July 28, 1971
A TENTATIVE WATER-BUDGET ANALYSIS OF
SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT
By Garald G. Parker, C. P. G. 1
A water-budget analysis is, for a given basin or other geographic area,
comparable to a family's financial budget. Let's look at the analogy,
then proceed to development of a water budget for the District. Working
up a water budget for any other area would be done the same way, differing
only in the values we plug into the equation of continuity we use, and
balances total income against total outgo.
Family Budget -- This must include income (salaries or wages, plus earnings
from savings accounts, rentals of owned properties, interest and dividends
from stocks, bonds or other securities, etc.) and outgo (regular of fixed
charges, such as house payments, car payments, utility bills, clothing,
food, dental and doctor's bills, etc. plus money set aside for savings and
investments). We might show it in formula form as:
Salary + Interest on savings +
Dividends on investments
= Fixed and regular costs +
The water budget includes as income precipitation on the area plus ground-
water inflow and surface-waterTinflow into the area; as outgo it includes
evapotranspiration losses, ground-water outflow from the area and surface-
water outflow from the area, plus changes in aquifer and soil-moisture
storage and in surface-water storage (rivers, lakes and reservoirs). This
might be shown in an equation of continuity as follows:
P (precipitation) + I- (ground-water
inflow) + I (surfa e-water inflow)
Shortened this appears:
= R (runoff) + Et (evapotrans ration) +
Ogw (ground-water outflowp+- 0
(surface-water outflow)*+ 4l (ground-
water storage changes) +A w (surface-
water storage changes) +sm (soil-
moisture storage changes).
P + I R + I Et+ +O + +A +A
gw sw gw sw gw sw sm
Fortunately for us at this time we are not going to have to work with all the
values in the equation given above. By use of a year's records, a full cycle
of weather can be included, thus we start and end, theoretically, with the
same general weather conditions so that storage of water in the aquifers, the
streams and lakes and in the soil is relatively the same at the end of the period
of study as it was in the beginning. Thus we can eliminate all the "deltas" in
the equation. This could not be done in a short-term study, nor can it be done
I/ Chief Hydrologist, Southwest Florida Water Management District~
Brooksvillc, Florida 33512.
: ..- ...... ...
where known large changes in storage have occurred. However, for our
District, this can be reasonably done at the present time, thus simplifying
Further simplification can be done when working with a hydrologic unit,
such as a discrete river basin, in which no ground- or surface-water inflow
takes place (nor any importations into the unit from foreign basins). The
District approximates such a unit, so we can drop from our equation the
terms "I and "Isw". Additional simplification can be accomplished by
dropping O and "O "because, in our area, these values are either
included in '' by measurement at the farthest downstream surface-water
gage or are estimated from tidal-flow measurements at or near the mouths
of streams where they enter estuaries or the ocean; additionally, "O may
be estimated by use of slope-area discharge studies and include this value
also in "R". Thus, we can reduce the cumbrous original formula simply to:
P = Et + R
Although we have lost some refinement in our calculation, for purposes
such as estimation of supply available versus demand expected, the ball-park
figure we obtain will be quite satisfactory. This value will be every bit as
reliable, in the present state of our knowledge of water use, water losses,
and water requirements. These, too, are only "ball-park" figures and as
such, constitutes one of our big "water problems".
Estimation of Quantities of Water in a Water Budget for Southwest Florida
Water Management District --
Given: (1) 1 inch of water = 17.4 million gallons per square mile
(2) 10, 000 square miles in the District (approximate)
(3) P = 55 inches per year
(4) Et = 40 inches per year
(5) R = 15 inches per year
1. How Much Water Income Do We Have on an Annual Basis?
P = 55"/yr. = 958. 3 mil/gal/ i2 X 10, 000 n'2 = 9. 58 tgy over SWFWMD
Et = 40"/yr. = 697 mil/gal/mi X 10, 000 mi = 6. 97 tgy over SWFWMD
R = 15"/yr. = 261 mil/gal/mi2 X 10, 000 mi2 = 2. 61 tgy over SWFWMD
But, pumping may lower water table below reach of many plant roots, thus
saving estimated 10% of Et losses.
Thus, Et losses would be reduced to 90% of 6. 97 tgy = 6. 92 tgy.
And, 9. 58 tgy (P) 6. 92 tgy(Et) = 2. 66 tgy (R).
Runoff (R) is the only part of the water in the budget that we can capture for
our use. It is the outflow from our aquifers and streams and can be captured
by means of wells (90+% of our supplies in SWFWMD) or from streams (10-'%,
in SWVFWMD). But, we cannot capture for use all of this 2. 66 tgy in our
budget. This is water that flows in the streams and from the springs as
I I II
. z- .:-..
measured flow, and that seeps or discharges into the bays and oceans as
unmeasured flow. It is this unmeasured part that introduces uncertainty
in the amount of water we call R.
We cannot capture all of R because water must be retained to sustain
stream flow. We need it for esthetic purposes, as well as for fish and
wildlife habitat, for our pleasure to boat upon or to swim in, to carry
barges or other river traffic, to dilute and sweep away pollutants from
the land -- municipal, industrial, agricultural, and human wastes, and
to help prevent inland encroachment of salt water. If we can capture for
our consumptive use more than one-third of R, we will be fortunate in-
deed. Thus, one-third of the runoff from the entire SWFWMD would be:
1/3 x 2. 66 tgy = 0. 89 tgy.
It should be emphasized that the value given above is a ball-park figure,
therefore, should be rounded off to about 900billion gallons. It is a long-
term estimated average of the "water crop" upon which we could reasonably
rely as being available for withdrawal for consumptive use. If we with-
draw more water, we dry up or greatly reduce stream flow, lower the
water level of lakes and swamps, the levels of the ground water, and
actually begin to "mine" stored water from the aquifers. A' little of heavy
overdraft can be tolerated and the shorter the time involved, the better.
Large amounts of overpumping will result in salt-water encroachment and
further deterioration of aquifer supplies as permanent damage,
2. How much water do we need?
The best way we can presently make a reasonable estimate of water this
region will require is to base our estimate on per capital water-use values.
Reliable, comprehensive data are not available to sum up water withdrawals
by the major users: (1) agriculture; (2) self-supplied industry; (3) munic-
ipal; (4) commercial; and (5) self-supplied dwellings, motels and hotels,
and lawn-watering supplies.
The U. S. Geological Survey in their 1965 water-use study of the entire
nation found that the nationwide per capital water use was 1, 600 gpcd (gallons
per capital per day) or 584, 000 gpcy (gallons per capital per year). This
figure was derived by dividing total water withdrawals from streams, lakes,
springs and aquifers (but not including hydroelectric plant water uses), by
the total number of U. S. residents in 1965. I suspect that if we were to
make an exhaustive study of water uses in the Distric t we would find our
per capital use to be about 1, 000 gallons. This contrasts with uses of private
homes in the District of about 80 to 150 gpcd and of municipal water supplies
in the District ranging from about 125 gpcd to 250 gpcd, chiefly depending
upon the amount of water the municipalities supply to industry and to the
amount of lawn and shrub sprinkling done. But our District includes large
users, particularly self-supplied industry (the phosphate industry is a prime
example) and agricultural irrigation, chiefly for watering of citrus groves,
vegetable farms and improved pastures. Altogether the per capital use is
thus much greater than one might surmise from casual considerations. The
1,000 gpcd figure is, I believe, conservative.
At 1, 000 gpcd, or 365, 000 gpcy (gallons.per year per person), the
SWFWMD population of about 1. 75 million persons would require,
for single, or one-time, use only, about:
1. 75 mil persons x 1, 000 gpcd = 1. 75 bgd x 365 da/yr =
638. 75 bgy or, rounded = 0. 64 tgy.
However, single, one-time uses of water occur chiefly only in the shore
area along the Gulf where used water is discharged into the ocean. It is
possible that this 0. 64 tgy is reused at least once. If used once again the
total water crop of 0. 90 tgy is not diminished at all because the amount
returned becomes recharge, or, in a sense is "new water" for the budget.
3. Is there enough water for future needs ?
If all the 0. 90 tgy of the water crop were consumed, it would support a
population of nearly 2. 5 million users at 1, 000 gpcd, and this should see
us through the year 2000 at expected population increases. If, however,
we cannot count on at least one reuse, but might instead get something less,
say 0. 69 tgy, this would support only 1. 9 million additional persons and
we'd be using up all our water income by about 1985 to 1990. Whenever
this time comes we'll be using more water each year than nature supplies
us and we'll begin "mining" water. What might we do to augment our
4. Water Shortage Remedial Measures -- what we might do to augment
When demand (withdrawals) exceeds the average annual replenishment
from nature there are several courses of action that we can take to obtain
the water needed, No attempt is made to place these in order of prefer-
ence (greatest advantage) for each must be evaluated. This has not yet
been done, but must be accomplished at the earliest possible moment.
A. Augment present sources by:
a. Reducing runoff losses to the ocean and Gulf. Some devices might be:
1. Establish and utilize more flood-retention reservoirs.
2. Create recharge facilities in association with such
reservoirs to hurry floodwaters into aquifer storage.
3. Establish salt-water control dams on tidal canals and streams
and place these dams as near the shoreline as feasible. Hold
a fresh-water head behind each dar at least 2 feet above
mean high tide 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 fresh-water head can be held
above msl (mean sea level) -- each foot of fresh-water above
msl depresses encroaching salt water by 40 feet.
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 and human enjoyment.
c. Reduce waste of water:
1. Increase charges for water, including water severance taxes
for large, non-public supplies, so as to obtain the joint bene-
fits of augmenting income (needed to pay for increased
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 those industrial and agricultural
effluents that are practicably reusable. Once-through-the-mill
and then discharge to the ocean 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.
4. Thousands of abandoned artesian wells are now flowing
to waste, depleting the aquifers and causing salt-water
intrusion. Each of these wells should be plugged securely
from bottom to top.
d. Augment present supplies:
S1. Reuse of water (c-2, above) is a means of augmenting
2. Preventing salt-water encroachment is an augmentation
of existing supplies.
3. Recycling sewage wastes is our biggest source of "new"
water. Most municipal sewage is 997o reusable water.
Being run through "tertiary" (extended secondary treat-
ment) 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 ques-
tion is only when shall we do it?
4. 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 discharge 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 SWFMWD, but the
largest potential storage is in the areas of large drawdown
around the phosphate production and irrigational areas,
mostly in Polk, eastern Hillsborough and eastern Manatee
5. Locate and operate well fields and recharge facilities so as i
to manage withdrawals and replacements (recharge) scien-
6. In the shore-zone region which has been invaded by salt-
water encroachment, unlimited supplies of saline water are
available. This water ranges from nearly as salty as ocean
waters to only slightly more salty than normal ground
water. Most of it a mile or so inland is only mildly saline
and can be reclaimed for use. This will be more costly
than use of fresh water (if it were locally available), -but
comparable to the cost of reclaiming sewage effluent. Some
day we will do this, and it may not be far off, particularly
if the coastal counties cannot obtain (or control) the fresh water,
they will have to obtain from outside their county boundaries.
We must remember that the coastal: strip is the downstream end of
nature's pipeline; the upstream end is in those inland counties,
mostly to the east, and generally included within the bound-
aries of the Southwest Florida Water Management District.
7. 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 previously mentioned. Engi-
neering studies will need to be made to evaluate just how
much these alternatives will cost us. Then, with such knowl- i
edge, the taxpayers will be in a position to make the necessary
t I '1l
\ Page Seven
B. Mine the aquifer.
The Floridan aquifer and its associated overlying shallow system of
water-table aquifers contain far more ground water in storage than
all the Great. Lakes combined. Inthe District,for example, the upper
2000 feet is generally filled with fresh water inland from the 40 foot
contour on the potentiometric surface. However, salt water under-
lies 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,
have already lost their downtown wells to salt-water encroachment.
And thousands of private wells in the shore-zone that extends
generally inland to about the 10 foot contour on the potentiometric
surface 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 hydro-
logic understandings that will serve to guide us until better until
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.