WUP Manual Part C - Southwest Florida Water Management District

Effective rainfall and supplemental irrigation provide a crop’s water requirements. Effective rainfall is
the amount of rain stored in a crop’s root zone. It is based on soil and crop root zone. Supplemental
irrigation is the difference between crop water use (ET) and effective rainfall, divided by the assigned
irrigation efficiency. Equations to calculate effective rainfall are from USDA - NRCS TR-21 “Irrigation
Water Requirements.”
Field conditions and cropping patterns can affect the amount of effective rainfall. The amount of usable
rain on plastic mulched beds could be limited. Most of the crop’s roots are in the bed and cannot directly
utilize rain. Therefore, for permitting purposes, effective rainfall for plastic mulched crop production is
assumed to be zero. This assumption will be used until additional data is available.
c. The supplemental irrigation is calculated as follows:
where
(5) SUP = U – re(d 2/10, s)
SUP = supplemental crop requirement for the growing period in inches
re = effective rainfall, (amount of rain stored in the root zone)
normalized to the 2-in-10 year drought (d 2/10), and the soil
type(s).
The gross irrigation quantity equals the supplemental irrigation requirements divided by the assigned
irrigation efficiency. By using the assigned efficiency the applicant is allocated quantities that include
crop requirements and water lost during delivery. These losses include water supply, conveyance, system
and application.
The assigned supplemental irrigation efficiency is based on the application efficiency. This irrigation
efficiency is a measurement of the effectiveness of an irrigation system to deliver water to a crop’s root
zone. It is the ratio of the water in the root zone to the water pumped. The District generally uses the
average application efficiency values cited in IFAS publications. However, in WUCA’s, where there is
increased competition for the resources, higher than average application efficiencies are used.
d. The allocated supplemental irrigation is calculated as follows:
where
(6) GIR = (SUP/EFas)
GIR = total allocation for supplemental irrigation (inches per acre)
EFas = assigned supplemental irrigation efficiency.
The District uses a computer program to calculate the allocated supplemental crop water requirement.
The program approximates equations (3) through (6) as follows:
(7) f(m) = (t(m) * p(m)) / 100
(8) kt(m) = (0.0173 * t(m)) – 0.314
(9) k(m) = kt(m) * kc(m)
(10) u(m) = f(m) * k(m)
(11) rtl(m) = (0.70917 * (rt(m) ** 0.82416) – 0.11556)
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