green cover surface, actively growing, and well-watered (such as turf or a
grass-covered area).
Historical daily averages of Penman-method ETo values are available for
six Florida regions expressed in units of acre-inches and gallons per acre
(Table 4). While these values are provided as guidelines for management
purposes, actual values may vary above and below these values, requiring
individual site adjustments. Actual daily values may be as much as 25%
higher on days that are hotter and drier than normal or as much as 25%
lower on days that are cooler or more overcast than normal. Real time ETo
estimates can be found at the Florida Automated Weather Network (FAWN)
internet site (). For precise management, SWT or soil
is the ratio of ETc to the reference value ETo (Eq. 2). Because different
which were derived using the same ETo estimation method as will be used
to determine the crop water requirements. Also, Kc values for the appropriate
stage of growth (Table 5 and 6; Fig. 3-3) and production system (Tables
7 and 8) must be used.
With drip irrigation where the wetted area is limited and plastic mulch
mulch use. Plastic mulches substantially reduce evaporation of water from
the soil surface. Associated with the reduction of evaporation is a general
increase in transpiration. Even though the transpiration rates under mulch
may increase by an average of 10-30% over the season as compared to
no-mulched systems, overall water use values decrease by an average of
10-30% due the reduction in soil evaporation. ETo may be estimated from
factor. During days without rainfall, ETo may be estimated from evaporation
from an ET gauge (Ea) as ETo = Ea/0.89. On rainy days (>0.2 in) ETo =
Ea/0.84.
Eq. 2 Crop water requirement =
ETc = Kc x ETo
SOIL WATER HOLDING CAPACITY AND THE
NEED TO SPLIT IRRIGATIONS
Appropriate irrigation scheduling and matching irrigation amounts with
the water-holding capacity of the effective root zone may help minimize
the incidence of excess leaching associated with over-irrigation. In Florida
sandy soils, the amount of water that can be stored in the root zone and be
available to the plants is limited. Usually, it is assumed that approximately
0.75 in of water can be stored in every foot of the root zone. Only half of
that should be used before next irrigation to avoid plant stress and yield
reduction (this will help maintain SWT below 15 cb). Any additional water
will be lost by deep percolation below the root zone.
Table 8 gives approximate amount of water that can be applied at each
event in Florida sandy soil under different production systems. When the
calculated volume of water to be applied in one day exceeds the values
in Table 7, then it is necessary to split applications. The number of split
irrigations can be determined by dividing the irrigation requirement (Eq. 1)
Table 3.4. Historical Penman method reference evapotranspiration (ETo) for six Florida regions expressed in (A) inches per day and (B) gallons per acre per day1.
Month Northwest Northeast Central Central West Southwest Southeast
Inches per day (A)
JAN 0.06 0.07 0.07 0.07 0.08 0.08
FEB 0.07 0.08 0.10 0.10 0.11 0.11
MAR 0.10 0.10 0.12 0.13 0.13 0.13
APR 0.13 0.14 0.16 0.16 0.17 0.17
MAY 0.16 0.16 0.18 0.18 0.18 0.18
JUN 0.17 0.16 0.18 0.18 0.18 0.17
JUL 0.17 0.16 0.17 0.17 0.18 0.18
AUG 0.15 0.15 0.17 0.16 0.17 0.16
SEP 0.13 0.13 0.14 0.14 0.15 0.14
OCT 0.09 0.10 0.11 0.11 0.12 0.12
NOV 0.07 0.07 0.08 0.08 0.09 0.09
DEC 0.05 0.06 0.06 0.07 0.07 0.07
Gallons per acre per day2 (B)
JAN 1629 1901 1901 1901 2172 2172
FEB 1901 2172 2715 2715 2987 2987
MAR 2715 2715 3258 3530 3530 3530
APR 3530 3801 4344 4344 4616 4616
MAY 4344 4344 4887 4887 4887 4887
JUN 4616 4344 4887 4887 4887 4616
JUL 4616 4344 4616 4616 4887 4887
AUG 4073 4073 4616 4344 4616 4344
SEP 3530 3530 3801 3801 4073 3801
OCT 2444 2715 2987 2987 3258 3258
NOV 1901 1901 2172 2172 2444 2444
DEC 1358 1629 1629 1629 1901 1901
1
2 Calculation: for overhead or seepage irrigation, (B) = (A) x 27,150. To convert values for drip irrigation (C) use (C) = (B) x bed spacing / 435.6. For example, for 6-ft bed
spacing and single drip line, C in Southwest Florida in January is C = 2,172 x 6/ 435.6 = 30 gal/100ft/day.
2019 Vegetable Production Handbook of Florida 15