Wind
Using the value of u* just calculated, u(0.03) = 3.13 x 0.16 m! s =
0.5 d s .
5.5 Wind Within Crop Canopies
The equations given so far deal only with wind above the plant canopy.
These equations are useful for determining the wind in the microenvironment of many living organisms, but cannot be used for leaves within a
canopy or for animals or insects that live within a crop or forest canopy.
Equation (5.1) predicts that wind speed is zero at z = d + z,. It gives
no information about wind speeds below this level, but presumably they
would remain at zero to the bottom of the canopy. From experience,
however, we know that the wind does blow inside canopies, so Eq. (5.1)
is apparently wrong. In fact, Eq. (5.1) is only useful above the canopy.
Within the canopy a different model must be used.
In order to model the wind in canopies, the canopy is divided into
at least two layers. In most of the canopy the wind speed decreases
exponentially with depth. Figure 5.6 shows the lower part of the wind
profile fiom Fig. 5.3 for the 50 cm tall canopy along with the wind profile
throughout most of the canopy. The equation for the top 90 percent of the
canopy is:
where a is an attenuation coefficient for the crop. The wind speed at the
0
1
2
3
4
Wind Speed (mls)
FIGURE 5.6. Wind speed within and just above a crop.
Using the value of u* just calculated, u(0.03) = 3.13 x 0.16 m! s =
0.5 d s .
5.5 Wind Within Crop Canopies
The equations given so far deal only with wind above the plant canopy.
These equations are useful for determining the wind in the microenvironment of many living organisms, but cannot be used for leaves within a
canopy or for animals or insects that live within a crop or forest canopy.
Equation (5.1) predicts that wind speed is zero at z = d + z,. It gives
no information about wind speeds below this level, but presumably they
would remain at zero to the bottom of the canopy. From experience,
however, we know that the wind does blow inside canopies, so Eq. (5.1)
is apparently wrong. In fact, Eq. (5.1) is only useful above the canopy.
Within the canopy a different model must be used.
In order to model the wind in canopies, the canopy is divided into
at least two layers. In most of the canopy the wind speed decreases
exponentially with depth. Figure 5.6 shows the lower part of the wind
profile fiom Fig. 5.3 for the 50 cm tall canopy along with the wind profile
throughout most of the canopy. The equation for the top 90 percent of the
canopy is:
where a is an attenuation coefficient for the crop. The wind speed at the
0
1
2
3
4
Wind Speed (mls)
FIGURE 5.6. Wind speed within and just above a crop.