ABSTRACT
Erosion on the 4 million hectares of non-irrigated cropland of the Northwestern Wheat and Range Region (NWRR) of the U.S.A. is largely the result of winter rainfall and snowmelt events and frequently involves thawing soil. A successful erosion prediction model must predict the occurrence and severity of these winter events as well as how they are affected by crop management. Winter conditions in the NWRR are particularly difficult to model because of the transient nature of the freezing and thawing process. In the NWRR the soil does not remain frozen throughout the winter but instead generally freezes to a depth of 10-20 cm and then thaws completely; this cycle may repeat from one to several times during the winter. In addition there are numerous diurnal ffeeze/thaw cycles. In much of the area there is sufficient soil water that freezing decreases the soil permeability (Lee 1983). During the thawing process the soil is very weak (Formanek et al. 1984; Van Klaveren and McCool 1998), usually at high moisture content (Kok and McCool 1990), and will erode with very small amounts of rainfall or snowmelt.
