Note: the original post was edited 2/27/16 to reflect updated data:

During 2016, IIHR Hydroscience and Engineering along with USGS had 14 real-time nitrate sensors deployed in locations that captured water quality conditions for 82.5% of the water leaving the state of Iowa. From this measured data (average of 217 days per site), I calculated the daily N load for each site and extrapolated that to 366 days, and then extrapolated the total to 100% of the state’s area. This amount is 1.04 billion pounds, the total amount of nitrate-N leaving the state in 2016. This amounts to an overall statewide N loss of about 29 pounds per acre.

The yearlong N loading for the 14 watersheds is shown in the graph below.

The Des Moines/Raccoon and Iowa/Cedar basins account for more than half of the statewide total, but remember this is mainly because they are the two largest of the 14 basins evaluated here. The Floyd River actually had the highest overall N loss on a per acre basis.