Title: Water Requirements in Florida
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 Material Information
Title: Water Requirements in Florida
Physical Description: Book
Language: English
Publisher: Industrial Development Magazine
 Subjects
Spatial Coverage: North America -- United States of America -- Florida
 Notes
Abstract: Richard Hamann's Collection - Water Requirements in Florida
General Note: Box 12, Folder 1 ( Materials and Reports on Florida's Water Resources - 1945 - 1957 ), Item 17
Funding: Digitized by the Legal Technology Institute in the Levin College of Law at the University of Florida.
 Record Information
Bibliographic ID: WL00002903
Volume ID: VID00001
Source Institution: Levin College of Law, University of Florida
Holding Location: Levin College of Law, University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Full Text





Agricultural Water Requirements

by
D. E. McCLOUD*


Irrigation of crop land accounts for the largest
use of water in Florida. About one-half million acres
of crops are now irrigated in the state using over 5
million acre-inches yearly. This consumption of
water accounts for almost one half of the state's total
annual water use for all purposes including industrial
and municipal.
Agricultural water use is expanding tremendously.
Recent research has shown that vintage crops of to-
bacco can be grown every year with irrigation. Fruit
and truck crops produce increased yields and higher
quality when supplemental water is added. Pastures
have a longer grazing season when water is controlled.
As a result, irrigation of all agricultural crops is ex-
pected to increase as shown in Figure 1. Estimates in-
dicate that by 1970 agriculture will consume up to
70 per cent of the total water usage in Florida. The
larger part of this increase will come through the ex-
pansion of livestock production on lands where water
control is feasible.
While the magnitude of agricultural water use, as
related to the total water consumption in Florida, is
very important we are also vitally concerned with
the dynamics of crop water use. How much water is
required by a growing crop and when should it be
applied are the questions most frequently asked of
agriculturalists. An understanding of drought is es-
sential in answering these questions. Yet there are a
multitude of definitions of drought. For instance
some workers have said that a drought begins after a
14-day period in which the daily precipitation is less
than .05 inches. Other workers use a 10-day period
with less than .10 inches per day. Still other in-
terpretations are based on a 5-day period with no
significant precipitation, leaving open the meaning of
significant precipitation. All these drought charac-
terizations are purely meteorological accounts of the
time moisture distribution. They have no direct re-
lationship to the point at which a crop plant is
restricted by a lack of available water. Agricultural
drought is characterized by the limitation of water to
the growing plant. Three interacting parameters
define this limit. The quantitative time distribution
of precipitation is only one. Water storage in the root
zone, a function of the rooting depth and the soil's

*Associate Agronomist, Agricultural Experiment Station, Uni-
versity of Florida.


water-holding capacity, is another. The third is the
reverse of precipitation, or the upward movement of
water vapor from the soil through the plant and into
the atmosphere. Plants pump immense quantities of
water from the soil into the atmosphere by this process
which is called transpiration. It is a biophysical pro-
cess which proceeds largely by the physical mechanism
of evaporation. Hence we have the combined word
evapotranspiration which is the reverse of precipitation
and simple the upward transport of water vapor from
a vegetated surface into the atmosphere. A knowledge
of the evapotranspiration rate allows a delineation of
the first question: How much water do agricultural
crops require? Water is needed by crops in quantities
sufficient to maintain evapotranspiration.
The question as to when water should be applied to
agricultural crops depends upon the rate of evapotran-
spiration and also on the water storage in the root
zone. This storage serves as a tank from which the
plant withdraws water. It is refilled by rainfall and
depleted by evapotranspiration. Soil moisture becomes
a resultant of the two-way water transfer. Crop
drought then is determined by agrohydrologic balance
as water moves down by precipitation and returns to
the atmosphere by evapotranspiration. When the
supply tank is emptied soil water is not available in
the root zone to meet plant needs, and a drought
ensues.
Of all the factors influencing drought, evapotran-
spiration is the least understood. It is governed both
by biological factors such as the opening and closing
of the vapor passing holes in the plant's leaves, the


MILLION
ACRE
INCHES


30 1-


20


tnn


Prtpored from
Estimates by:
Depts.of
Agr. Econ.
Agr. Eng.
Agron.





S1970
I 1950


FRUIT FIELD PASTURE
TRUCK
Figure 1.-Agricultural Water Use in Florida.


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I


Figure 2.-Cross Section of a Battery of Thornthwaite Evapotranspirimeters


stomata, as well as by morphological features of the
plant itself. The physical aspects however seem to be
most important. Vapor pressure gradients, tempera-
ture, air movement, each influence evapotranspiration.
Solar radiation, which supplies the energy, is the pri-
mary factor when large masses of vegetation supplied
with ample water are considered. For small plots or
for evaporating pans, advective heat transfer from the
air to the evaporating surfaces brings about exaggera-
ted evaporation rates.
Evapotranspiration can be measured by a very
simple instrument (Fig. 2). Known quantities of water
are added to the tanks containing growing plants to
maintain optimum moisture conditions and allowing
some percolation. The percolate is subtracted from
the original quantity of water added, the difference
being evapotranspiration. Similar plants surrounding
the tanks and watered in like fashion prevent the ad-


Two Inch
Storage '




AGROHYDROLOGIC BALANCE

GAINESVILLE, FLORIDA


vective heat transfer experienced in meteorological
pans. Since evapotranspiration is not widely meas-
ured, it must be related to other commonly measured
factors to be of greatest usefulness. This approach has
been used by several investigators-Thornthwaite,1
Penman2 and others. McCloud and Dunavin3 have de-
rived an empirical formula for the relation of mean
temperature and evapotranspiration:
Potential evapotranspiration = K'W(T-32)
where K and W are constants depending on time and
rate of water-loss and T is the mean temperature in OF.
This formula can be used 1o determine the agro-i
hydrologic balance so important to crop production.
For Gainesville in 1953, the wettest year on record
with over 73 inches total rainfall, a comparison is
easily made between the fluxes of moisture transfer
(Fig. 3). The actual evapotranspiration, with a two-
inch soil storage, was about 53 inches. The evapo-


'00,


1953


Figure 3.-Agrohydrologic Balance for Gainesville, Florida, 1953.


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iraion deficit was almost 18 inches, even in
wet year. Percolation loss was likewise high,
-ting to 20 inches. These values reflect a non-
m distribution of precipitation with respect to
growth. The year 1954 was one of Gainesville's
I" ith a little over 36 inches of rainfall as evi-
Sby the potential evapotranspiration deficit of
inches (Fig. 4). Every month except January
,water shortage. The percolation loss amounted
,about four inches. An assessment of moisture
-y, with respect to crop production, can readily
by this method.
quantitative aspects of Florida's water prob-
Salso be delineated using the evapotranspira-
cept. On an average day following a rain, a
ked evapotranspiration rate of 0.15 inches would
ipt for a total upward movement of water from
~L into the atmosphere of 142 billion gallons per
i~r the state of Florida. This tremendous upward
if water may be compared to an average stream
of about 40 billion gallons per day for all of
Wda's streams. Or it compares to a daily spring
"of less than four billion gallons per day for the


myriad springs in Florida. This infinitely large rate
of evapotranspiration makes our total consumption
of water in the state for municipal, industrial, and
agricultural uses of about 1% billion gallons per day
seem infinitesimally small. The magnitude of this
tremendous upward surge of water from vegetated
areas is not generally realized. Yet this huge water
transport must be maintained to obtain top crop
yields. Supplemental irrigation must be used to supply
that amount which precipitation fails to provide. An
understanding of evapotranspiration is essential also
to all other water problems. This important element
of the hydrologic cycle has a direct bearing on surface
and subsurface waters, water control and even salt-
water intrusion.
I. Thornthwaite, C. W., "An approach toward a rational classi-
fication of climate." Geogr. Rev. Vol. 38 (1): 55-94. 1948.
2. Penman, H. L., "Natural evaporation from open water, bare
soil and grass." Proc. Roy. Soc. London Vol. 193 A): 120-145.
1948. /
3. McCloud, D. E. and Dunavin, L. S. Jr. "Agrohydrologic balance
studies at Gainesville, Florida." The Johns Hopkins University
Laboratory of Climatology Publications in Climatology Vol.
8 (1) 55-68. 1954.


i44


Two Inch
Storage


;ROHYDROLOGIC BALANCE

GAINESVILLE, FLORIDA


P IP'.

41>


q04


1'0-


1954


Figure t.-Agrohydrologic Balance for Gainesville, Florida, 1954.


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