Heavy rain drives one-third of nitrogen runoff, according to a new study

According to a new study by scientists at Iowa State University, heavy rain events that occur only a few days a year can account for up to one-third of the annual nitrogen runoff from farmland in the Mississippi River basin.

Research funded by the US National Science Foundation, published in Earth and environment communications, uses innovative computer modeling techniques to quantify nitrogen runoff from terrestrial ecosystems into rivers and streams. Nitrogen fertilizers applied to Midwestern farmland can make their way along the Mississippi River, where they contribute to a hypoxic (low or depleted oxygen) zone in the Gulf of Mexico.

The findings could inform agricultural management practices such as timing and application of nitrogen fertilizers to reduce these effects, said lead author of the study, Chaoqun Lu.

Lu used computer modeling to simulate various ecological processes. His previous work looked at how farmland stores carbon, a process known as carbon sequestration. For this article, Lu and his colleagues used modeling approaches and publicly available water quality data dating back to the 1980s to analyze how rainfall affects the nitrogen load in Midwestern waterways.

Lu said previous studies have shown that years with high rainfall totals also see higher nitrogen load from the land to water bodies. But the new study took a close look at extreme rainfall events across the Mississippi River basin, defined as any rainfall that exceeds the 90th percentile of historical daily precipitation records for a location in a month.

According to the data, basin-wide extreme precipitation events occurred on average only 8.6 days per year, but contributed to about one-third of total nitrogen annual yields.

“What attracted us to these questions is that heavy rain events can be very localized,” Lu said. “You see rains here and there and they occur at different times of the year. long-term monitoring and modeling tools that take those differences into account and get a better look at how they affect nitrogen loading. “

Laura Lautz, program director of the NSF’s Earth Sciences Division, added: “This research shows that although extreme rainfall events can only occur a few days a year, they can have a huge influence on nitrogen loads. Knowing this, we can take an informed and focused approach to managing agricultural practices to mitigate nutrient dumping in the Gulf. ”