The average monthly effective rainfall calculated by Equation 24 can-
not exceed either average monthly rainfall or average monthly ET. If the
application of this equation results in an ER which exceeds either, then
ER must be reduced to the lesser of the two.
Equations 24 and 25 were developed using a daily soil moisture balance
procedure. Such a procedure necessarily fails to consider two factors
which may affect ER. These factors, soil infiltration rates and rainfall
intensities, were not considered because sufficient data were not avail-
able, and they are too complex to be readily considered. If, in a specific
apphcation, infiltration rates are low and rainfall intensities are high,
large amounts of rainfall may be lost to surface runoff. Sloping land
surfaces would further reduce infiltration amounts. In such cases, the ER
values obtained from Equation 23 would need to be modified appro-
priately.
The Everglades Agricultural Area (EAA), as described in Section 3.1
and by Mierau (1974), was used as example to demonstrate the applica-
tion of Equations 24 and 25 for determining effective rainfall. The aver-
age available water capacity in the top 60 cm layer of Pahokee muck
(Lithic Medisaprists) is about 0.3 cm of water per cm of soil (Stewart et
al., 1963). If irrigations are scheduled at 50% of the available water
capacity, the usable soil water storage, D, is 9 cm.
Effective rainfall was computed from Equation 24, using rainfall data
(Table 5), potential ET from the Penman method (Table 6), and D = 9
cm. The results are shown in Table 11. Effective rainfall was computed to
be 825 mm compared with 1494 mm of rainfall (55% of rainfall). How-
ever, if only the irrigation period (Nov.-May) was considered, the effec-
tive rainfall was computed to be 312 mm compared with 468 mm of
rainfall (67% of rainfall).
Field data, based on a water budget for the EAA, showed that the
actual effective rainfall values were 305 mm during the irrigation season
and 805 mm on an annual basis. The good agreement between the field
data and the model predictions for effective rainfall implied that the
effective rainfall estimated from the Soil Conservation Service (1967)
method is a satisfactory approach for the south Florida organic soil
watershed.
4.3 IRRIGATION REQUIREMENT
Irrigation requirement is defined as the quantity of water, exclusive of
precipitation, that is required by a crop growing without water stress. It is
calculated as the difference between evapotranspiration and effective
rainfall, i.e.,
IRR = ET- ER (26)
47