Geology Reference
In-Depth Information
When R
n
> 0 (i.e., daytime),
G
¼
0
1R
n
ð
7
:
10
Þ
:
and when R
n
< 0 (i.e., nighttime),
G
¼
0
5R
n
ð
7
:
11
Þ
:
With a daily time step,
0
408
D
ð
R
n
G
Þþc
T
hr
þ
273
u
2
ð
900
e
0
ð
T
hr
Þ
e
a
Þ
:
ET
0
¼
ð
7
:
12
Þ
D
þ cð
1
þ
0
:
34u
2
Þ
where ET
0
is the reference evapotranspiration (mm day
−
1
),
Δ
the saturation slope
C
−
1
), R
n
the net radiation at the
vapor pressure curve at daily average T
hr
(kPa
°
grass surface (MJm
−
2
day
−
1
), G the soil heat
flux density (MJ m
−
1
day
−
1
),
ʳ
C
−
1
), T
hr
the mean daily air temperature (
the psychrometric constant (kPa
C), U
2
the average daily wind speed (m s
−
1
), e
0
(T
hr
) the saturation vapor pressure at T
hr
(kPa), and e
a
the average daily actual vapor pressure (kPa).
The psychrometric constant,
°
°
ʳ
, can be calculated using (
7.13
):
10
3
P
c ¼
0
665
ð
7
:
13
Þ
:
C
−
1
]; and P atmospheric pressure [kPa].
The actual vapor pressure (e
a
) is calculated by
where
ʳ
is the psychometric constant [kPa
°
RH
hr
100
e
0
e
a
¼
ð
T
hr
Þ
ð
7
:
14
Þ
where RH
hr
is the average hourly relative humidity (%).
The saturation vapor pressure at T
hr
, e
0
(T
hr
), can be calculated from the fol-
lowing equation:
27T
hr
T
hr
þ
17
:
e
0
ð
T
hr
Þ¼
0
6108 exp
ð
7
:
15
Þ
:
237
3
:
where e
0
(T
hr
)is the saturation vapor pressure at T
hr
(kPa), T
hr
the mean hourly air
temperature (
C
−
1
)
°
Δ
°
C), and
the slope of the saturation vapor pressure curve (kPa
at mean air temperature (T
hr
)[
20
,
27
] calculated by
h
i
27T
hr
T
þ
237
:
3
17
:
4098 0
6108 exp
:
D
¼
ð
7
:
16
Þ
2
ð
T
þ
237
Þ
