Threshold friction velocity influenced by wetness of soils within the Columbia Plateau

Windblown dust impacts air quality in the Columbia Plateau of the US Pacific Northwest. Wind erosion of agricultural lands, which is the predominate source of windblown dust in the region, occurs when the friction velocity exceeds the threshold friction velocity (u∗t) of the surface. Soil moisture influences the u∗t, but the u∗t of variably wet soils is virtually unknown in the region. The relationship between u∗t and water content was ascertained for five silt loams collected from field sites in eastern Washington. After moistening the soils to predetermined water contents, u∗t was assessed from wind velocity and PM10 (particles with an aerodynamic diameter ⩽10 μm) and TSP (total suspended particulate matter) concentration profiles above the soil surface inside a wind tunnel. Soil water content appeared to have little influence on u∗t until gravimetric water content rose to 6–14% (depending on the soil). Trends in the relationship between u∗t and soil matric potential (Ψ) suggested moistening soils had little influence on u∗t until the Ψ was in the range of −25 to −1 MPa. The u∗t rose sharply with an increase in Ψ above −1 MPa. The results of this study suggest that u∗t is significantly influenced by adsorption and capillary forces that comprise the matric potential of soils.