Geoscience Reference
In-Depth Information
agricultural activity combines with a semi-arid climate,
highly erodible soils and seasonally active strong winds
(e.g. Sharratt, Feng and Wendling, 2007; Zobeck and Van
Pelt, 2006). Indeed, Nordstrom and Hotta (2004) report
that 90 % of wind erosion in the USA occurs west of the
Mississippi River with about 60 % in the Great Plains re-
gion (Ervin and Lee, 1994), where Lubbock, Texas, copes
with the national maximum of 47.5 dust days per year
(Hagen and Woodruff, 1973). The major wind erosion
problems on these agricultural lands occurs during ground
preparation for planting and after harvesting (Clausnitzer
and Singer, 1996). At these times the cotton and wheat
fields are left bare and far more susceptible to erosion, and
disturbance is at its greatest through tillage (ploughing)
operations, planting and weeding (Nordstrom and Hotta,
2004). Particular problems are evident during drought pe-
riods when the soil is much more erodible and protective
crop residue is far sparser. As a result, Merrill
et al.
(1999)
estimated that wind erosion losses in the Great Plains can
be up to 6100 times greater in drought years than in wet
years. The intense drought in the mid-west USA during
the 1930s was a catalyst in producing the devastating dust
storms of the 'Dust Bowl' (see boxed text (Box 23.1)).
The experience of the Dust Bowl promoted the develop-
ment and implementation of land conservation strategies
in the USA and stimulated the founding of both the US
Soil Conservation Service and the Agricultural Research
Service of the US Department of Agriculture. Both agen-
cies research and encourage the application of sustainable
agricultural practices and there are now many methods
by which farmers and land managers can attempt to con-
trol wind erosion from agricultural fields. These include
windbreaks, crop management and tillage operations.
There is a great deal of literature on the most efficient
design, construction and placing of windbreaks and shel-
terbelts (Cleugh, 1998; Brandle, Hodges and Zhou, 2004;
Cornelis and Gabriels, 2005). They usually consist of trees
and shrubs placed perpendicular to the prevailing wind to
reduce both upwind and downwind wind speeds, thus re-
ducing wind erosion. Upwind velocity reductions may be
experienced for a distance up to 2-5 times the height of the
windbreak while the downwind protected zone can extend
up to 10-30 times the windbreak height (Wang and Takle,
1995). The height, width and porosity of the windbreak
all have important influences on its effectiveness. Wind
tunnel experiments by Cornelis and Gabriels (2005) indi-
cate that porosities of between 0.20 and 0.35 m
2
/m are the
most efficient (see Figure 23.3). While protecting the soil
from erosion, windbreaks have also been shown to ame-
liorate the microclimate (soil temperature, evaporation)
so that crop growth response is generally positive, show-
100
80
60
Porosity
73%
62%
50%
36%
10%
40
20
0
-10
-5
0
5
10
15
20
25
30
Distance from windbreak (H)
Figure 23.3
The reduction in horizontal wind speed (
u
)rel-
ative to an upwind reference (
u
0
) with varying distance and
porosity (after Wang and Takle, 1997).
and Zhou, 2004). However, as discussed by Nordstrom
and Hotta (2004), windbreaks require labour and mainte-
nance in order to preserve their effectiveness and they also
take up valuable space that could be used for arable pur-
poses; therefore sometimes they are removed to make way
for modern agricultural machinery or irrigation systems.
Crop management systems for reducing wind erosion
include selecting crops that offer the maximum amount
of post-harvest protective residue (Hagen, 1996). Further,
crop rotation or strip cropping can be used to protect
the soil surface during the most hazardous erosive peri-
ods (Nordstrom and Hotta, 2004). However, one of the
most frequently used methods of wind erosion control is
tillage. While tillage can enhance wind erosion if it dis-
turbs a relatively stable soil surface, it can also reduce
erosion susceptibility if practised appropriately (Merrill
et al.
, 1999). One of the key techniques, often used as an
emergency wind erosion control method, is to overturn
the soil such that moist clods lie on the surface in furrows
perpendicular to the erosive wind (Nordstrom and Hotta,
2004). While the increased aerodynamic roughness result-
ing from the furrow structures has the effect of increasing
wind shear stress, it also raises the depth of air with zero
wind velocity (
z
0
), hence protecting finer particles from
erosion (see Chapter 18). The excess wind shear stress
is absorbed by the larger clods, which have much higher
erosion thresholds (Figure 23.4). Such a tillage technique
can be very successful at reducing wind erosion so long
as the clods are not broken down too quickly by the action
of saltation (Stout and Zobeck, 1996). Wiggs and Holmes
(2010) found that the impact of employing a ridge-furrow
ploughing strategy on erodible fields in South Africa
was a reduction in the frequency of erosion events by
almost 49 %.
Conservation tillage is also a practical method for ero-
