In April 2010, USDA’s Natural Resources Conservation Service (NRCS) released data estimating the rate of soil erosion on agricultural land in the United States.1 On the surface, the data from the 2007 National Resources Inventory (NRI) were reassuring. Erosion in Iowa averaged 5.2 tons per acre per year, only slightly higher than the allegedly “sustainable” rate of 5 tons per acre per year. Across the entire Corn Belt, erosion averaged only 3.9 tons per acre per year.

Scientists tracking soil erosion in Iowa, however, are producing compelling evidence that soil erosion and runoff from cropland are far worse than these estimates suggest. These new data count the amount of soil lost after each storm that hits the state and produce a far more detailed picture of the toll that erosion takes on soil and water. These numbers are far more informative than the superficially reassuring national, regional or statewide averages of soil erosion. There is every reason to expect that applying the Iowa project’s methods to other states would reveal the same disturbing picture.

Averaging soil erosion over states, regions or the nation obscures more than it reveals because erosion and polluted runoff do not occur “on average.” They occur when it rains. How much rain falls, how fast it falls, how wet the soil was before it started raining, how steeply the field slopes, how prone the field is to forming gullies, how much the soil is covered by a growing crop or crop residues, how well the field and adjacent streams are protected by conservation practices — all these factors determine how much damage is done by any one storm.

The new, more precise estimates of erosion — corroborated by EWG with aerial surveys that produced striking visual evidence of the damage — make it clear that farmers and policy makers must do much more to protect agricultural land from this old enemy. Indeed, it appears that the nation is losing ground in the decades-old fight to gain control over this most fundamental and damaging environmental problem in agriculture.

Events versus Averages

The Iowa Daily Erosion Project (IDEP), led by Iowa State University scientists working with a long list of partners, is producing compelling evidence that soil erosion is far worse than the National Resources Inventory estimates. Combining information on daily rainfall amounts, soil type, slope, crop rotation and conservation practices, the project is able to generate — for the first time ever — detailed estimates of erosion caused by each individual storm that hits Iowa over the course of a year.2 The scientists then estimate minimum, average and maximum rates of soil erosion and runoff that likely occurred on agricultural land (cropland, pasture, hay fields) in each of 1,570 Iowa townships after each storm (see sidebar).

The project has built the most significant new tool for measuring soil erosion since the NRI was developed in the 1980s. The Inventory, which set up a system of collecting data on wind, water, soil type and land use changes at some 800,000 points across the country, revealed for the first time that some farmed areas were eroding far more rapidly than others. That allowed policymakers and program managers to target conservation efforts on the most sensitive areas and led, for a time, to significant progress in slowing erosion rates.

The new techniques developed at Iowa State are an advance of similar importance, yielding dramatic new insights into the pace and location of severe erosion events. This far more precise information shows that larger storms can cause serious damage that is largely obscured by the long-term, statewide averages generated by the NRI. These new data produce a far more detailed picture of the toll erosion takes on soil and water — down to the level of individual townships — and make clear that farmers and policymakers must do much more to protect agricultural land from this old enemy.

Townships are key components of the Public Land Survey System (PLSS), which is regulated by the U.S. Department of Interior’s Bureau of Land Management.3 Each township is a square six miles long by six miles wide, made up of 36 smaller squares called sections that are each one square mile in size. A section encompasses 640 acres and a township encompasses 23,040 acres. (An acre is about the size of a football field.) The Daily Erosion Project does not report how many acres in each township are farmed, but agriculture dominates the Iowa landscape. In 2002, 88 percent of Iowa’s land was in cropland, pasture or hay. Trees or water covered 9 percent, and 3 percent was urban. The project does not report estimates of soil erosion and runoff from predominantly urban townships.4

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