2016 Nitrate Summary (Corrected) for Raccoon and Des Moines
Rivers (at Des Moines)
1.7 billion pounds of nitrate (left) has met 2.4 billion pounds of nitrate (right) since 1974.
I first posted this last week, and made some mistakes in process. I used English Tons
units for the load instead of my tradtional metric tons. Thus the 2016 loading numbers
were about 10% higher than what they should have been. This made 2016 at the Des
Moines River (at 2nd Ave in Des Moines) the second highest loading year ever (not the
highest as reported earlier). The rest of the post remains essentially the same. I have
inserted corrected graphs.
The past four years that have elapsed since the 2012 drought have all been pretty
remarkable for nitrate levels (with the possible exception of 2014 Des Moines River).
2013-2016 was the largest four-year N loading in the historical record for both rivers. In
addition, the ten years that have elapsed since 2007 is similarly the highest 10-year
loading period in the record. Graphs below illustrate this.
Total river discharge (flow) was 7th-highest in 43 years for the Raccoon; 4th-highest in
35 years for the Des Moines. In the graph below I show discharge normalized to
watershed area. In an average year, the Raccoon River watershed gets 866 mm of
precipitation and 24% of that (208 mm) leaves the watershed in the river. This ratio of
discharge (Q) to precipitation (P) is a strong predictor of river nitrate. Large values for
Q/P almost always mean large nitrate loadings. The past two years, Q/P has averaged
0.38 in the Raccoon Watershed, well above the 0.24 average.
I began working on paper about three years ago that showed a modest decline in
Raccoon River nitrate concentrations from 1999-2014. The paper finally got to print in
the Journal of Soil and Water Conservation in early 2016. By that time, it was apparent
that nitrate levels were on their way back up. This shows the peril associated with
talking about water quality trends. Looking backward on water quality does not enable a
person to predict the future.
1,678,464,652 pounds of nitrogen have been transported by the Raccoon River to its
confluence with the Des Moines River in downtown Des Moines since 1974. For the Des
Moines River, the figure is 2,385,314,891 pounds since 1982, for a combined total of
4,063,779,543 pounds, enough for a one-year fertilization of 27 million corn acres.