TEMPORAL DISTRIBUTION OF RUNOFF AND SOIL-EROSION AT A SITE IN NORTHEASTERN OREGON

Average annual erosion predictions have marginal success in the Pacifi c Northwest perhaps because of variability of natural features related to runoff and soil erosion. Hydrologic frequency analysis and probabi lity theory were used to analyze 12 years of runoff and erosion data f rom continuous fallow, fall-seeded winter wheat, and fall-plowed wheat stubble at a site in eastern Oregon. Relatively rare events are the m ajor contributors to long-term soil losses for both discrete erosion e vents and seasonal erosion volumes. The soil loss tolerance for this s oil type was never exceeded for discrete events on either the fall-see ded winter wheat or fall-plowed wheat stubble. Seasonal soil loss exce eded the soil loss tolerance in only one of twelve years for winter wh eat and was never exceeded for the fall-plowed treatment. Soil losses over the long-term should not exceed soil loss tolerance values at thi s site under the soil management conditions used during this experimen t.