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Understanding What Happens When Rain Meets Field

Spring rains may make flowers–and crops–grow, but when stormwater runs off fields it can carry topsoil, chemicals and nutrients such as phosphorus into rivers and streams. This spring, a University of Wisconsin-Madison scientist and his team will begin tests to understand the mechanisms of erosion by tracking sediment movement as well as the flow of water over fields.

“We’re very excited about this project because we will be using novel techniques to study an issue that is very important, from both an agricultural and an environmental standpoint,” says K. G. Karthikeyan, an assistant professor of biological systems engineering in the university’s College of Agricultural and Life Sciences.

The balance between promoting agriculture and preserving the environment can be difficult to find–especially when it comes to managing problems associated with stormwater runoff. When rain hits a field some of it filters down through the soil, nourishing crops and replenishing groundwater supplies. However, water can also flow off fields, stripping topsoil and carrying chemicals, including phosphorus from manure applications. When runoff enters watersheds it can cause larger environmental problems–such as the recent fish kills in a Dane County trout stream.

Karthikeyan has spent years studying stormwater runoff–everything from lab experiments in concentrating phosphorus by treating manure to field tests of how harvesting techniques affect erosion–and has just begun a three-year project with colleagues at Case Western University. The team, which includes Karthikeyan’s postdoctoral fellow Paul Miller, will use natural radioisotopes, soils labeled with rare earth elements, and surface runoff sensors to understand the mechanisms of erosion.

“The atmosphere contains natural radioisotopes and radioisotopes that were generated by human activity–including isotopes of beryllium, cesium and lead–that fall to earth with rain and mark soil with specific fingerprints,” which scientists can use to quantify soil loss and to trace sediment movement, Karthikeyan explains.

“We’re going to examine soil cores before and after rainfall and also sediments that collect at the edge of agricultural fields to determine the relative ratios of isotopes present, which will help us to discriminate erosion mechanisms.”

The team will also install surface runoff sensors so they can map exactly where water moves on the field, and will label patches of soil with “rare earth” elements–or elements that do not naturally occur in soil at high levels-to track how far the soil moves after rainfall.

“This work has major implications for water quality, as well as helping us to understand exactly how erosion works,” Karthikeyan says. “Different mechanisms of erosion can have different effects on phosphorus transport, which is a critical area of interest.”

The research will take place at the College’s Arlington Agricultural Research Station, located just outside of Madison. Karthikeyan calls Arlington crucial to his work. “We cannot do this without them,” he says. “It is a great resource for us.”

Karthikeyan says that he hopes to have some preliminary findings by the end of the year. His project is sponsored by the United States Department of Agriculture’s National Research Initiative Program and by the state of Wisconsin.