Agriculture Reference
In-Depth Information
8.1 INTRODUCTION
The pan evaporation method is widely used to schedule irrigation because it
is easy and inexpensive. The University of Puerto Rico Agricultural Experi-
ment Station (UPR-AES) is currently promoting this method in the “Tech-
nological Package [Spanish]” guidance publications for various crops [1]. A
number of studies have been performed to determine optimal irrigation
rates based on pan evaporation data in Puerto Rico for several crops, such as:
Tanier [2], bananas under mountain conditions [3], bananas under semiarid
conditions [4], plantains under semiarid conditions [5], watermelon under
semiarid conditions [6], and sweet peppers under humid conditions [7, 8].
Harmsen [9] presented a summary of these studies.
The evapotranspiration (ET) can be estimated using pan evaporation method
with the following equations:
ET pan = K c *Et o-pan
(1)
Et o-pan = K p *E pan
(2)
where: ET pan = Actual crop ET, based on the pan-derived reference ET, Et o-
pan ; K p = Pan coefficient; E pan = Class A pan evaporation; and K c = Crop coeffi-
cient. According to Allen et al. [10], estimates of ET from pan data are general-
ly recommended for periods of 10 days or longer. It is recommended that the Eqs.
(1) and (2) should be used for ET estimations usually for periods of four to seven
days in Puerto Rico. Most of the studies have recommended applying water
to plants at a rate equal to 1 to 1.5 times the pan-estimated ET rate to maxi-
mize crop yield. Because this approach is easy and inexpensive, these studies
represent valuable contributions to agricultural production in the tropics.
Problems, however, may result from this approach because of the inher-
ent differences in water loss from an open water surface and a crop [10]. An-
other potential limitation is that only a single value of crop coeffi cient is
commonly used, and by defi nition the crop coeffi cient varies throughout the
season. The magnitude of the crop coeffi cient depends on crop height, leaf
area, crop color, stomatal resistance, and crop maturity.
Although recommended irrigation application rates by this method may
maximize crop yields, the method may also result in the over application of
water early in the crop season, leading to the degradation of groundwater
resources from leaching of agricultural chemicals.
In Puerto Rico, the K p values commonly used were derived from a study
by Goyal and González [11] using data from the seven agricultural substa-
tions located at Adjuntas, Corozal, Juana Díaz (Fortuna), Gurabo, Isabela,
Lajas, and Río Piedras. Figure 1 shows the location of the substations and
the Climate Divisions established by the National Oceanic and Atmospheric
Administration (NOAA). These data were developed on the basis of the ratio
of long-term monthly average reference evapotranspiration (estimated from
an equation) to pan evaporation:
 
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