Geoscience Reference
In-Depth Information
Reference level
2m above forest
1.0
0.9
z
*
Z
0
= 10
0.8
z
*
= 50
Z
0
Figure 22.7
Height average
reduction in aerodynamic
resistance in neutral
conditions for different
relative reference levels with
(
z
*
/
z
0
)
z
*
0.7
= 100
Z
0
0.6
10, 50 and 100. The
height of 2 m reference
expressed relative to the
aerodynamic roughness
length for grass and forest are
also shown.
=
Reference level
2m above grass
0.5
0
25
50
75
100
125
150
Height of reference level [above(
z-d
) in units of z
0
]
for turbulent transfer. However, the magnitude of the reduction depends strongly
on the height at which the reference level is defined relative to the underlying
surface. Figure 22.7 illustrates the height-average reduction in aerodynamic
resistance in neutral conditions as a function of (normalized) reference level, with
(
z
*
/
z
0
) assumed to be 50. The relative height of a reference level that is 2 m above
crop height for a 0.12 m high grass crop and a 10 m high forest stand are also
shown (expressed relative to the aerodynamic roughness length for these two
surfaces). With the assumption (
z
*
/
z
0
) ≈ 50, the height-average reduction in
aerodynamic resistance for the grass crop is around 10%, but there is a reduction
of almost a factor two for a forest stand.
Wet canopies
When leaves and other components of a plant canopy are wet during and shortly
after rainfall, the source of the water evaporated from the wet portions of the can-
opy is no longer inside the leaves, rather it is from the water surfaces on the outside
of the leaves. Consequently for wet leaf surfaces the stomatal resistance is 'shorted
out' and is zero. Strictly speaking, the canopy average surface resistance in such
conditions should be calculated from the average surface area covered with water.
However, in practice, using the wetted area is not feasible and models of evaporation
from wet and partly wet canopies have adopted the alternative of describing the
evaporation in terms of the depth of water (in mm) stored on the canopy.
The most successful model of wet canopy evaporation is the Rutter model
(Rutter
et al.
, 1971; 1975) and derivatives thereof (e.g., Gash, 1979). This model,
