Geoscience Reference
In-Depth Information
Table 7.1
Typical values for aerodynamic resistance and canopy resistance for
low and high vegetation (grass and forest) and the resulting contributions of the
radiation and aerdynamic term for dry and wet vegetation
Grass
Forest
Dry
Wet
Dry
Wet
r
c
(s m
-1
)
50
0
100
0
r
ah
and
r
av
(s m
-1
)
40
40
10
10
r
c
/
r
a
(-)
1.25
0
10
0
L
v
E
(radiation term, W m
-2
)
247
343
83
343
L
v
E
(aerodynamic term, W m
-2
)
124
195
140
780
L
v
E
/(
Q
*
- G
)
0.74
1.08
0.45
2.24
H
(W m
-2
)
129
-
38
277
-
623
Available energy (
Q*- G
) has been set to 500 W m
-2,
ambient temperature to 20 ºC and rela-
tive humidity to 60%.
only on the vegetation type, but also on the duration of periods that the vegetation is
wet. If a forest is located in a climate with frequent, light, rains the canopy will be
wet for most of the time, and water loss will be large as compared to grass. If, on the
other hand, the same amount of rain falls in infrequent but intense events, the canopy
will be dry most of the time, and a forest would lose less water than grassland. The
magnitude of the interception reservoir is also relevant in this respect, as it determines
how long evaporation from a wet surface can continue before the liquid water on the
canopy is depleted (see
Chapter 6
). These differences in evapotranspiration between
grass and forests have important consequences for water management (e.g., which
part of the precipitation is available for ground water recharge or runoff).
Question 7.7:
Evaporation is different for different surface types and surface conditions.
The table below gives surface properties for various surface types and conditions.
Give typical values or qualitative indications (e.g., high, low, not relevant) for the sur-
face properties mentioned in the irst column. Do this for each of the surface types listed
in the header, that is, wet and dry forest, wet and dry grassland and lakes.
Also explain in one sentence for each surface property the origin of the differences
between the different surface types.
Forest
(dry)
Forest
(wet)
Grassland
(dry)
Grassland
(wet)
Lake
Canopy resistance,
r
c
Aerodynamic resistance,
r
a
Roughness length
z
0
Albedo
r
Evaporation of intercepted water
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