When heavy rainfall sweeps the countryside, waterways are flooded with peak levels of the nutrient phosphorus, which can trigger algal blooms and create dead zones in rivers, lakes, and coastal waters, says a newly published study. The lead author says the surge of farm runoff from “a really big storm” can overwhelm buffer strips and cover crops — common practices for trapping nutrients — so additional efforts will be needed.
The lead author, Steve Carpenter, director emeritus of the University of Wisconsin’s limnology center, said in a National Science Foundation release that some farmers are having success managing their soil phosphorus, and “and we could learn from them.”
Climate change is forecast to create more intense storms in the Farm Belt. The study of rainfall and phosphorus runoff in two tributaries of Lake Mendota in southern Wisconsin, published in the journal Limnology and Oceanography, looked at two decades of records beginning in the early 1990s. Big rainstorms were followed by big pulses of phosphorus, which is found in manure, fertilizer, sewage, and urban runoff, and also leaches from natural sources.
“Our analysis shows that precipitation is strongly associated with extreme P [phosphorus] load events,” says the study. “Thus, management efforts to reduce P runoff and transport during and after extreme precipitation events are crucially needed to mitigate eutrophication.”
A National Science Foundation official said the study was “an important example of how changes in one aspect of the environment, in this case precipitation, can lead to changes in other aspects, such as phosphorus load.”
Contour farming, filter strips, and cover crops are widely used in agriculture to prevent erosion and slow nutrient runoff. Carpenter said those practices help but that “eventually a really big storm will overwhelm them.” Other researchers have reported that the incidence of heavy rainfall — one, two, or three inches of rain in 24 hours — increased significantly in southern Wisconsin from 1950 to 2006. Cover crops are being promoted as a way to control runoff when fields lie idle during the winter and to improve soil health.
Separately, the Environmental Working Group, which says farmers should be required to protect water quality as a condition for federal subsidies, said its analysis of satellite imagery found that Iowa farmers in the Raccoon River drainage were planting more cover crops but that “it was still less than 2.5 percent of the amount needed to clean up the watershed.” The group also said “similar satellite data showed a net loss in the number of acres of buffer strips along fields that border streams within the watershed.”
The EWG looked at the Raccoon River because of a 2015 lawsuit by the Des Moines Water Works against three counties in northwestern Iowa. The utility sought, unsuccessfully, to hold the counties accountable under federal clean water laws for high nitrate levels in the river, which provides drinking water for Des Moines and its suburbs.