A Preliminary Study of
SHillsborough County's Water-Supply Situation
'h Garald G. Parker C.P.G. 1/
This analysis is based on the water-budget method as a means of determining how
much water is available for use in Hillsborough County. Then, comparing the quantity
available with the quantity required now and as forecast for future needs, it is
readily apparent how well or badly the supply matches the expected requirements.
The water budget is based on the equation of continuity which, in its simplest
form is: P (precipitation) = Et (evaporation) + R (runoff). Other elements, in-
cluding ground-water and surface-water inflow and outflow, and changes in ground-
water and surface-water storage must also be included if they are of such magnitude
as.to be of consequence. By choosing an area that is a hydrologic unit, that is,
one that is surrounded by water divides across which no flow occurs, inflow and
outflow factors can be ignored. And, if the budgeted period is long enough to
begin and end at the same time of the year, normally the changes in storage can
also be ignored because a complete cycle of wet and dry seasons will have been
completed and the water balance at the end of the period is the same as at the
Hillsborough County is not a hydrologic unit. Both ground-water and surface-
water inflow take place across its northern, eastern and southern borders from,
respectively, adjacent parts of Pasco, Polk and Manatee Counties. But by drawing
hydrologic boundaries on the potentiometric surface map to coincide with ground-
water and, incidentally and fortunately with surface-water divides, a hydrologic
unit as shown in figure one, can be drawn that is not greatly larger than the
county. Actually, it is this larger area from.which Hillsborough County draws its
1/ Chief Hydrologist and Senior Scientist. Southwest Florida Water Management District
I 6i L
entire water supply, derived solely from precipitation on its surface, and there-
fore is the area we must consider in this study. It is an area encompassing about
1,633 square miles, as compared with the county's land area of 1,038 square miles.
FIGURE ONE NEAR HERE
The Florida Geological Survey, in 1961, published a report (R.I. No. 25)
entitled "Water Resources of Hillsborough County, Florida", prepared by C. G. Menke,
E. W. Meredith and W. S. Wetterhall of the U. S. Geological Survey. It was one of
the earlier water-budget studies made in Florida, preceded only by that of the
Kissimmee River Basin, reported in U. S. Geological Survey Water-Supply Paper 1255,
by Garald G. Parker and others, and published in 1955.
The present report derives essentially the same water budget values as that of
Menke and others but reaches a vastly different conclusion. Menke and others state
(p.17) that "an average of 1,400 mgd (million gallons a day) is potentially avail-
able". This is enough water to supply 1,250,000 persons IF ALL THE FLOOD WATERS
COULD BE STORED FOR USE"(the italics are mine). And this is the fly in the ointment!
All the flood water cannot be stored, in fact very little of it can be saved for
later use. Practically, we cannot expect to harvest a water crop exceeding one-
third of this 1,400 mgd or, based on Menke and others, about 467 mgd. And, for
once-through-the-mill uses,we will be lucky to capture this much of our potential
water crop. We have no means of storing much of our flood waters, as is commonly
done elsewhere by use of surface reservoirs because our entire District lacks large
and deep valleys in which capacious and economic reservoirs can be built to hold
such flood waters. Likewise, in non-flood times we cannot withdraw all, or even
much, of the streamflow. We must leave most of it to keep the streams flowing, to
prevent the marshes and swamps from drying out, to provide for recreational water
uses such as swimming, boating, and fishing, to provide water to dilute and carry
off industrial, agricultural and municipal wastes, to benefit fish and wildlife,
and last but not least, to help maintain a beauty of the landscape that we all
enjoy and treasure.
However, we do have at our disposal a natural subterranean reservoir of vast
potential for storage of billions of gallons of excess water, but to date we are
only beginning to investigate its uses. By means of artificial recharge some of
our otherwise wasted flood waters and our cleaned-up and reclaimed previously used
waters could be stored underground for subsequent reusal. Such recycling and reuse
of waters is both practical and needed. We need to get on with this method of ex-
tending and augmenting Nature's water crop as soon as the experimental tests indi-
cate the best ways to do it. As an example, if our entire water crop of 488 mgd (see
p.4)were used over only once the water crop would be doubled -- to 976 mgd. This
is a goal to be sought.
Reliability of Quantity of Water Data
Precipitation over Hillsborough County, as a long-term average, is estimated
by the U. S. Weather Bureau to be about 53.8 inches annually. Runoff and ground-
water outflow to the Bay is estimated from U. S. Geological Survey data to be about
18.8 inches of which 18.3 inches is streamflow and 0.5 inches is ground-water flow.
Evapotranspiration, the other major item in the hydrologic budget, is the residual--
35 inches, or about 65 percent of the precipitation. To put this in the equation of
continuity: P = R + Et; P (53.8") = R (18.8") + Et (35") and the equation is in
balance. However, none of these values can be measured with precision. For in-
stance, precipitation stations are widely scatteredof relatively short duration,
and may contain a 10 percent error; the streamgaging is probably incapable of being
measured better than with a 5 percent margin of error; additionally, the gaging-
station records are generally shorter than the precipitation records; and, finally,
evapotranspiration cannot be measured directly at all but must be derived by sub-
tracting R from P when all other items in the water budget can be balanced out.
The reliability of our derived values of the water crop are thus limited by the
reliability of the data going into the equation, probably of about 90 percent
,JUll 1, i 1i ,l li
As regards our water crop, runoff (R) is the upper limit of cropping and, as
we have said earlier, this is about 1/3 R.
Quantity of Water Available for Use
To estimate the quantity of water available for use, based on long-term hydro-
logic data averages, we go about it like this: One inch of runoff (R) from one
square mile during one year amounts to 17.4 mgy (million gallons a year), and the
Hillsborough County water-catchment area covers 1633 square miles. Precipitation
(P) over this area averages about 53.8 inches per year, of which 35 inches, or 65
percent, are lost shortly after falling on the land surface. This leaves a potential
water crop of 18.8 inches of which, as we've previously said, we would be lucky to
capture more than one-third. One-third of 18.8 inches is 6.27 inches per year per
square mile. Multiply 6.27 inches by 17.4 mgy and we find the water yield to be
109.1 mgy per square mile. Multiply this by the total number of square miles in
the water-contributory area to Hillsborough-County and we obtain 178,160 mgy or about
488 mgd. However, about 60 mgd of this is currently being exported to St. Peters-
burg and Pinellas County, thus reducing the available and harvestable water crop in
Hillsborough County to 428 mgd as shown in figure two.
FIGURE TWO NEAR HERE
Quantity of Water Needed
Until better data are available on consumptive use of water in Hillsborough
County, our best means of deriving a reasonable estimate is to base our values on
selected per capital use figures. Reliable, comprehensive data are not available to
sum up water withdrawals by the following major uses: (1) agriculture, particularly
citrms irrigation; (2) self-supplied industry, particularly phosphate and citrus;
(3) municipal (although this is fairly well documented); (4) commercial; (5) and
self-supplied hotels, motels and dwellings plus lawn-watering supplies.
Figure three is a graph showing U. S. Geological Survey's derived data regarding
water use on a nation-wide basis, 1955-1970, with my projections to 1990. Per capital
uses were derived by dividing total reported withdrawals from both surface- and ground-
water sources (except for hydroelectric power generation) by the total number of inhabi-
tants of the United States. An increasing use-curve is noted beginning with 900 gpcd
'in 1955 and standing at 1800 gpcd in 1970 -- a two-fold increase in only 15 years
Here in Hillsborough County we do not have the large uses of the industrial East
or the agricultural West, but industry, agriculture and commerce in Hillsborough County
are large enough to have caused the U. S. Geological Survey (in the Menke and others
report) to estimate the per capital use then (1960) to be 1,100 gpcd. Based on the
U. S. Geological Survey canvass of water use in Hillsborough County during their 10-
year recurring national water-use study, the Survey now estimates the Hillsborough
County per capital use at 600 gpcd/. For our current values I am using two enveloping
curves as shown on figure two. The higher, maximum-use curve A is based on a per/capita
use of 800 gpcd and the lower, minimum-use curve C on 500 gpcd. Between these two is
the estimated actual-use curve B of 600 gpcd based on the U. S. Geological Survey
water-use inventory of 1970.
Next, to estimate current use and to project future water demands, population
forecast data as developed by the Tampa Bay Regional Planning Council were utilized to
develop the following table and the demand curves shown on figure four:
Year PopUlation Water use at Water use at 600 Water use at
500 gpcd t4inimum) gpcd (1970 Value, 800 gpcd (Maximum)
1960 397 788 a/ 198.9 mgd 238.7 mgd 318.2 mgd
1965 453 000 b/ 222.3 mgd 266.9 mgd 355.9 mgd
1970 490 265 a/ 245.2 mgd 294.2 mgd 392.2 mgd
1975. 536 294 a/ 268.2 mgd 321.8 mgd 429.0 mgd
1980 590 855 c/ 295.5 mgd 354.9 mgd 472.7 mgd
1985 654,936 c/ 3?7.5 mgd 392.9 mgd 532.9 mgd
1990 724,416 c/ 362.2 mgd 434.6 mgd 579.9 mgd
a/ from U. S. Census
b/ from curve, figure 4
c/ from TBRPC Cohort survival projection
;il ; A 1.
Comparison of Water Needs with Water Availability
A comparison of water-supply curve E with the three demand curves on figure
two shows that curve A, which I regard as the one most likely to be applicable as
the county becomes more urbanized and industrialized, crosses the supply curve at a
point in late 1975. Demand curve C crosses the supply curve E only by extended pro-
jection, perhaps about the year 2010. But demand curve B, based on 1970 reported
uses of water in the county, crosses the supply curve E in 1990. Probably the real
value lies somewhere between curves A and B and the cross-over point may well be in
the 1980-1985 interval. At whatever time the demand curve crosses the supply curve
we will then be using up all our natural, annual supply of fresh water and we will
then begin "mining" the ground-water. But there are, as mentioned earlier, some
means available to increase or augment the water crop. What are they?
Ways and Means of Increasing Our Natural Water Crop
When demand (withdrawal) exceeds the average annual replenishment from nature,
there are several courses of action that can be taken to obtain the additional water
needed. No attempt is made here to place these in order of preference (greatest
advantage) for each must be evaluated. This has not yet been done, but is a hydrolo-
gic duty that must be accomplished as soon as possible.
A. Augment present sources by:
a. Reducing runoff losses to the Bay. Some devices might be:
1. Establish and'utilize more flood-retention reservoirs.
2. Create aquifer recharge facilities in association with such reservoirs to
hurry flood waters into aquifer storage.
3. Establish salt-water control dams on canals and streams entering the Bay,
and place these dams as near the Bay as feasible. Hold a fresh-water head
behind each dam at least 2k feet above msl (mean sea level) and higher if
possible. These dams will not only prevent bleeding off of fresh-water,
but will prevent salt-water encroachment both in the dammed-off section of
the canals and streams and in the aquifer at depths directly related to the
height to which(~esh-water head can be held abcn msl -- each foot of fresh-
water above msl depresses encroaching salt water by about 40 feet. By holding
fresh-water to 2k feet above msl, salt-water would thus be held to -100 feet
msl in the aquifer.
b. Reduce Et evapotranspirationn) losses. This can best be accomplished
by lowering the water table in swampy and marshy places below the reach
of water-wasting plants. Choices of areas will have to be made to decide
what areas can be utilized and what ones not used. Some areas must be
saved from lowering the water level in order to preserve natural forest and
swamp environments for esthetics as well as sanctuaries for wildlife. Our
large well-fields are prime examples of how efficiently this works.
c. Reduce waste of water:
1. Increase charges for water, particularly for large users, so as to
obtain the joint benefits of augmenting income (needed to pay for in-
creased costs of water supply and management) and causing water users
to be concerned with wasting. The more costly the water, the less
the people are likely to waste it.
2. Insist on reuse of water for industrial and those agricultural uses
that permit reuse. Once-through-the-mill and then discharge to the
Bay should not be tolerated.
3. Many irrigators now put far too much water on their crops. Educate
irrigators to crop needs and allow only what is really required. To
avoid excessive irrigation losses due to evaporation, spray irrigation
-should be done at night, preferably in the pre-dawn hours.
4. Hundreds of abandoned artesian wells are now flowing to waste in
Hillsborough County, particularly in the Ruskin area, depleting the
aquifers and causing salt-water intrusion. Each of these wells should
be plugged securely from bottom to top.
d. Augment present supplies:
1. Recycling sewage wastes is one of our biggest source of "new" water.
Most municipal sewage is 99% reusable water. Being run through "tertiary"
(extended secondary treatment) to reduce impurities of all kinds to:.
be at least as good as water naturally available in the aquifers and
streams of the area, would make such water available for reuse and
essentially make this region's water supply self-sufficient for the
next thirty years or so. This can be done, but at a cost. It is a
cost that, eventually, we must pay. The question isn't if we should
do it, the question is only when shall we do it?
2. Capture as much of flood flow as we can and inject it into the only
large storage reservoir we have -- the Floridan Aquifer. This can be
accomplished best by developing flood retention reservoirs with dis-
charge channels and works leading to those parts of the Southwest
Florida Water Management District where large drawdowns of water
level have created billions of gallons of available storage volume.
Some such storage capacity exists in the areas of pumping influence
from every large well field in the TBR, but the largest potential
storage is in the areas of large drawdown around the phosphate pro-
duction and irrigational areas, mostly in Polk, eastern Hillsborough
and eastern Manatee Counties, where over hundreds of square miles the
potentiometric surface of the Floridan Aquifer has been lowered 60
feet or more since 1949.
3. Locate and operate well fields and recharge facilities so as to manage
withdrawals and replacements (recharge) scientifically. The well
fields should all be part of a regional water-supply system, hooked
together much as the electrical industry has regionalized their elec-
4. In the shore-zone region which has been invaded by salt-water encroach-
ment, almost unlimited supplies of brackish water are available. This
water ranges from nearly as salty as ocean waters to only slightly more
" ,. i l I I
salty than norm ground water. Most of it extens a mile or so inland
from the Gulf of Mexico, is only mildly saline, and can be economically
reclaimed for use. This will be more costly than use of fresh water
(if it were locally available), but has recently become comparable to
the cost of transporting fresh water from distant well fields. The
new reverse osmosis (RO) method now in use at the 500,000 gpd Rotunda
West water-treatment plant in southwestern Sarasota County was installed
at a total cost of $385,000 in the summer of 1972 and is expected to
produce fresh water at about fifty cents a thousand gallons. More
such plants are needed in our coastal areas.
5. Import water from great distances, such as from Weekiwachee Springs,
Chassahowitzka Springs, Homosassa Springs and others. But this will
be extremely costly, probably much more costly than other means pre-
viously mentioned. Engineering studies will need to be made to eval-
uate just how much these alternatives will cost us. Then, with such
knowledge, the taxpayers will be in a position to make the necessary
B. Mine the aquifer.
The Floridan Aquifer and its associated overlying shallow system of water-
table aquifers contains far more ground-water in storage than all the Great
Lakes combined. In the TBR, for example, the upper 2,000 feet is generally
filled with fresh water inland from the 25-foot contour on the potentiometric
surface. However, salt-water underlies this aquifer everywhere and bounds it
on the west all along the shore. If the aquifer is overpumped, salt-water
encroachment follows. Tampa and St. Petersburg, to name only two large users,
lost their downtown wells to salt-water encroachment in the late 1920's. And
thousands of private wells in the shore-zone that extends generally inland to
about the 10 foot contour on the potentiometric surface all along the Bay and
our Gulf Coast either have been lost to salt-water encroachment or are in
imminent danger of becoming lost.
Great care must be taken that the aquifer not be mined of its fresh water with
resultant salt-water encroachment. Detailed research must be made to develop
better knowledge of the aquifer's hydrologic characteristics so that realistic,
effective management decisions can be reached.
Right now we have some usable generalized information and hydrologic under-
standings that will serve to guide us until better and more detailed data are
available. We can make do, then, for a while. But, we can't afford to dally.
The situation is upon us now.
Garald G. Parker C.P.G.
Ii / ,*
Pull I" .
I Ayict "Sj _-. *,-, .. I S 5 ~
-'' 5* ,. 55
Bigl Bayou Pil H-l 11Y1 1 :L.
Caad's O z one,,, '
jrearater -~~ .: '5 II /: -:-
rda C., hi ~ 4
Aricfof j .:.;:h i\.5
Ii .~i I.: '.'.
tl .. t
P c ,'.S '2"i\!.:- .
D LC i n
,lea rwaiet-C" -------
i I M
Trasr W n Ll .
It ,?he/u' Beac IA .
Pass-a-Grille Beach N r7-j-
Egmo'st Kyey -t; !6 s 62 'r I
'04 A ......
Anna Maria h\
Holmes 'ea I 51' yd
DEP N I OJE :'lWH OF lU ofF PE i/F MI tEA
YIELDS 17.4 M -YI. AREA jOF LAN q PRo>UCID)&J HILLSBOo-H4
CouTY, T po coL I S I1G33M / PIREClT PIATTIOI OV E' THIS
AE. .___..EAAV.^l RA S 3U. .ttU.MPF,; &LJt.n ^ '3 YLO
AND O.5 CU-1oELJD-L/A .- FLOW I5 1,./ E V. POTrAF iIFVATIOP \
Lo.ssIs ARE3 WE2 MAY BE A'LE T Ce.APTU rL rE s3 oP U~Lf=.)
S/3x 189. 6.H.7A THE ATTA I A'SLbi.z 1 or R 0M 01 M 17
6.27'x /7.-4 r ""al AY.- P-ii J /M1fM MXI rH I
-76 6I MAGY 35 DAYS /YRY 4?8. 7TH 1IS IS OUR LONCG-T ER
AV5-RAC E WATER ROP. ISUT 60.'i-D 1S EXPCrTE- T sT*T PT
P!JM-LLA!, CO. T7 U$, u, cUR RLR- T H iO ?FP IS 42z. M&D .
:TH RE-E DE-!AK) CYRVE .S ARE -SHOCN CUrLIE 13, GCO CPC.D, 15 TQOM
THE U S S;' CURVES: A. C ARE EST.I:ATE D LVAi-'/ M /L P
SlNiL VALL/ ,
o USE IJ crmD T
.y PROTExCTED IN.
E- rPOR TD eY'
Y~"le !57 L.)t)1
U ~,~i- -*
-- -- -- -- t -- -- -- --i -- -- ----- -------- -----------------4 --
_ I i
5.3 NATIONAL PATE OF WATET USE
S-i 19 -2EC 9I
1z 3. POLK 44
00 4. POLK 45
I 90 ---
CC' h ^
5. POLK 51
HYDROGRAPHS OF SELECTED WELI
WITH LONG-TERM RECORDS
Observation well measured in 0-
1949 and 1969
Number indicates obsercntion uwll
for which hydrugraph is sho'n
Boundary of Southwest Florida 0 10 20 30MILES
Water Management District -- I -
CHANGE OF POTENTIOMETRIC SURFACE,
0 Note: Decline-in potentiometric surface is based partly on actual differences in water levels
measured in 18 wells during the period and partly on the difference between potentio-
metric surfaces shown on maps of September 1949 and May 1969.
The 1949 piezometric map of the principal artesian aquifer in Florida and Georgia
was prepared by the U.S. Geoloi;ical Survey in cooperation with the Florida Geological.
Survey and the Georgia Division of Mines, Mining, and Geology and published by the
Florida Geological Survey. September 1949. The 1949 map represented the aggregate
of water-level information as of 1949