Consequences of Historic Rainfall on Western Iowa Farmland

Rainfall on five soil and water conservation research watersheds near Treynor, Iowa, varied from 18 to 22 inches for the period of May 28 to June 27, 1967. This unique series of events has a return period that exceeds 100 years, based on the 97-year Weather Bureau record at nearby Omaha, Nebraska. Surface runoff from two corn cropped watersheds planted on an approximate contour approached or exceeded 50% of the storm rainfall during the period. Surface runoff was 8% or less for all rainfall events on a corn cropped, terraced watershed and 17% or less for all but one storm on a grassed watershed. These rains occurred a few weeks after planting when the corn plants were only 6 inches tall; consequently, little or no erosion protection was provided for the bare, loose soil. Sheet rill erosion rates were 75 to 100 tons per acre on the contoured corn watersheds. By contrast, conservation practices on two other watersheds limited the sheet rill erosion to 2.5 tons per acre. Gullies on the contoured corn watersheds eroded severely, whereas conservation practices on the other watersheds reduced gully erosion to an insignificant amount.