Page 120 - Soil Degradation, Conservation and Remediation

108 4 Wind Erosion

C = climatic factor
L = equivalent unsheltered distance across the field along the prevailing wind
erosion direction
V = equivalent vegetative cover

4.4.1 Soil

Soil erodibility refers to the ease of detachment and transport by wind. From wind
tunnel tests, Chepil (1950) determined relative erodibility of soils reasonably free
from organic residues as a function of apparent specific gravity and proportions of
dry soil aggregates in various sizes. Clods larger than 0.84 mm in diameter were
non-erodible in the tests. Since then, the non-erodible soil fraction greater than
0.84 mm, as determined by dry sieving, has been used to indicate erodibility of soil by
wind (Chepil and Woodruff 1954). A dimensionless soil erodibility index, I, (Chepil
and Woodruff 1954, 1959) was based on the non-erodible fraction (percentage
of clods exceeding 0.84 mm diameter). The quantity of soil eroded in a tunnel is
governed by the tunnel’s length and other characteristics; therefore, erodibility was
expressed on a dimensionless basis so that for a given soil and surface condition,
the same relative erodibility value would be obtained regardless of wind tunnel
characteristics (Chepil 1960). The soil erodibility index was expressed as
X
I = 2
X 1
where X 1 is the quantity eroded from soil containing 60 % of clods exceeding
0.84 mm, and X 2 is the quantity eroded under the same set of conditions from soil
containing any other proportion of clods exceeding 0.84 mm. Soil erodibility index, I,
gave a relative measure of erodibility, but actual soil loss by wind was not known. When
a field is smooth, bare, wide, unsheltered, and noncrusted, its relative erodibility is
equivalent to the soil erodibility index defined the above equation.
Soil characteristics that affect erodibility of soil due to wind are texture, structure,
and water content. Texture is considered as the most dominant factor, and it is
commonly agreed that particles smaller than 0.25 mm and larger than 0.08 mm are
most easily eroded by wind. Soils with characteristics like fine sand particles having
low organic matter contents, single grains to massive in arrangement, with friable
and nonsticky consistence, etc. are more easily subjected to wind erosion. Dong
et al. (2000) classified the soil in the arid and semiarid areas of China into seven
primary types according to material composition in relation to wind erosion: Gobi
desert, sandy desert, loess deposits, residuum, flood deposits, salinized deposits,
and irrigation deposits, and hypothesized that the sandy desert, desert/loess transitional
zone, and flood deposits along abandoned river channels are the most easily eroded
by wind. Chen (1991) suggested that soil structure, degree of compaction, and vegetation
coverage are the main determining factors of soil resistance to wind erosion and