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Title: FREEZE/THAW EFFECTS ON RILL AND GULLY EROSION IN THE NORTHWESTERN WHEAT AND RANGE REGION

Author
item Mccool, Donald
item Williams, John

Submitted to: ASAE Pacific Northwest Section Meeting
Publication Type: Proceedings
Publication Acceptance Date: 1/10/2004
Publication Date: 9/30/2004
Citation: McCool, D.K., and Williams, J.D. 2004. Freeze/thaw effects on rill and gully erosion in the Northwestern Wheat and Range Region. In: Proceedings of the ASAE Pacific Northwest Section Meeting, September 23-25, 2004, Baker City, Oregon. Paper No. PNW04-1002.

Interpretive Summary: Hydrology of the cropland of the Northwestern Wheat and Range Region (NWRR) of the Pacific Northwest USA is uniquely defined by the mixture of rainfall and snowmelt runoff events that can occur on frozen, thawing or non-frozen soil during any of the winter months of November through March. Runoff plot and small watershed studies first initiated in 1931 at Pullman, WA, followed by more recent studies, form the basis for an understanding of the hydrology of the region and the resulting runoff, erosion, and channel characteristics. Fifty-five to sixty-five percent of the annual precipitation occurs from November through March, and approximately eighty-five percent of the annual erosion hazard occurs during this period. Impermeable frost is a major factor in rill and gully formation in the region. Saturated zones can form above tillage pans, freeze solid, and significantly reduce infiltration. Frost-heaved surface soils thaw and weaken with warming temperatures or rain. Runoff from rain or snowmelt, or a combination of the two, concentrates in rills and channels and carries with it the loosened soil. Concentrated flow channels form because of collection of water from impervious areas such as conventionally tilled fall seeded small grain and bare grass seed fields. Gullies are also created by seepage from saturated layers above permanent restrictive layers in the soil. Rill measurements on conventionally tilled fields in Oregon, Washington, and Idaho were used to determine coefficients for commonly used relationships between slope length and steepness and rill size. Steepness relationships differed from those found in the eastern US. Concentrated flow channels may also exhibit unique characteristics. Consideration of the unique hydrological characteristics of the region will improve erosion and sediment transport modeling.

Technical Abstract: Hydrology of the cropland of the Northwestern Wheat and Range Region (NWRR) of the Pacific Northwest USA is uniquely defined by the mixture of rainfall and snowmelt runoff events that can occur on frozen, thawing or non-frozen soil during any of the winter months of November through March. Runoff plot and small watershed studies first initiated in 1931 followed by more recent studies form the basis for an understanding of the hydrology of the region and the resulting runoff, erosion, and channel characteristics. Fifty-five to sixty-five percent of the annual precipitation occurs from November through March, and formation of impermeable frost, intensified by excessive tillage and tillage pans, is a major factor in rill and gully formation in the region. Saturated zones can form above tillage pans, freeze solid, and significantly reduce infiltration. Frost-heaved surface soils thaw and weaken with warming temperatures or rain. Under these conditions, runoff from rain or snowmelt or a combination of the two is inevitable, concentrating in rills and channels and carrying with it the loosened soil. Concentrated flow channels form because of collection of water from impervious areas such as conventionally tilled fall seeded small grain and bare grass seed fields. Gullies are also created by seepage from saturated layers above permanent restrictive layers in the soil. Rill measurements on conventionally tilled fields in Oregon, Washington, and Idaho were used to determine coefficients for relationships between slope length and steepness and rill size. The steepness relationships differed from those found in the eastern US. Concentrated flow channels may exhibit unique characteristics as well. Consideration of the unique hydrological characteristics of the region will improve erosion and sediment transport modeling.