Page 122 - Soil Degradation, Conservation and Remediation

110 4 Wind Erosion

Chepil et al. (1962) considered the climatic elements wind velocity, precipitation,
and temperature as the dominant factors relative to wind erosion rates and introduced
a combined wind erosion climatic factor C, which was modified by Woodruff
and Siddoway (1965) as

v 3
C = 34 483 2
.
−
( PE)
where v means annual wind velocity for a particular geographic location corrected
to a standard height of 30 ft, and P − E = Thornthwaite’s P − E ratio = 10 P − E = 115
(P/T − 10) 1.111 . Later FAO (1979) revised the equation as

−
1  ETPP 
C = ∑ u 3   d
100  ETP 
where C is the wind erosion climatic factor, u (m s ) is mean monthly wind speed at
−1
2 m height, ETP (mm) is potential monthly evaporation amount, P (mm) is monthly
precipitation, and d is the number of days in the month concerned. Dong and Kang
(1994) selected meteorological data from 233 stations covering parts of 12 provinces
of northern China to calculate the wind erosion climatic factor in arid and semiarid
China. The annual C value in arid and semiarid areas in China ranged from 10 to 150.

4.4.3 Field Width and Slope

It was shown by wind tunnel simulation (Dong 1994; Li 1999) that there are two
effects of slope on wind erosion. Firstly, the wind velocity increases with slope
along the upslope direction of a landform and strengthens the wind erosion on the
slope, and secondly, the threshold velocity of grain increases with slope and weak-
ens the wind erosion.
The rate of soil flow increases with distance downwind across an eroding field
until, if the field was large enough, it reached a maximum that a wind of a given
velocity can carry. Beyond that point the rate of flow remained essentially constant
(Chepil 1957a). This maximum was about the same for soil of any texture about
2 t per rod width per hour for a 40 mph wind at 50 ft. The distance required for soil
flow to reach the maximum that a wind of a given velocity can carry varies inversely
with erodibility of a field surface. The more erodible the surface, the shorter the
distance to reach maximum flow (Chepil and Woodruff 1959).

4.4.4 Vegetation

Vegetative covers reduce the wind velocity at the soil surface and also generally
decrease the soil erodibility. The relationship between vegetation coverage and wind
erosion rate is an exponential function, i.e., with the increase of vegetation coverage