Geoscience Reference
In-Depth Information
Radiation-based estimation of
E
RC
As previously stated, when estimating evaporation rates, it is recommended that
the starting point should be a measurement or estimate of the energy available to
support evaporation; because this has a strong influence on evaporation rate
when water is not limiting. Equation (22.30) can be calculated if a measurement
of the available energy (
R
n
-
G
), and temperature and pressure are available.
Temperature is needed to calculate
Δ
from Equation (2.18), and pressure is also
needed to calculate
from Equation (2.25). As Fig. 22.11 shows, for a range of
surface resistances around 70 s m
−1
, the value of surface resistance selected for the
reference crop, Equation (22.30) can provide a reasonable estimate of evaporation
with
γ
1.26.
However, it is important to recognize that this result only applies if there is
surface-ABL coupling over an extensive area with a similar surface resistance.
It could be that the area of reference crop for which an estimate of evaporation
is sought is not extensive. It could be a limited area of irrigated reference crop in
an otherwise arid landscape, for example. Therefore, some means of quantify-
ing the aridity of the advected atmosphere is required. In this context, it is
relevant that the Penman-Monteith equation (Equation (21.33) can be
re-written in the form:
α
=
rr
+
a
c
l
E
=
lim
Δ
A
(
Δ+
g
)
r
+
g
r
(23.17)
a
s
where
r
clim
is the climatological resistance which is defined by:
⎛
r
cD
⎞
ap
−
2
r
=
⎜
(if
A
is in Wm
)
⎟
c
lim
Δ
A
⎝
⎠
(23.18)
or:
187219
g
D
⎛
187219
g
D
⎞
r
=
≈
⎜
(if
A
is in mmd
−
1
)
(23.19)
⎟
(
)
c
lim
⎝
C
⎠
C
(275
+Δ
TA
)
Δ+
A
1
0.622
e P
(273.15
+
T
)
The value of
r
clim
provides a measure of the relative influence of the (advected)
vapor pressure deficit and available energy on reference crop evaporation rate.
Equating the estimated evaporation rates given by Equations (22.30) and
(23.15), rearranging, and substituting
r
s
=
70 s m
−1
and
r
a
=
208/
u
2
gives
a
effective
, the
value of
α
required to give a good estimate of reference crop evaporation, thus:
(
Δ+
g
) (208 /
⎡
ur
u
)
+
⎤
⎣
⎦
2
c
lim
a
=
(23.20)
effective
(
Δ+
g
)(208 /
)
+
70
g
2
