Agriculture Reference
In-Depth Information
FIGURE 5
Example of the cumulative irrigation and ET c plotted with time for a crop season.
Note :
This graph is not related to the example problem in this chapter.
Step 2: Determine the reference evapotranspiration for the five day period.
Figure 3 shows the estimated reference evapotranspiration for Puerto Rico on
February 15, 2012 obtained from the web address provided in Table 2. Note
that the preferred reference evapotranspiration method is used (i.e., Penman-
Monteith method). The estimated ET o for the site location on Feb. 15, 2012 is
2.95 mm. Using a similar procedure, the ET o for Feb. 16, 17, 18 and 19 is 2.8
mm, 3.1 mm, 3.5 mm and 3.7 mm, respectively. Summing up the ET o values
comes to a total crop water requirement (for the five days) of 16.1 mm.
Step 3: A rain gauge is not available on or near the farm for the example prob-
lem; therefore it is necessary to obtain rainfall information from the NEXRAD
radar. Figure 4 shows the NEXRAD rainfall for Puerto Rico for February 15,
2012. At the site location no rainfall was estimated from the NEXRAD radar.
Checking the other maps for the other days reveals that no significant rainfall
occurred at the site. Therefore, all of the crop water requirement will have to
be satisfied with irrigation.
Step 4: The crop water requirement for the time period can now be estimated
as follows:
ET c = K c ET o = (0.85)(16.1 mm) = 13. 7 mm (slightly greater than one-half of
an inch).
Step 5: Determine the number of hours that the pump should be run to apply
the 13.7 mm of water. A form of the well-known irrigation Eq. (3) [2] can be
used which is shown below:
 
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