Modeling of a nutrient reduction constructed wetland in an Iowa watershed using hydrological simulation program -- FORTRAN

The Gulf of Mexico dead zone — a large area with low dissolved oxygen levels that kills marine life like fish, mussels, and crabs — is caused each summer in part from the nutrients that runoff from farmlands and enter the gulf. Iowa’s agricultural landscape, where nitrogen fertilizer and manure is applied to grow crops, is a significant contributor of the nutrient nitrate to the Gulf of Mexico. Building artificial wetlands is one way to reduce nitrate concentrations in streams and rivers downstream. We used a computer model to predict the effect of constructed wetlands within an Iowa watershed. First we first simulated conditions for an existing wetland. Using three years of data collected at the wetland’s inlet and outlet, we found model parameters that yielded the best simulation of water temperatures, dissolved oxygen, and nitrate leaving the wetland. Next, we used these model parameters to simulate the effect of building thirty-nine potential wetlands identified within the Headwaters North English, a fifty-six square mile watershed in east-central Iowa. The results shown that nitrate is reduced by 36 to 94% at the wetlands themselves. Further downstream, the model predicts the nitrate loads leaving the watershed is reduced by about 12%. Although nutrient reduction wetlands can significantly help reduce nitrate loads in Iowa rivers and streams, additional practices will still be needed to achieve the Iowa Nutrient Reduction Strategy’s goal of a 45% reduction in nitrate.