Operator`s manual

60
Installation, Operation and Maintainance of Buoy Operated Automatic Meteorological Stations
However, in line with its name, the psychrometric constant y is virtually constant, with a typical value of 0.066 [kPa
°C
-1
]. Thus:
y = 0.066 [kPa °C
-1
]
Net Solar Radiation R
n
Net Solar Radiation is calculated with:
R
n
= R
short-wave-in
– R
short-wave-out
+ R
long-wave-in
– R
long-wave-out
Short wave radiation has a wavelength between 0.2 and 2 m, while long wave radiation encompasses the
spectrum from 4 to 50 m. Incoming and reflected solar radiation is typically measured with a pyranometer. The
Net-Short-Wave radiation is then calculated with:
R
net-short-wave
= R
short-wave-in
x (1 – a)
in which:
a = albedo [-]
A typical value for the reflection coefficient of short wave solar radiation on an open water surface, the albedo, is
0.08.
As with short-wave radiation, long-wave radiation can be measured. The Net-Long-Wave radiation is calculated
with:
R
net-long-wave
= R
long-wave-in
– R
long-wave-out
= -f ’ (T + 273.2)
4
in which:
f = adjustment for cloud cover [-]
’ = net emission between atmosphere and the ground
= Stefan-Blotzmann constant (4.903 x 10
-3
J m
-2
°K-4 day
-1
)
T = temperature of the water body [°C]
The net emission is a function of the vapor pressure, and is expressed by:
’ = 0.34 – 0.14 √e
d
in which:
ed = vapor pressure [kPa]
The coefficient f, adjustment for cloud cover, can be set to 1 for the Lake Nasser area, where the skies are essen-
tially cloudless.
Change in Energy Storage ES
The change in energy storage in the lake is found from volumes and average temperatures of the lake water at the
beginning and end of the selected time step. It is expressed by:
Annexes