Operator`s manual

58
Installation, Operation and Maintainance of Buoy Operated Automatic Meteorological Stations
= gradient of the saturated vapor pressure versus temperature curve
y = psychrometric constant (function of air temperature and barometric pressure)
R
n
= net solar radiation
ES = change in energy storage in the water body
The semi-empirical coefficient a varies per climate zone. In humid areas, research supports a value of 1.26 for a
free water surface, while for arid locations a value of 1.74 was found more appropriate. It is clear that the exact value
of a is a function of local conditions and needs to be determined through further research.
3.2 Left-Hand Terms of the Priestley-Taylor Equation
LE is represented by the following formula:
LE =
v w
E
In which:
v
= is Latent Heat of Vaporization [J g-
1
]
w
= mass density of water [g m-
3
]
E = evaporation rate [m s-
1
]
Consequently, LE is expressed in [J m-
2
s-
1
]
The Latent Heat of Vaporization
v
is a function of the air temperature T
a
, and is represented by the following
formula:
v
= (2500.8 – 2.361 x T
w
)
in which:
v
= Latent Heat of Vaporization [J g-
1
]
T
w
= temperature of water surface [°C]
The mass density of fresh water is a function of the water temperature. For a water temperature of 20 °C it has a
value of 0.99836 x 10
6
[g m
-3
]
BOX 1: The energy associated with the evaporation of a 1 cm water layer with a temperature of 20 °C
The evaporation of a 1 cm water layer with a temperature of 25 °C would require the following amount of
energy:
LE =
v w
E
v
= (2500.8 – 2.361 x 20) = 2454 [J g-
1
]
w
= 0.99836 x 106 [g m-
3
]
E = 0.01 [m]
LE = 2442 x 0.9972 x 10
6
x 0.01 = 2450 x 10
4
[J]
Given a sample net radiation input of 500 watt per square meter [J s-
1
m-
2
], this would require
2450 x 10
4
/ 500 = 49,000 seconds » 13,5 hours
Annexes