Cropland that sheds soil also sheds a great deal of water, often polluted with fertilizers, manure and pesticides. The volume of polluted water running off agricultural land varies dramatically depending on the time of year. In 2009 — the wettest year between 2002 and 2010 — runoff from agricultural land in 1,502 townships (encompassing 34.6 million acres, essentially the entire state) exceeded 136,000 gallons per acre. Runoff in 956 townships (22.0 million acres) exceeded 271,000 gallons per acre, and in 141 townships (3.2 million acres) it exceeded 543,000 gallons per acre. Total runoff was greater in 2009 and 2007 than in 2008, the year of devastating floods in eastern Iowa, which illustrates the importance of the timing and location of storms. In 2006, a dry year, runoff from agricultural land never exceeded 136,000 gallons.

Over time, almost all agricultural land sheds large quantities of runoff, just as it suffers large amounts of erosion. Between 2002 and 2010, for example, runoff from agricultural land in 1,393 townships (32 million acres) exceeded 543,000 gallons per acre; runoff exceeded 1 million gallons per acre in townships encompassing 9.4 million acres.

Perfect Storms

Storms that cause soil erosion and polluted runoff far above average are frequent events in Iowa and are becoming more so all across the Corn Belt.6

A severe storm over poorly protected soil can cause permanent and irreversible damage in a single day — or even a few hours. Spring is the most dangerous season for soil erosion and runoff. The danger is greater if melting snow has saturated the soil, which means more of the rain runs off rather than soaking in. No crops are growing to stabilize the soil and take up water. Unless good soil conservation practices are in place, spring storms can, and often do, result in heavy runoff accompanied by severe soil erosion.

Over three days in 2007 (May 5-7), such a storm passed over large portions of southwest Iowa. According to IDEP, average erosion exceeded “sustainable” rates in 198 townships (4.6 million acres). On May 6, the worst day, 182 townships encompassing 4.2 million acres suffered erosion exceeding the “sustainable” rate for an entire year. In 69 townships (1.6 million acres), soil eroded at twice the “sustainable” rate, an average of 10 tons per acre. In 14 townships (323,000 acres), the rate was more than 20 tons per acre.

The estimates of soil erosion averaged over a township can obscure much more extreme damage to the most vulnerable cropland. The maximum rates of erosion reported by IDEP occur on the single most vulnerable and poorly protected crop field represented by an NRI sample point in the township. The same storm will cause far less erosion on a pasture or a hayfield than on cropland because the grass cover provides much more protection than crop residue. Steeply sloping cropland, unprotected by good conservation practices, will erode terribly. And the amount of polluted runoff will be much greater.

Figure 4: In early May 2007, a single storm eroded up to 100 tons of soil per acre.

The worst-case scenario painted by IDEP for the May 5-7, 2007, storm is truly sobering (Figure 4). In just three days, the single most vulnerable and poorly protected agricultural field in each of 665 townships (encompassing 15.3 million acres) may have eroded at rates that exceeded the estimated annual T value. In 446 townships (encompassing 10.3 million acres), that much erosion occurred on a single day — May 6. On that day, the single most vulnerable agricultural field in each of 230 townships (encompassing 5.3 million acres) may have eroded at rates above 20 tons per acre. The single most vulnerable and poorly protected field in 10 townships (encompassing 230,000 acres) may have eroded at a catastrophic rate of 100 tons per acre in a single day.

IDEP statistical analyses cannot determine how representative the single most vulnerable and unprotected field is of other fields in each township. It is possible that there are no other fields that are as vulnerable and poorly protected. It is also possible that there are many. What the Project’s results do tell us is that a single storm can cause catastrophic damage on any poorly protected field.

Gullies Ignored

Soil erosion and runoff is actually worse — likely far worse — than even the alarming IDEP estimates because the currently available models cannot account for the erosion caused by ephemeral gullies. Such gullies are called “ephemeral” because tillage temporarily obliterates them, but they quickly reappear when the next storm occurs.

Surprisingly little research or monitoring has been done to determine how much erosion occurs in ephemeral gullies. A 2008 study published in the Journal of Soil and Water Conservation that simulated erosion in ephemeral gullies reported rates ranging from 2.23 tons to 4.91 tons per acre per year.7 A survey conducted by the Natural Resources Conservation Service found that the erosion in ephemeral gullies ranged from 1.22 tons per acre per year in Michigan to 12.8 tons in Virginia.8 The same report concluded that including ephemeral gully erosion in national estimates could more the double the amount of soil loss thought to be occurring.

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