Page 121 - Soil Degradation, Conservation and Remediation

4.4 Factors Affecting Wind Erosion 109

found out that the threshold velocities of soil particles depend on the average diameter
of soil aggregates instead of the grain size of single particles. After conducting
experiments in a wind tunnel, Dong and Li (1998) discovered that the relationship
for wind erodibility of aeolian sand as a function of its grain size follows a discon-
tinuous function, with 0.09 mm sand being the most susceptible to wind erosion.
The erodibility of aeolian sand can be divided into 3 categories: difficult to erode
at >0.7 and <0.05 mm, moderately erodible at 0.7–0.4 mm and 0.075–0.05 mm, and
most erodible at 0.4–0.075 mm. With similar grain size, a mixture of sizes is more
susceptible to wind erosion than is a uniformly sized material (Skidmore 1974).
Chepil and Milne (1941) investigating the influence of surface roughness on
intensity of drifting dune materials and cultivated soils found that the initial intensity
of drifting was always much less over a ridged surface. Ridging cultivated soils
reduced the severity of drifting, but ridging highly erosive dune materials was less
effective because the ridges disappeared rapidly. The rate of flow varied inversely
with surface roughness. Because it was difficult to determine surface roughness
by measuring surface obstructions, Zingg and Woodruff (1951) devised a method
to determine surface roughness from pressure relationships in a wind tunnel
duct. The roughness was controlled by constructing ridges of non-erodible gravel.
Armbrust et al. (1964) studied the effects of ridge roughness equivalent on total
quantity of eroded material from simulated, cultivated soils exposed to different
friction velocities. A ridge roughness equivalent of 6 cm reduces wind erosion
50 %. As roughness increases to about 11 cm, the soil ridge roughness factor remains
about constant; then, with additional roughness, the effectiveness of ridges gradually
decreases. As roughness increases to about 11 cm, the soil ridge roughness factor
(K) remains about constant; then, with additional roughness, the effectiveness of
ridges gradually decreases (Skidmore 1974).

4.4.2 Climate

Climate influences wind erosion by determining wind velocity, precipitation, and
temperature that, in turn, determine evaporation and soil moisture content. Wind
erosion occurs when the shear stress exerted on the surface by the wind exceeds
the ability of the surface soil materials to resist detachment and transport. Strong
winds erode, and dryness increases the susceptibility of the surface soil to erosion.
The aridity of an environment is often evaluated by the Budyko dryness ratio
(Budyko 1958; Hare 1983). The dryness ratio at a given site indicates the number of
times the net radiative energy could evaporate the mean annual precipitation.
Semiarid zones where wind erosion is likely to be a serious problem have a dryness
ratio between 2 and 7 (Hare 1983). Areas with dryness ratios larger than 7 are in the
desert and desert margin zones. Most of the Great Plains of the USA has dryness
ratios between 2 and 5. The Sahara Desert in North Africa has a maximum dryness
ratio as high as 200 (Henning and Flohn 1977). The higher the dryness ratio, the
greater is the risk of wind erosion.