The researchers studied water quality across nearly 50 watersheds in Wisconsin from 2008 to 2020. They correlated ammonia and phosphorus concentration data from the Water Quality Portal with the location of livestock farms and crop acreages, and they determined nutrient levels after ½ inch, 1 inch, and 2 inches of rainfall.
They found spikes in nutrient concentrations immediately after extreme precipitation events, and the effect increased with the amount of precipitation. For example, within five days of an inch of precipitation, ammonia was 49% higher and phosphorus was 24% higher. If there was at least one day in a month with over an inch of precipitation, monthly ammonia was 28% higher and monthly phosphorus was 15% higher.
“We observe a significant interaction between rainfall, agricultural production, and runoff. It is not just a short-term spike in nutrient levels; at the end of the season, we still see persistent increases in phosphorus and ammonia attributed to those extreme precipitation events months earlier,” Skidmore stated.
However, the researchers found that agricultural management practices can help mitigate the effects.
“Our results show that cover crops planted in the winter can lower the amount of nutrients in the water. Areas with cover crops have significantly lower spikes in ammonia and phosphorus, and the effect persists until the end of the growing season. We already know cover crops are great for soils and nutrient management, but this is additional empirical evidence showing that cover crops are climate-smart practices that can help agriculture be resilient into the future,” Skidmore said.
The researchers also observed the presence of legacy nutrients, which are left behind from agricultural practices decades or even centuries ago.
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