Geoscience Reference
In-Depth Information
1.4
1.2
β
e
α
e
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
θ
5
/
θ
f
Scaled Soil Moisture,
Fig. 4.5
Variation of (
β
e
α
e
) with the water content in the upper 5 cm of the soil
θ
5
, expressed relative to its
value at field capacity
θ
f
for a sandy loam covered with perennial ryegrass in Ontario. The curve
represents the function
β
e
α
e
=
1
.
26[1
−
exp(
−
10
.
563
θ
5
/θ
f
)]. (After Davies and Allen, 1973.)
One difficulty in applying the formulation (4.33) with an apparent potential
evaporation
E
pa
is that, as the surface dries out, the two quantities on the right-hand
side of (4.33) move in opposite directions. Indeed, whenever
β
e
approaches zero,
E
pa
tends to become large; this may lead to an unstable product of a large with a small quan-
tity, each with considerable noise. On the other hand,
E
pe
depends mainly on radiation
and temperature, and not on the dryness of the air; hence application of (4.33) with
E
pe
islikely to be more robust and therefore preferable.
Surface resistance concept
A second procedure of reducing
E
p
to
E
is based on the realization that the release of
water vapor from a vegetation is controlled by the stomata of the leaves. This is illustrated
schematically inFigure 4.6. The underlying idea is that the air is assumed to be saturated
with water vapor inside the stomatal cavities but not at the outside surface of the leaves, and
the stomata provide an obstruction or a resistance to the diffusion of the water vapor from the
inside to the outside of the leaves. This is often referred to a stomatal resistance
r
st
. Because
evaporation also takes place from the soil surface, beside the leaves, the basic idea is usually
extended to include this transport as well; thus the soilair at some depth below the surface
can be assumed to be saturated and the soil pores can then be visualized as providing
a resistance to the diffusion of water vapor to the soil surface. Hence more generally,
the resistance approach is based on the concept of one or more resistance parameters
in parallel and/or in series, which may account for the moisture stress in the vegetation
and/or soil, and which relate saturation specific humidity
q
s
, at the temperature
T
s
of the
evaporating surface, to the actual (non-saturated) specific humidity
q
s
at the evaporating
surface.
Several such resistance parameters have been used for this purpose (see, for example,
Monteith, 1973). The one given by Thom (1972) is instructive as an illustration and can be
