0 R REVIEW
ONLY NOT FOR RELEASE
Preliminary Report of the
Committee on Climatology and Meteorology
FLORIDA WATER RESOURCES STUDY COMMISSION
Werner A. Baum Chairman
Stanley E. Asplund
James P. Clawson
D. E. McCloud
Keith D. Butson
A. 0. Patterson
Homer W. Hiser
Lawrence A. Farrer
John S. Telfair
A. L. Danis
U. S. Allison
Meteorology Dept., Florida State Univ.
Meteorology Dept., Florida State Univ.
Central & Southern Fla. Flood Cone Dist.
Agri. Experiment Station, Univ. of Fla.
U. S. Weather Bureau
Fla. Section, American Water Works Assn.
Weather Radar Laboratory, Univ. of Miami
Corps of Engineers
Florida Engineering Society
Engrg. & Ind. Exp. Station, Univ. of Fla.
Amer. Soc. of Agri. Engrs., Fla. Section
THE ORIGIN AND MOVEMENT OP FLORIDA'S WATER RESOURCES
Water is always on the move. We realize this when we observe the
movement of clouds, or the fall of rain, or the runoff from rainfall on its
way to a nearby stream or lake, or the flow of a stream on its way to the sea.
We also sense the movement of underground water when we see vast quantities
flowing from the numerous springs which dot Florida's countryside. This
movement distinguishes water from most other natural resources. It makes the
state's water resources a reDlenishable one. If this were not true, our
streams, lakes and springs would quickly vanish, leaving only those water
resources which are found in the tidal estuaries and along the ocean shores.
This movement of water is called the hydrologic cycle. Let us examine
the components of this cycle so that we may understand how each acts to
replenish or to delete the water resources of Florida. Later sections of
the report present a more thorough discussion of what is known about each
component and how such knowledge is obtained.
A diagrammatic sketch of the hydrologic cycle is shown in Figure 1.
As the term implies, the cycle of water is without beginning and without end.
Precipitation, surface runoff, ground water, evaporation, and transpiration*
are all stages of the cycle. Of the rain which falls upon the earth, part
falls on water surfaces; part flows over the ground surface until it reaches
a stream, lake or sinkhole; part is shortly returned to the atmosphere by
evaporation from land surfaces and vegetation; and the remainder percolates
into the ground.
* Transpiration is evaporation or loss of water vapor from living plants.
It is the equivalent of perspiration in animals. Since it is difficult to
distinguish the losses resulting from evaporation and transpiration,
measurements of such losses are often combined under the single term
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...... .. ~- -- --- -" "*w WF-.-
A part of the water that enters the ground is retained as soil mois-
ture pear the surface, from where it may be evaporated directly into the at-
mosphere, or it may be taken up by vegetation for transpiration. Another
part of the ground water filters through the soil and forms part of the under
lying free ground water or else enters the rock formations to become artesian
water. Most of the ground water is eventually discharged at the earth's
surface as springs or seepage water, or flows into streams, lakes, canals, or
into the sea. In Florida the water may enter and leave the ground several
times before it is discharged into the sea.
The water flowing in surface streams is derived either directly from
rainfall as surface runoff or indirectly from the discharge of ground water
to the surface. It is this latter source that supplies the streams and lakes
with water during dry weather.
Although the cycle of water is continuous it does not create a uniform
rate of water movement. The average annual value for rainfall over a given
area, for the flow of a stream, for the level of a lake, or for the water
table elevation in a well tells us only what we may expect on the average.
In any year the actual value may fall far short of the average value or it
may greatly exceed that figure. Such variations from the average become even
more pronounced when we study the monthly or the weekly values. It is this
factor of variability that prompts us to exercise management over our available
water resources--to dispose of excess waters in times of flood and to conserve
water for use in times of drought.
Measurement of Water Movements. Measurements are made of several
components of the hydrologic cycle to give us quantitative information on the
movement of water. The instruments used for these measurements include rain
gages, stream gaging stations, evaporation pans, and evapotranspirational
tanks. Because the quantities we are investigating are so great none of thes6
.instruments (except the stream gages) measure the entire amount of water which
we are studying. For example, the Standard 8-inch rain gage has an area of
only 1/80,000,000 of a square mile, and thus records a very small portion of
the total rainfall. There are many areas in Florida where a single rain gage
is used to measure the precipitation on an area of several hundred square
miles. Obviously many rains occur which are not recorded by the gages.
Evaporation pans are used to measure the rate of water lost by direct
evaporation from open water, and evapotranspiration tanks tell us how much
water is returned to the atmosphere by evaporation and transpiration from the
land. These instruments have only a few square feet of area and give us a
very small sample of the total amount of water moving in this component of
the water cycle.
There are other instruments upon which we depend for related informa-
tion but which do not measure the amount of water movement directly. These
are observation wells, lake level gages, and tide gages. Each is useful,
however, and furnishes information from which we may estimate the amount of
water moving in the particular component of the water cycle under study.
Thus we see that it is generally necessary to rely upon estimates and
extremely small samples when we talk about the quantity of water moving in
the hydrologic cycle. Hydrologists and other scientists are able by shrewd
analysis to describe the water movements in Florida quantitatively, but the
need for additional measurements in most areas is apparent.
The primary source of rainfall information is the monthly record
published by the U. S. Weather Bureau. This publication gives rainfall data
from 194 stations in Florida and in watersheds of streams common to the state.
Figure 2 shows the distribution of these stations. In addition there are 257
stations, the data of which are not published. These stations are operated
by various federal, state and private interests, and the records may be
obtained upon request.
-------- --' *'..- ----^-- *t.^.'.-------
An examination of the records discloses many interesting facts.
Florida and the adjacent land areas drained by streams that pass through the
state on their course to the sea (Western Georgia and Southeastern Alabama)
experience precipitation conditions that are quite varied both in annual
amounts and seasonal distribution. Despite the absence of large scale topo-
graphic influences, point rainfall averages, based upon the 25-year period
1931-55, range from as high as 66 inches to as low as 45 inches annually.
Some locations, such as the Florida Keys and not included in this discussion,
average less than 40 inches of rainfall annually. (See Figure 3 for the
climatological discussion used for this report.) The main areas of high rain-
fall are in northern Georgia, extreme northwest Florida, and southeastern
Alabama and at the southern end of the Florida peninsula. Along the lower
east coast of the peninsula, annual point rainfall totals have exceeded 100
inches and have been less than 40 inches. In northwest Florida annual point
totals have ranged from less than 40 inches to nearly 95 inches.
The distribution of precipitation within the year is quite uneven and,
although the seasonal distribution changes markedly from one section to
another, there is an orderly transition from north to south. Figure 4 shows
the maximum, median and minimum values for monthly rainfall in the various
climatological divisions. In Georgia and Alabama there are two high points
(late winter and mid-summer) and one pronounced low point (mid-fall). On the
Florida peninsula the most striking aspects of the precipitation regime are:
1. The dominance of summer rainfall generally more than one-half of the
annual total is observed in the four-month period June through September;
and 2. The rather abrupt entry into and departure from the so-called "rainy
season". June median precipitation is about double that of May, and in the
fall, the median amount for the last month of the wet season is generally
twice that of the following month. Northwestern Florida exhibits a precipi-
tation pattern that, in general, combines the salient features of the areas
to the north and to the south. Here the feature of the double maximum is
preserved but the summer months are still dominant.
Extreme variations in annual area rainfall totals may occur in
consecutive years. 1953 ranks among the wettest years in the 25 year period
1931-55 while 1954 ranks among the driest in a number of sections of the area.
Figures 5, 6 and 7 show the rainfall distribution for 1953, 1954 and 1955,
respectively. It is noteworthy that opposite extremes may occur in different
sections of this large area within the same year. 1954 is generally the
driest of the series in the northern sections of the area, but ranks among
the 5 wettest years in the series in southwest Florida. Wet years or dry
years also can occur in succession. In the northern portion of the area 1947
and 1948 are among the wettest and 1954 and 1955 are among the driest in the
25 year span.
DRIEST AND WETTEST YEARS,
(All values are in inches
TABLE 1. -
Table 1 continued.
Most of the summer rainfall on the Florida peninsula is derived from
local air mass shower or thundershower activity. Numerous locations average
more than 80 thunderstorms per year and some will average nearly 100 annually.
Although most of these thunderstorms are of short duration, occurring in late
afternoon or early evening, relatively large amounts of precipitation in a
short time are not uncommon. Most locations have experienced two-hour rain-
fall totals in excess of 3 inches at one time or another. Because most of
the summer showers are local in character, large differences in monthly and
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annual totals for points relatively close to one another are common. To a
large extent these differences disappear when a comparison is made on the
basis of a long term average; however, large differences in the long term
averages do exist within very short distances. The normal annual rainfall
for Miami Beach, Miami City and Miami Airport are 42.90, 47.20 and 56.41
inches respectively, yet it is less than 10 airline miles from the Airport
to the Beach location. Similar conditions undoubtedly exist elsewhere along
the immediate Florida coast.
In the more northerly areas, precipitation results from two general
causes: 1. Summer rainfall is mostly of the shower and thundershower type;
and 2. Winter and early spring precipitation is more the widespread general
type that results from the interaction between the warm moist tropical air
masses and the colder air masses from the northern interior of the continent.
In the areas north of 30 degrees North Latitude, the greatest positive de-
partures from monthly average rainfall usually occur in the winter or spring--
quite a contrast to the peninsula where the greatest departures generally are
observed in summer or fall.
The tropical storm, although not an annual visitor to this area, does
on occasion release copious amounts of rainfall over relatively large areas.
All portions of the area have felt the effect of a tropical storm of hurricane
or lesser intensity at one time or another. Years without tropical storms,
although more likely to be "dry" than "wet", are not necessarily destined to
be classed as a "dry" year. Recall 1954 when there were no tropical storms,
yet this year shows up to be the fourth wettest in the 25-year period along
the southwest coast and Everglades portion of Florida, and the sixth wettest
in the lower east coast area.
The importance of rainfall as it influences the water cycle, drouth,
and even plant growth is universally recognized. Yet in attempting to balance
the water cycle an unknown quantity, the opposite of precipitation, assumes
major importance. This salient factor is evapotranspiration. It is simply
the upward transfer of water vapor from the soil and plant surfaces into the
atmosphere. It is the combined evaporation/and transpiration by growing
plants. Plants pump a continual stream of water from the soil into the atmos-
phere. From irrigated areas, or from natural areas well supplied with water,
the magnitude of this upward movement of water is tremendous. On a summer
day, with abundant soil moisture, average rates of evapotranspiration in
Florida have been measured at 0.15 inches per day. This rate would account
for a total upward movement of water from the soil into the atmosphere of 142
billion gallons per day from the surface of the state if sufficient moisture
were available. This tremendous upward surge of water may be compared to an
average flow from all of Florida's streams, including the Apalachicola, the
St. Johns and all less/ones of about 40 billion gallons per day. Or it com-
pares to the average spring flow of a little over 3J billion gallons per day
for the myriad springs ranging from Silver Springs down to the smallest in
Florida. This infinitely large rate of evapotranspiration makes our total
usage of water in the state for municipal, industrial, and agricultural uses
of about 3.7 billion gallons per day seem relatively small. Figure 8 gives
a graphic comparison.
The magnitude of this tremendous upward surge of water from vegetated
areas is not generally recognized. Of course, under natural conditions, a
high rate of evapotranspiration is not maintained continuously. Yet for agri-
cultural areas 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. It is the largest single form of
water use. Evapotranspiration is a consumptive use--the water thus used is
lost to the state.
In spite of the importance of evapotranspiration very little is known
about it. Early evapotranspiration studies at the Everglades Agricultural
Experiment Station were among the first in the entire humid eastern United
States. This work is now being carried on in cooperation with the United
States Department of Agriculture, Agricultural Research Bervice, Soil and
Water Conservation Research Branch, Watershed Hydrology Section at Ft. Lauder-
dale and Belle Glade.
Evapotranspiration, in addition to its importance in the movement of
water from the ground into the atmosphere, is important in a similar energy
movement, since large amounts of heat are required to bring about the evapo-
ration process. Thus evapotranspiration is much more than the reverse of
precipitation--it is also a reverse flow to the downward movement of radiation
from the sun. Since solar radiation furnishes the energy required for the
evaporative moisture movement, radiation data can furnish estimates of poten-
tial evapotranspiration. Figure 9 shows estimated potential losses at Miami
for the four-year period 1952-55, togehter with rainfall during the same
Even with these studies little is known of the variations in evapo-
transpiration for the state of Florida--despite its importance in the water
cycle. With the rapid development of Florida its mushrooming population, its
developing industries and agriculture, increased attention must be given to
evapotranspiration commensurate with its importance in the water cycle.
--~~- ~-~--.- ----IL~---C----~--~
The great quantities of water lost by evaporative and transpirative
action in Florida are explained in part by the state's temperature (solar
energy). In general, higher temperatures cause greater rates of water loss
from evaporation and transpiration. Figure 10 shows the average annual
temperatures of the state and illustrates the high annual temperatures of the
peninsular section where these losses are the greatest. In some areas of
central and southern Florida the evapotranspirative losses may equal or exceed
the rainfall. The section of this report dealing with surface water shows
little surface runoff in these areas, indicating the importance of this
component of the water cycle in Florida.
ARTIFICIAL INDUCEMENT OF PRECIPITATION
Ten years have passed since it was demonstrated in the laboratory
that there is a scientific basis for attempting to induce increases in the
yield of rain-bearing clouds by artificial means. The economic potential of
the general porposition led to immediate development of a commercial rain-
making technology. The desire to leave no stone unturned in alleviation of
many real water shortages is responsible for the present situation, in which
a great deal of commercial activity is being carried on without a proper
foundation of basic knowledge and engineering research. Various governmental,
academic and private research organizations have endeavored to carry out the
kinds of studies which are normally prerequisite to technological application!
of a scientific advance, and simultaneously perform functions of evaluation
of the success of the existing technology. Determination of the real effects.
and best methods has suffered by premature adoption of fixed operational
The initial optimistic claims made for artificial weather modification
are now generally considered to have been based upon insecure grounds. The
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chief difficulties in determination of the efficacy of commercial rainmaking
lies within the field of statistics. As a consequence of greater understand-
ing developed in the course of controlled scientific experimentation, however,
there is now little doubt that under proper circumstances of climate and
terrain it will be possible to obtain real and economically important increases
of precipitation by artificial inducement. A very important problem that does
remain as yet unsolved is the characterization of the various sets of circum-
stances in which cloud seeding can hope to succeed, and the development of
operational procedures to fit those categories. Present indications are that
there are in some instances very basic differences between existing commercial
techniques and those which are best suited to the circumstances.
With reference to the climate of Florida in particular, again there
is basis in experimental fact for expectation that effective rainfall-increas-
ing techniques can be worked out. As to the success of past attempts, the
unfortunate fact is that there is too little real evidence. Statistical
evaluation of net change in rainfall for this region simply does not rest
upon a proper basis of background information, controlled experimentation, and
an adequate number of trials. Even in the case of seeding projects in other
regions, where the circumstances are most favorable for the success of present
methods, such shortcomings of the experimental data and data evaluation pro-
cedures are present. These shortcomings place severe limitations on the
confidence in results obtained, even those that apparently show increases in
the rainfall. For Florida the results of reliable evaluation have been
questionable or else have not indicated definite rainfall increases.
Even if a net increase in Florida's rainfall could be demonstrated as
a result of human efforts, there would still remain P question of the economic
wisdom of depending upon such an effect in water resource planning. The natural
rainfall over the long run is quite abundant, and can be husbanded from the
time of feast against the time of famine. The greater expense of taking care
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of periods of rainfall deficiency by use of stored water may be easily compensated by
its reliability. Artificial inducement is not capable of causing rain when it would
otherwise be dry. It can at best only augment precipitation, and that in rough propor-
tion to the natural rainfall potential of the weather situation.
The present state of development is such that artificial inducement is not a
practical tool for use in water resource planning. However, because it has potential
value for this purpose, it merits further attention by the State of Florida.