Putting Soil Movement into Perspective

Abstract

Soil erosion is the detachment and movement of soil. Variations in landscape, soil type, and available energy cause a continuum of detachment and deposition resulting in most soil particles moving only a few feet. The Universal Soil Loss Equation (USLE) and Wind Erosion Equation (WEE) were developed from small uniform areas that minimized chances for deposition. Estimated sediment delivery to rivers in most agricultural areas ranges from 1 to 30% of total reported watershed erosion. Some of this sediment is from channel or gully erosion not measured by the USLE, indicating that “soil loss” values from the USLE seriously overestimate net erosion. Although notable exceptions exist, 137Cs studies have confirmed that most eroded soil remains in fields where erosion occurs. Some basins with high average rates of erosion experience little net loss or, in some cases, net gains of soil. Deposition from wind erosion can account for a net gain. Some basins experiencing little net erosion still suffer serious erosion problems if deposition occurs on a small proportion of the landscape. Depositional studies have shown that high erosion rates predicted by the USLE on long steep slopes do not always conform to this expectation, and that deposition occurs in areas where it is not always anticipated. Because of these inconsistencies, the USLE and WEE may have their greatest utility where several sites over larger geographic areas are sampled. An important contribution of erosion equations has been to identify alternative cultural practices for erosion control. The equations also have aided in identification of regions experiencing high relative erosion rates. Erosion control strategies based solely on gross erosion are often inefficient in meeting objectives and will profit from consideration of other variables known to affect the problem. The terms “soil erosion” and “soil loss” are not interchangeable, but the paucity of information on deposition allows few specific statements on the fate of eroded soil. Future efforts need to focus on the interdependence of erosion and deposition in field environments.