Wind - Agroecology: The Ecology of Sustainable Food Systems - page 94

FIGURE 7.6 Windbreaks in the arid region near Eilat, Israel. These windbreaks reduce evapotranspirational water
loss for the irrigated annual crops grown between them.

The primary effect of a windbreak is reduction of wind
velocity. A good windbreak can reduce wind velocity as
much as 80% for a distance of up to ten tree heights
downwind from the windbreak, and often for a distance
as long as two tree heights to the windward side. The area
in the lee of the barrier is known as the “quiet zone,” a
wedge-shaped area of greatly reduced wind speed with
moderate turbulence and small eddies. Above the quiet
zone and for a distance of several tree heights more down-
wind, there is a “wake zone” of large eddies, more turbu-
lence, and less reduction in wind speed (Figure 7.7).
Since a windbreak creates an obstacle to the wind,
flow is actually deflected upward as it approaches the
barrier. Near the top of the windbreak, flow is compressed
and accelerated. Just downwind and behind the barrier,
flow is reduced to close to zero with a solid windbreak,
and to intermediate speeds with a porous barrier. There
is a zone of strong velocity shear just above the top of
the windbreak that widens and follows the flowline as
the air moves downwind, eventually mixing with the
air in the zone of turbulence until it returns once again to
its normal speed at as much as 20 to 30 heights to the
leeward.

The density and porosity of a windbreak have a sig-
nificant effect on the distance over which the windbreak
can alter wind flow. Denser barriers produce the largest
velocity reductions directly to the leeward, but the largest
wind shear between the retarded air behind the windbreak,
and the accelerated zone above. Denser barriers also create
more turbulence, since kinetic energy loss from the orig-
inal flow must be balanced by an increase in kinetic energy
in the eddies. This leads to a quicker recovery of wind
speed behind the barrier, and therefore, a reduced pro-
tected area. A barrier with a porosity of 40% has been
shown to reduce wind speed effectively for a distance of
30 heights downwind (Tibke, 1988).
Besides reduction of soil erosion, the most tangible
effect of windbreaks is enhancement of the final yield of
the crop. Higher yield volume is the most obvious gain,
but earlier harvest time and better harvest quality are
important benefits as well. Less stress in the lee of the
barrier allows crops to allocate more energy to vegetative
or reproductive growth and less to maintenance. Less
physical damage occurs, transpirational losses are mini-
mized, and higher temperatures and humidity contribute
to better quantity and quality of production.