Adapting the Universal Soil Loss Equation to the Pacific Northwest

This is the authors' abstract. We don't add key points for this paper.

THIS paper reports a first-generation adaptation, based on limited data, of the Universal Soil Loss Equation to the Dryland Grain-growing Region of the Pacific Northwest. A preliminary analysis of the data in-dicated the Wischmeier-Smith slope length and steep-ness relationships overpredicted erosion for the steep slopes of the region. Revised relationships were pro-posed. The rainfall erosivity factor, based on rainfall intensity and energy, did not adequately account for erosive forces during the winter months when surface runoff from rain-fall and snowmelt dominates the erosion process. Field data were used to determine an equivalent winter R fac-tor to add to the spring, summer, and fall EI. A com-bined relationship based on the winter precipitation and the 2-yr return interval, 6-h duration precipitation was used to determine the areal distribution of a rainfall and runoff erosivity factor. The rainfall and runoff erosivity factor was distributed throughout the year so that cover and management fac-tors could be calculated for given rotations. Approxi-mately 85 percent of the erosion hazard occurs during the winter months when the fields planted to winter wheat are often bare and unprotected. The resulting adaptation of the Universal Soil Loss Equation is considered to be applicable to the Pacific Northwest Dryland Grain Region and extendable to the Intermountain Dryland Grain Region and other non-mountainous areas east of the Cascade ranges in Wash-ington, Oregon, and Idaho.