Geoscience Reference
In-Depth Information
crop evapotranspiration (
ET
0
). However, this required retention of the two step
approach because it was still necessary to multiply
ET
0
by the relevant crop
coefficient to obtain actual evapotranspiration (
ET
c
). Later in this chapter a recent
approach for estimating daily average evaporation is described that is more realis-
tic because it uses the Penman-Monteith equation to calculate estimates using
crop-specific values for unstressed surface resistance, and an aerodynamic
resistance that reflects the height of the crop. However, the FAO crop factor
method currently remains in widespread use.
The FAO definition of reference crop evaporation rate is the standard
Penman-Monteith equation used to calculate a daily average evaporation rate
assuming a specific value of surface resistance and a specific expression for
aerodynamic resistance. On the basis of field calibrations for short, well-watered
grass,
r
s
is set equal to 70 s m
−1
in Equation (21.33), while
r
a
(which is assumed the
same for both latent and sensible heat transfer) is calculated from Equation (22.8),
assuming the wind speed, temperature, and specific humidity and therefore, by
implication, vapor pressure deficit are all measured 2 m above 0.12 m high grass.
On the basis of field studies, the zero plane displacement,
d
, and aerodynamic
roughness,
z
o
, for the grass crop are assumed given by:
z
=
0.123
h
(23.13)
o
and
d
=
0.67
h
(23.14)
where
h
is the height of the grass crop. Substituting these values into Equation
(22.8) gives
r
a
208/
u
2
. With these assumptions, the short grass crop-specific
version of the Penman-Monteith equation that can be used to estimate reference
crop evaporation,
E
RC
, in mm d
−1
is:
=
⎛
⎞
⎛
⎞
⎛
Δ
g
900
275
⎞
E
=
A
+
u D
(23.15)
⎜
⎟
⎜
⎟ ⎜
⎟
RC
22
Δ+
g
Δ+
g
T
+
⎝
⎠
⎝
⎠
⎝
⎠
m
m
2
where
A
is the energy available to evaporate water in mm d
−1
,
T
2
, u
2
, D
2
are respec-
tively the temperature in
C, wind speed in m s
−1
, and vapor pressure deficit in kPa
measured at 2 m, and
g
m
is the 'modified psychrometric constant' which is given by;
°
g
=+
g
(1
0.33
u
)
(23.16)
m
2
Commonly
A
(
R
n
-
G
), where
R
n
is the net radiation and
G
is the soil heat flux
both in mm d
−1
for the short grass crop.
Equation (23.15) is strongly recommended as the preferred method for
estimating reference crop evaporation whenever daily average values of all the
required weather variables are on hand. But sometimes not all the required varia-
bles are available, and less reliable estimates of
E
RC
then have to be made which are
described in the next sections.
=
