Geoscience Reference
In-Depth Information
Table 23.2
Demonstration of the sequence of steps undertaken to calculate daily average net radiation as described in the
text applied in the three cases A, B, C specified previously. In two cases (A and C) cloud cover is measured and in the third
case (B) the number of bright sunshine hours is measured.
Origin
Variable
Units
Site A
Site B
Site C
(Data)
Day of year
(none)
196
135
46
Equ. (5.5)
Eccentricity fctor
(none)
0.9679
0.9774
1.0232
Equ. (5.8)
Solar declination
(radians)
0.3773
0.3254
−0.2355
Equ. (5.12)
Sunset hour angle
(radians)
2.0964
1.7849
1.5838
(Data)
Latitude
(deg)
51.7
32.2
−3.1
Latitude
×
π
/180
Latitude in radians
(radians)
0.9023
0.5620
−0.0541
Equ. (5.15)
Extraterrestrial solar radiation
(mm day
−1
)
16.45
16.36
15.61
(Data)
Cloud fraction
(none)
0.50
-
0.70
Equ. (5.16)
Solar at ground (cloudy sky)
(mm day
−1
)
8.23
-
6.24
(Data)
Number of bright sunshine hours
(hours)
-
13.00
-
Equ. (5.13)
Maximum daylight hour
(hours)
-
13.64
-
Equ. (5.17)
Solar at ground (cloudy sky)
(mm day
−1
)
-
11.89
-
Selected from above
Solar at ground (cloudy sky)
(mm day
−1
)
8.23
11.89
6.24
(Data)
Selected value for albedo
(none)
0.23
0.23
0.23
Equ. (5.18)
Net solar radiation
(mm day
−1
)
6.33
9.16
4.81
Table 23.1
Vapor pressure
(k Pa)
1.538
0.821
2.809
Equ. (5.23)
Effective emissivity
(none)
0.166
0.213
0.105
Equ. (5.16) (with c
=
0)
Solar at ground (clear sky)
(mm day
−1
)
12.34
12.27
11.70
(Assigned)
Assigned site humidity
(none)
Humid
Arid
Humid
Equ. (5.24) or (5.25)
Cloud factor
(none)
0.667
0.958
0.533
Table 23.1
Average temperature
(
)
17.50
27.00
26.50
°
C
Equ. (5.22)
Net longwave
(mm day
−1
)
−1.58
−3.30
−0.90
Equ. (5.26)
Net radiation
(mm day
−1
)
4.76
5.86
3.90
Open water evaporation
All the methods recommended for estimating daily evaporation in this text are in
some way derived from the Penman-Monteith equation. In the case of open water
evaporation, the equation is used with surface resistance set equal to zero because
air is assumed saturated at the evaporating water surface. When calculating open
water evaporation the available energy,
A
w
, used in the Penman-Monteith equation
must be calculated with an albedo appropriate for a water surface (often 8% is
assumed), and should allow for any change in the energy stored in the water body
as a result of heat advection. Heat advection might result if water enters and leaves
the water body with temperatures that differ.
The most appropriate form of expression to be used for the aerodynamic resist-
ance for open water,
r
a
ow
, has been debated over the years, but the empirical form
originally (implicitly) defined by Penman (1948) is considered appropriate and is
selected here. It takes the form:
