Geology Reference
In-Depth Information
D
ð
Þ
ð
D
þcÞ
þ
R
n
G
0
268
D
ð
D
þcÞ
ð
a
w
þ
b
w
u
2
Þ
VPD
:
ET
0
¼
ð
7
:
18
Þ
k
where ET
0
is the evapotranspiration (mm h
−
1
), u
2
the wind speed at 2 m (m s
−
1
), R
n
the net radiation (MJ m
−
1
h
−
1
), G is set equal to zero with standard CIMIS usage,
and
C
−
1
) calculated from
ʳ
is the psychrometric constant (kPa
°
c ¼
0
000646
ð
1
þ
0
000946T
hr
Þ
P
ð
7
:
19
Þ
:
:
where P is barometric pressure (kPa) at the study area, and
e
0
VPD
¼
ð
T
hr
Þ
e
a
ð
7
:
20
Þ
cit (kPa) and e
a
and e
0
(T
hr
)are from (
7.6
) and
where VPD is the vapor pressure de
C
−
1
)at
(
7.7
), respectively,
Δ
the slope of the saturation vapor pressure curve (kPa
°
mean air temperature, (T
hr
) is from (
7.8
), and
ʻ
is the latent heat of vaporization
(MJ kg
−
1
), using (
7.21
):
k ¼
2
:
501
ð
2
:
361
10
3
Þ
T
hr
ð
7
:
21
Þ
Doorenbos and Pruitt [
7
] developed coef
cients a
w
and b
w
for predicting hourly
reference ET
0
.
They suggested the coef
cients as
a
w
= 0. 29 and b
w
= 0.53, for Rn > 0 (daytime)
a
w
= 1.14 and b
w
= 0.40, for Rn < 0 (nighttime)
Hourly estimations of ET
0
can be calculated by applying these values to the
modi
ed form of the Penman equation (CIMIS PM) in (
7.18
).
7.4.1.4
''
Copais Approach
''
for Hourly Time Steps
Alexandris and Kerkides [
3
] developed the Copais equation for estimating ET
0
on
an hourly basis (mm h
−
1
):
C
6
RH
R
s
2
C
3
RH
2
C
4
T
2
ET
0
¼
C
0
þ
þ
þ
þ
þ
C
5
R
s
þ
ð
þ
Þ
C
1
RH
C
2
T
C
7
T
C
8
T
2
þ
ð
7
:
22
Þ
flux density in (MJ m
−
2
h
−
1
), T the mean hourly air
where R
s
is the solar radiation
temperature (
°
C), and RH the relative humidity (%).
