Operator`s manual

62
Installation, Operation and Maintainance of Buoy Operated Automatic Meteorological Stations
BOX 4: Sample Calculation of ES
Table 1 presents the temperature changes in a fictitious water body with a total depth of 20 m with a time
step of 1 day.
Table 2 presents the average temperature change per depth layer.
Consequently, per square meter:
(D
i
T
i2
– D
i
T
i1
) = 0.2 + 0.3 + 0.25 = 0.75 [M
3
°C]
ES = c
w
x
w
S (D
i
T
i2
– D
i
T
i1
)
c
w
= 4.182 [J °C
-1
g
-1
] (for water with a temperature of 20 °C)
w
= 0.99836 x 10
6
[g m
-3
] (for water with a temperature of 20 °C)
ES = 4.182 x 0.99836 x 10
6
x 0.75 = 3.13 x 10
6
[J M
-2
Day
-1
]
4 Sample Calculation of Open Water Evaporation Using Priestley-Taylor
This paragraph presents a sample calculation of open water evaporation using Priestley-Taylor, given:
= Priestley-Taylor coefficient = 1.74 (arid zone)
T
w
= average daily temperature of water surface = 20 [°C]
T
a
= average daily air temperature = 20 [°C]
RH = average daily relative humidity = 35 [%]
R
short-wave-in
= total daily incoming short wave radiation = 22.8 x 10
6
[J M
-2
day
-1
]
a = albedo = 0.08 [-]
For reasons of convenience, the Priestley-Taylor equation is again presented below:
LE = 1.74 x / ( + y) x (R
n
- ES) and LE =
v w
E
resulting in:
E = (1.74 /
v w
) x / ( + y) x (R
n
- ES)
Annexes
Depth
0
2
5
10
15
20
T1 [°C]
19.9
19.8
19.8
19.6
19.6
19.5
T2 [°C]
20
19.9
19.9
19.6
19.6
19.5
T [°C]
0.1
0.1
0.1
0
0
0
Layer
0 – 2
2 – 5
5 – 10
10 – 15
15 - 20
Layer depth [M]
2
3
5
5
5
Tavg[°C]
0.1
0.1
0.05
0
0
Depth x
Tavg[M °C]
0.2
0.3
0.25
0
0