Long-Term Erosion Trends on Cropland in the Pacific Northwest.
Dennis Roe, USDA_NRCS, S717Hillcrest Dr., Cofax, WA 99111
Winter erosion from non-irrigated corpland in northeastern Oregon, southeastern Washington, and northern Idaho appears to have decreased during the past 20 years. Assuming this effect is real and not just apparent, is there a single cause? Is this the effect of climate change, changes in the result of the 1985 Farm Bill, or a combination of these and other factors? We can't answer all these questions, but a unique 43-year data set obtained from monitoring winter erosion on a large number of sample fields in Whitman County, WA from Water Year 1940 through 1982, provides an opportunity to examine historic trends in erosion and corresponding climate change conditions. During this period, the winter wheat/summer fallow rotation was used on much of the area, including the higher precipitation zone more suitable for annual cropping. There were several consecutive years in the 43-year data set when erosion was low. Weather records for these years indicate reduced freeze/thaw activity with rain or snowmelt during the period of thaw. We examined diurnal freeze/thaw cycles, length and severity of frozen periods, snow-melt accumulation during cold periods, and rain during early stages of the thawing process. Our analysis of the 1983 through 2005 climate data indicate reduced erosion hazard from freeze/thaw effects. U. S. Department progress records for 1979 and 1994 indicate increased application of conservation practices in 1994 as compared to 1979, with a large estimated reduction in erosion. Measurement of sediment at the mouth of the Palouse River indicated a large reduction from the 1962 through 1971 period to the 1993 to 1996 period. Sediment in runoff and flooding from snowmelt on frost impacted soil occurred in water Year 1997, but erosion rates were not catastrophic.