Geoscience Reference
In-Depth Information
2.50
2.25
2.00
1.75
Arid conditions
1.50
Humid conditions
1.25
1.00
0.75
Figure 23.1
Variation in the
value of
a
effective
required for
Equation (22.28) to give an
estimate of reference crop
evaporation rate consistent
with an FAO estimate as a
function of
r
clim
.
0.50
0.25
0.00
0
25
50
75
100
r
clim
125
150
175
200
Figure 23.1 shows the calculated variation of
a
effective
as a function of
r
clim
when
T
2 m s
−1
and reveals that, in these conditions,
a
effective
is within 10% of the value 1.26 used in the Priestley-Taylor equation when
r
clim
is
between about 40 to 70 s m
−1
. When the atmosphere is more arid, the value of
a
effective
must be higher to give a reasonable estimate of reference crop evaporation.
Jensen
et al.
(1990) proposed
=
20
°
C,
P
=
100 kPa and
u
2
=
1.74 as being the value required for a reasonable
estimate of reference crop evaporation in arid conditions and Fig. 23.1 shows that
when
T
α
=
=
20
°
C,
P
=
100 kPa and
u
2
=
2 m s
−1
,
a
effective
is within 10% of 1.74 when
r
clim
is between about 90 to 140 s m
−1
.
Thus, if measurements or estimates of either vapor pressure deficit, or wind
speed (or both) are not available, but an estimate of daily average net radiation
can be made, the best available estimate of reference crop evaporation is from
the equation:
Δ
E
=
a
A
RC
effective
Δ+
g
(23.21)
with
a
effective
set to 1.26 if the climate of the area is considered to be generally humid,
or to 1.74 if the climate of the area is generally arid.
Temperature-based estimation of
E
RC
A temperature-based estimate of reference crop evaporation should only be made
when the available data is limited to measurements of maximum and minimum
temperature. There have been several empirical equations proposed for relating
