Agriculture Reference
In-Depth Information
The aerodynamic resistance r a between z r and the water vapor and sensible heat source
height, h a , [21], can be defined as
H
z
1
1
r
r
=
dz
+
dz
,
(13)
a
K
K
h
s
H
s
a
where K s is the turbulent transfer coefficient (momentum/moisture/heat) inside and above the
canopy in the intervals ( h a , H ) and ( H, z r ), respectively. The aerodynamic resistance in canopy
air space, r d , can be written in the form
h
H
1
1
a
r
=
dz
+
dz
,
(14)
d
K
K
s
s
z
h
g
where z g is the eff ective ground roughness length, while the area-averaged bulk boundary
layer resistance, r , has the form [21]
()
H
1
L
u
z
d
=
dz
,
(15)
r
C
P
b
h
s
s
a
where
(
)
LAI d = ; u ( z ) is the wind speed; C s is the
transfer coefficient [21]; and P s the leaf shelter factor. Eqs. (12)-(15) can be modified to take
into account the effects of nonneutrality. According to Sellers et al. (1986) [21], the position
of t he canopy source height, h a , can be estimated by obtaining the center of gravity of the
r
L is related to leaf area index LAI as
L
H
h
integral. Thus,
1
/
h
H
H
L
L
1
L
1
a
d
d
d
dz
=
dz
=
dz
=
.
(16)
r
r
2
r
2
r
h
b
h
b
h
b
b
a
We may obtain h a by successive estimation [21,23] until the foregoing equality is
reached.
2.3. Calculating the Wind Profile Inside Tall Grass Canopies
We consider the canopy to be a block of constant-density porous material sandwiched
between two heights, H and h [21,22]. The differential equation describing the wind profile
within such a “sandwiched” canopy architecture can be written in the form of Eq. (11). To
solve this equation, we have to know how K s depends on parameters representing the
canopy's aerodynamic and morphological features. K-theory is a commonly used method in
modeling the turbulence within a plant canopy. Although its use may be physically unrealistic
for this application, it yields reasonable results, so we shall use this method until suitable
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