Unformatted text preview: 533 Figure 5.69: Class A Evaporation Pan
pan are not standard equipment and should preferably not be used. Pans should be protected by
fences to keep animals from drinking. Panevaporation rates are generally greater than evaporation
rates from large bodies of water, where numerous studies have shown that large water bodies have
evaporation rates far higher near the edge of the water than towards the center where the air is
more saturated and able to absorb less water vapor. The small size of an evaporation pan means
that the whole pan is eﬀectively an ‘edge’ and will have higher evaporation rates than a much
larger bodies of water (Davie, 2002). A second, smaller, problem is that sides of the pan and the
water inside will absorb radiation and warm up quicker than a much larger lake, providing an extra
energy source and greater evaporation rate. Referencecrop evapotranspiration, ETo , is estimated
from pan measurements by multiplicative factors called pan coeﬃcients. Therefore
ETo = kp Ep (5.363) where kp is the pan coeﬃcient and Ep is the measured pan evaporation.
Pan coeﬃcients vary seasonally, are typically in the range of 0.35 to 0.85 (Doorenbos and Pruitt,
1975; 1977), and representative pan coeﬃcients for various site conditions are given in Table 5.47.
Key factors aﬀecting the pan coeﬃcient are the average wind speed, upwind fetch characteristics,
and the ambient humidity. The estimation of evapotranspiration using the pan coeﬃcients in Table
5.47 is commonly called the FAO24 pan evaporation method. This method was published by the
Food and Agriculture Organization (FAO) in paper number 24, hence the name. The pan coeﬃcients given in Table 5.47 are described by the following regression equations (Allen and Pruitt,
1991; Allen et al., 1988)
Surrounded by short green crop:
kp = 0.108 − 0.0286u2 + 0.0422 ln(FET) + 0.1434 ln(RHmean ) −
0.000631[ln(FET)]2 ln(RHmean ) Surrounded by dry area:
kp = 0.61 + 0.00341RHmean − 0.000162u2 RHmean − 0.00000959u2 FET + (5.364) 534 Wind
Light
(< 2 m/s) Table 5.47: Pan Coeﬃcients for Various Site Conditions
Case A: Pan
Case B: Pan
surrounded by
Surrounded by
short green crop
dry, bare area
Mean relative
Mean relative
humidity, %
humidity, %
Upwind
Upwind
fetch of
fetch
green
Low
Med
High
of dry
Low
Med
crop, m
< 40
40–70
> 70
fallow, m
< 40
40–70
1
0.55
0.65
0.75
1
0.70
0.80
10
0.65
0.75
0.85
10
0.60
0.70
100
0.70
0.80
0.85
100
0.55
0.65
1000
0.75
0.85
0.85
1000
0.50
0.60 High
> 70
0.85
0.80
0.75
0.70 Moderate
(2–5 m/s) 1
10
100
1000 0.50
0.60
0.65
0.70 0.60
0.70
0.75
0.80 0.65
0.75
0.80
0.80 1
10
100
1000 0.65
0.55
0.50
0.45 0.75
0.65
0.60
0.55 0.80
0.70
0.65
0.60 Strong
(5–8 m/s) 1
10
100
1000 0.45
0.55
0.60
0.65 0.50
0.60
0.65
0.70 0.60
0.65
0.70
0.75 1
10
100
1000 0.60
0.50
0.45
0.40 0.65
0.55
0.50
0.45 0.70
0.65
0.60
0.55 Very strong
(> 8 m/s) 1
10
100
1000 0.40
0.45
0.50
0.55 0.45
0.55
0.60
0.60 0.50
0.60
0.65
0.65 1
10
100
1000 0.50
0.45
0.40
0.35 0.60
0.50
0.45
0.40 0.65
0.55
0.50
0.45 Source: Doorenbos and Pruitt (1977). 0.00327u2 ln(FET) − 0.00289u2 ln(86.4u2 ) − 0.0106 ln(86.4u2 ) ln(FET) + 0.00063[ln(FET)]2 ln(86.4u2 ) (5.365) where u2 is the average daily wind speed at 2 m height (m/s), RHmean is the average daily relative
humidity (%) calculated by averaging the maximum and minimum relative humidity over the course
of a day, and FET is the upwind fetch (m). In applying Equations 5.364 and 5.365 the following
data ranges must be strictly observed (Allen et al., 1998)
1m≤ FET ≤ 1000 m (5.366) ≤ 8 m/s (5.368) 30% ≤ RHmean ≤ 84% 1 m/s ≤ u2 (5.367) The pan coeﬃcients given in Table 5.47 are applicable to shortirrigated grass turf. For taller and
aerodynamically rougher crops, the values of kp would be higher and vary less with diﬀerences in
weather conditions (ASCE, 1990). It is recommended that the pan be installed inside a short green ...
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 Spring '09
 Miralles
 Water vapor, pan coeﬃcients

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