Geoscience Reference
In-Depth Information
To enhance comparability with the FAO method for estimating reference crop
evaporation, Equation (23. 30) can be re-written to give crop evaporation,
E
C
, as:
⎛
⎞
⎛
⎞
⎛
Δ
g
187219
⎞ ⎛
D uD
⎞
E
=
A
+
50
22
50
(23.37)
⎜
⎟
⎜
⎟ ⎜
⎟ ⎜
⎟
C
Δ+
g
*
Δ+
g
*
T
+
275
D
R
⎝
⎠
⎝
⎠
⎝
⎠ ⎝
⎠
m
m
2
2
c
where
A
is the energy available to evaporate water from the crop in mm d
−1
,
T
2
, u
2
and
D
2
are respectively the temperature in
°
C, wind speed in m s
−1
, and vapor
pressure deficit in kPa measured at 2 m, and
γ
m
* is the 're-modified' psychrometric
constant, given by;
⎛
⎞
()
ru
R
g
*
=+
g
1
sc
2
(23.38)
⎜
⎟
m
50
⎝
⎠
c
with the values of (
r
s
)
c
and
R
c
50
taken from tables such as Table 23.5.
Table 23.6 demonstrates example calculations of daily average evaporation from
unstressed crops calculated using the Matt-Shuttleworth approach and the FAO
crop factor method at the three example sites A, B and C specified previously, with
values of
K
c
, (
r
s
)
c
and
R
c
50
taken from Table 23.5 for three example crops, i.e., cereal
crops, an alfalfa crop and small vegetables. These results show there are differences
between the estimated daily evaporation rates when calculated using the more real-
istic Matt-Shuttleworth approach relative to those calculated using the FAO crop
factor method. At the two humid sites, the estimated daily average evaporation rates
given by the Matt-Shuttleworth approach are slightly less than those given by the
FAO crop factor method. However, at the arid site the estimated daily average evapo-
ration rates given using the Matt-Shuttleworth approach are significantly greater
than those given by the FAO crop factor method. This is to be expected and is real-
istic: the higher evaporation rate reflects the fact that for crops with crop height
greater than the reference crop, the aerodynamic resistance is less. Consequently the
evaporation rate will necessarily be more sensitive to atmospheric aridity, in general,
and will be greater in arid conditions because the advection term in the Penman-
Monteith equation becomes more significant in comparison with the radiation term.
Evaporation from water stressed vegetation
The approach used to represent the effect of soil water restrictions on evaporation
rate which applied in conjunction with the simple models of daily average evapo-
ration described in this chapter is usually not complex. Typically, a volume of soil
is defined that is considered to be accessible to the atmosphere via plants, the
depth of which may be related to the nature of the overlying vegetation through an
assumed rooting depth. A running water balance is then made for this soil sample.
The primary input to this water balance is precipitation, and the primary output is
