In response to ongoing hypoxia in the Gulf of Mexico, several states in the Mississippi River basin have adopted nutrient reduction plans in recent years designed to arrest the flow of nitrogen (N) and phosphorus (P) from both point and non-point sources to the stream network. Iowa's Nutrient Reduction Strategy, implemented in 2012, aims to reduce stream loading of these nutrients by 45% within a yet-to-be-defined time frame. Because the state has chosen to integrate accountability into the strategy through the numerical objective, ongoing water monitoring is necessary to credibly measure progress. The primary objective of this research was to use water quality monitoring and discharge data to update statewide nitrate-nitrogen (NO3-N) loading using the combined data sets generated by in situ water quality sensors and traditional grab sample monitoring conducted by state government. Our research shows that the 5-year running annual average of nitrate-nitrogen loading continues to increase, and after the 2018 water year is 73% higher than that calculated in 2003. Loads from Iowa areas draining to the Missouri River are increasing more rapidly than loads from areas draining to the upper Mississippi River: 132% versus 55% since 2003. This shows that best management practices designed to stem the loss of nutrients from the corn-soybean system must be widely adopted and robustly designed for extreme environmental conditions if Iowa is to meet its water quality objectives.