ESTIMATION OF PAN EVAPORATION COEFFICIENTS - Management, Performance, and Applications of Micro Irrigation Systems - page 111

• Reflectionof solar radiation from the water surface might be dif-
ferent from the assumed 23% for the grass reference surface.
• Storage of heat within the pan can be appreciable and may cause
significant evaporation during the night while most crops tran-
spire only during the daytime.
• Thereare differences in turbulence, temperature and humidity of
the air immediately above the respective surfaces.
• Heattransfer occurringthrough the sides of the pan can affect the
energy balance.
Monthly average pan coeffi cients were estimated for each month at each
of the seven experimental substations on the basis of pan evaporation data
from 1960 (approximate) to 1980 and from 1981 to 2000. (For convenience,
hereafter the earlier period will be referred to as 1960 to 1980 and the latter
period as 1981).
A Student t-Test analysis indicated that the difference between the
mean K p based on the two time periods was highly signifi cant. Table 2 pres-
ents the results of the t-Test. The difference in the mean K p for all locations
for the two time periods was 0.15. The average K p equaled 0.75 for 1960 to
1980 and 0.91 for 1981 to 2000. A comparison was also made between the K p
values of Goyal and González [11] and the 1981 to 2000 K p values from this
study (See Table 5).
A signifi cant difference was observed between the two datasets at the
0.01% probability level, with a difference in the mean K p of 0.08. The average
value of the K p of Goyal and González [11] was 0.82.
To understand whether the difference in the mean pan evaporation be-
tween the two periods [1960 to 1980 and 1981 to 2000] is signifi cant on a
practical level (independent of statistical signifi cance), Eq. (2) was used to
estimate the difference in the reference ET for a given amount of pan evapo-
ration. Suppose the annual pan evaporation for a certain location was 1500
mm; then the K p difference of 0.15 is equivalent to [ 0.15 × 1500 mm] = 225 mm
in the annual reference ET. For an average farm size of 18 hectares in Puerto
Rico [21], this is equivalent to 40,500 m 3 of water (or 10.7 million gallons).
Because there was a signifi cant difference between the mean K p for the
last 20 years and that of the subsequent 20-year period, this study recom-
mends that crop water use estimates use K p values from the most recent 20
years. Tables 3, 4 and 5 give the average monthly reference ET, pan evapora-
tion and pan coeffi cients, respectively.
The methodologies used in this paper can be considered suffi ciently general and
therefore could be applied at other locations throughout the world.