Threshold friction velocity of soils within the Columbia Plateau

Wind erosion only occurs when the friction velocity exceeds the threshold friction velocity (TFV) of the surface. The TFV of loessial soils found across the Columbia Plateau region of the U.S. Pacific Northwest is virtually unknown even though these soils are highly erodible and a source of atmospheric particulates that reduce air quality. The TFV’s of a sandy loam and four silt loams collected from field sites in eastern Washington were determined by systematically measuring the temporal variation in wind speed, saltation activity, and PM10 (particles with an aerodynamic diameter of ⩽10 μm) and TSP (total suspended particulate matter) concentrations above the soil surface inside a wind tunnel. The erodible fraction of each soil, obtained by drying and screening the soil, was placed in a tray inside the wind tunnel. The TFV for the sandy loam was 0.139 m s−1 whereas the TFV for the four silt loams ranged from 0.180–0.239 m s−1. The sandy loam was aerodynamically smoother than that of the silt loams, possibly due to a smaller size fraction of larger soil aggregates or particles on the surface of the sandy loam. The TFV’s measured in this study were lower than those previously observed in the field in the Columbia Plateau and also lower than the minimum TFV required to initiate erosion in some wind erosion models. While these TFV’s are representative of the erodible soil fraction, the low TFV’s may contribute to the occasional poor performance of wind erosion models in the region.