Size Distribution of Windblown Sediment Emitted from Agricultural Fields in the Columbia Plateau

Windblown sediment resulting from erosion of agricultural land has impaired visibility and threatened human health in the U.S. Inland Pacific Northwest. There is, however, a lack of information on the size distribution of windblown sediment originating from these lands. Passive collectors were used to trap windblown sediment from heights of 0 to 1.5 m above eroding agricultural fields. Sediment collected during one high‐wind event in each of 7 yr was separated into ≤ 10‐, 11‐ to 32‐, 33‐ to 45‐, 46‐ to 100‐, and >100‐μm‐diameter size fractions. Windblown sediment trapped nearer the soil surface was more characteristic of the erodible portion of the in situ parent soil. Vertical size gradation of windblown sediment was evidenced by a decrease in mass of the 46‐ to 100‐ and >100‐μm size fractions and increase in mass of the ≤ 10‐ and 11‐ to 32‐μm size fractions with height from the soil surface. Trends in the sediment size distribution with height suggested that sediment ≤ 32 μm in diameter was transported primarily by suspension, sediment 33 to 45 μm in diameter was transported by saltation and suspension, and sediment >45 μm in diameter was transported primarily by creep and saltation. In addition, the size of windblown sediment observed at all heights above the soil surface was found to increase with increasing wind velocity. A large fraction of windblown sediment was comprised of suspension‐sized aggregates and particles, thus mitigation strategies to control wind erosion in the Columbia Plateau must promote aggregation of the suspension component (≤32‐μm diameter) of the parent soil or shelter the suspension component at the soil surface from high winds.