LARGE STORM EFFECTS ON TOTAL SOIL-EROSION

Soil erosion measurements on small upland watersheds totaling 229 watershed-years show that most of the total erosion occurring over a long-term period of record comes from a few large storms. Nine 1-ha (2.5-acre) watersheds containing residual soils with silt loam surfaces on B and C slopes were farmed under a corn-wheat-meadow-meadow rotation with moldboard plowing, disking, and harrowing in April of the corn years for seedbed preparation and cultivation for weed control. After corn harvest, stalks were disked prior to winter wheat seeding. There was no tillage in the wheat or meadow years. Four of the watersheds were farmed in straight rows across the slope; five were contoured. More than 92% of the erosion came in the corn years; the rest came in the wheat years. Erosion from the contoured watersheds averaged 30% of that from the straight-row treatment. With more than 4,000 rainfall events during the study period, the five biggest erosion-producing events on each watershed accounted for 66% of the total erosion over 28 years. On one watershed, one storm caused more than half of the long-term measured erosion.