Critical Slope Lengths for Unanchored Cornstalk and Wheat Straw Residue

ABSTRACT A rainfall simulator and field plots were used to study how unanchored cornstalks failed on corn fields that had not been tilled since last year's crop. Inflow was add-ed at the upper end of the plots to simulate long slopes. The simulated slope length where mulch movement began, significant rill erosion beneath the mulch began, or loss of erosion control began was defined as a critical slope length. At low mulch rates (0.2 kg/m2), the mulch was washed away piece by piece. At heavy rates (> 0.4 kg/m2), the mulch floated on the runoff water and failed en masse — a section a meter or longer washed down-slope a distance and lodged against other mulch or stub-ble. Significant rill erosion occurred beneath the mulch on a soil that was susceptible to rilling. On soils not sus-ceptible to rilling, critical slope lengths ranged from 45 to 200 m for unanchored cornstalks at mulch rates rang-ing from 0.2 to 0.9 kg/m2 on 7 and 9 percent slopes. On a soil that was susceptible to rilling critical slope lengths ranged from 40 to 150 m for cornstalk rates ranging from 0.6 to 1.3 kg/m2 on a 6 percent slope. These critical slope lengths, based on a runoff rate of 50 mm/h, represent the beginning of failure rather than complete loss of ero-sion control. Similar, but limited, test were conducted with wheat straw.