ERODIBILITY AND CRITICAL SHEAR OF A PREVIOUSLY FROZEN SOIL

Many areas of the northern United States and southern Canada, and particularly the four million ha of nonirrigatedcropland of the Northwestern Wheat and Range Region in the United States, suffer severe erosion under thawingsoil conditions. Ability to predict soil erosion in these areas is hampered by lack of knowledge of soil properties andhydraulic resistance of thawing soils. This study was conducted to determine erodibility parameters of soil frozen andthawed under controlled moisture tension. A tilting flume was designed to allow near-natural freezing and thawing of asoil mass and apply shear stress from flowing water. Tests were conducted under soil moisture tensions of 50, 150, and450 mm. A linear relationship was found between detachment and shear stress. Critical shear stress values showed littlechange with time. Rill erodibility was related to soil moisture tension and changed rapidly during the course of a 90-mintest flow. At a tension of 450 mm, the erodibility was similar to that found in field tests under summer conditions. Thisstudy indicates the transient nature of rill erodibility during soil freezing and thawing that will influence the accuracy ofcontinuous simulation erosion models for winter conditions.