Iowa Is Getting Sucked into Scary Vanishing Gullies

A challenge for Iowa is the largely invisible crisis of topsoil, which appears to be eroding at a much higher rate than US Department of Agriculture numbers indicate—and, more importantly, at up to 16 times the natural soil replacement rate.

That disturbing assessment comes from Richard Cruse, an agronomist and the director of Iowa State University's Iowa Water Center. Cruse's on-the-ground research has shown a particular kind of soil erosion clearly connected to last year's heavy rains.  Cruse told me that with current methods, the USDA measures a kind of soil loss called "sheet and rill erosion," wherein water washes soil away in small channels that form at the soil surface during rains.  Under that measure, Iowa farmland loses an average 5.2 tons of topsoil per acre every year, according to the USDA's latest numbers, which are from 2007.

The USDA sees five tons per acre as a "magic number," Cruse said, because it's generally accepted to be the rate at which soil renews itself.  So the prevailing view has been that "if we can limit erosion to five tons per acre, we can do this forever," Cruse said. But he added that the "best science" indicates that the real sustainable erosion rate is closer to a half ton per acre—meaning that even by the USDA's own limited measure, Iowa's soils are eroding much faster than they can be replaced naturally.

But here's where we get to the scary part.  Using stereo-photographic techniques, Cruse and his team have been measuring a different form of erosion that occurs through what are known as "ephemeral gullies"—that is, large gashes in farm fields formed by water during heavy rains, bearing soil rapidly away and dispersing it into streams and rivers. This kind of erosion is not included in conventional soil-loss measures, and as a result, the USDA is "way underestimating" erosion in Iowa, he said.

Iowa Is Getting Sucked into Scary Vanishing Gullies