WEB-BASED IRRIGATION SCHEDULING - Management, Performance, and Applications of Micro Irrigation Systems - page 146

Step 2. Go to the following website address to obtain the appropriate ET o
map(s) for the specific location: http://academic.uprm.edu/hdc/GOES-
PRWEB_RESULTS/reference_ET/. Note that if you are irrigating every day,
then you need only to obtain the ET o for yesterday's date. If, however, one is
irrigating once per week, for example, then one will need to obtain the ET o
values from the maps for the previous week. In this latter example, one will
need to sum up the daily values of ET o to obtain a value of the weekly ET o .
Step 3 . From the K c curve obtained in Step 1, determine a representative value
of K c for the current growth stage of the crop.
Step 4. Estimate the crop water requirement (crop evapotranspiration) using
Eq. (1): ET c = K c × ET o .
Step 5. Estimate the required amount of irrigation in depth units: Irrigation =
[ET c - Rainfall]. If the estimated Irrigation is negative, then one does not need
to irrigate.
It is recommended that rainfall be measured on the farm with a rain gauge, how-
ever, if measured rainfall is not available, the approximate value of the rainfall (de-
rived from NEXRAD radar) can be obtained at the following website: http://academic.
uprm.edu/hdc/GOES-PRWEB_RESULTS/rainfall/. It will also be necessary to mea-
sure the irrigation volume. A digital or mechanical fl ow meter, which measures the
cumulative volume in gallons, is recommended.
The irrigation scheduling approach described above is based on various simplify-
ing assumptions (e.g., surface runoff and deep percolation are ignored). The FAO [1]
has suggested corrections to K c ini for time interval between wetting events, evapora-
tive power of the atmosphere and magnitude of the wetting events, and corrections
to K c mid and K c end for air humidity, crop height and wind speed; however, these
corrections have been ignored in order to preserve simplicity in the approach pre-
sented above. Despite the simplifying assumptions, the approach should signifi cantly
improve water management on a farm if currently there is no irrigation scheduling
method being used.
11.3 DETAILED EXAMPLE PROBLEM
A detailed example problem is presented here to illustrate the use of the proposed
methodology. In this problem, we will determine the irrigation requirement for the
5 day period [February 15-19, 2012] for a tomato crop being grown in Juana Diaz,
Puerto Rico, USA. Table 1 summarizes the information used in the example problem.
Table 2 provides the important web addresses necessary for obtaining data for use in
the example problem.
Step 1: With the information in Table 1, it is now possible to construct the crop
coefficient curve by consulting the FAO Document No. 56, Table 11. (Lengths
of crop development stages for various planting periods and climatic regions)
and Table 12 (Single time-averaged crop coefficients…). FAO Document No.
56 is available online at the web address given in Table 2. Table 3 summarizes
the crop stage and crop coefficient information. The crop coefficient curve
constructed from the data in Table 2 is shown in Fig. 2. The approximate