, . /2-/7-72-Hillsborough County's W accr-Supply Situation by Garald G. Parker -C.P.G. !/ Introduction 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-eluding ground-water and surface-water inflow and outflow, and changes in groundwater 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, 0n.e th~ t is. surrounded by water divi.dP~ across which no flow occu:cs, i_nflow 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 beginning. Hillsborough County is not a hydrologic unit. Both ground-water and surfacewater 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 groundwater 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 !/ Chief Hydrologist and Senior Scientist. Southwest Florida Water M'lnagement District -1-
entire wat e r supply, derived solely from precipit ~tion o n its surfa c e , and therefore is the area w e mus t conside r in thi s study. It is a n area encompassing about 1,633 square miles, a s compared with the county's land are a of 1,038 squar~ m iles. 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 bythat of the Kissinnnee 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 available". This is enough water to supply 1,250,000 persons IF ALL THE FWOD WATERS COULD BE STORED FOR USE" (the italics are mine). And this is the fly in the ointment: All the flpod 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 onethird 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 connnonly done elsewhere by use of surface reservoirs1because 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. -2-
However, we do h ave 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 som~ 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 extending and augmenting Nature's water crop as soon as the experimental tests indicate 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 groundwater 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 instance, precipitation stations are widely scattered1of relatively short duration, and may contain a 10 percent error; the streamgaging is probably incapable of being measure.d better than with a 5 percent margin of error; additionally, the gagingstation records are generally shorter than the precipitation records; and, finally, evapotranspiration cannot be measured directly at all but must be derived by subtracting 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 validity. -3-
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-tenn hydrologic 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. Petersburg and Pinellas County, thus reducing the available and harvestable water crop in Hillsborough County to 428 FIGURE TWO NEAR HERE mgd as shown in figure two. 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 capita use figures. Reliable, comprehensive data are not available to sum up water withdrawals by the following major uses: (1) agriculture, particularly citrus 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 capita uses were derived by dividing total reported withdrawals from both surface-and ground--4-
water sources (except for hydroelectric power generation) by the total number of inhabitants 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 capita 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 for 1970 . County per capita use at 600 gpcdl. 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 Con 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 -ti.n_imum) gpcd (1970 Value, 800 gpcd (Maximum) USGS data) 1960 397,788 ~, 198.9 mgd 238.7 mgd 318.2 mgd 1965 453,000 222.3 mgd 266.9 mgd 355. 9 mgd 1970 490,265 :.I 245.2 mgd 294.2 mgd 392.2 mgd 1975 536,294 ~, 268. 2 mgd 321.8 mgd 429.0 mgd 1980 590,855 s;_I 295.5 mgd 354.9 mgd 472. 7 mgd 1985 654,936 327. 5 mgd 392.9 mgd 532.9 mgd 1990 724,416 s../ 362.2 mgd 434.6 mgd 579.9 mgd a/ from U. S. Census b/ from curve, figure 4 ,! from TBRPC Cohort -survival projection -5-
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 projection, 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, som~ means available to increase or augment the water crop. What are they? Ways and Means of Increasing Our Natural Water Crop When demand ~vithdrawal) 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 hydrologic duty that must be accomplished as soon as possible~ A. Augment present sources by: a â€¢ . 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 2\ 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 dannned-off section of the canals and streams and in the aquifer at depths direc,._tly related to the -6-
height to whi c h fresh-water head c a n b e held above msl e a c h foot o f fresh water above m s l d epresses encroaching s alt wate r by about 40 feet. By holding fre sh-water t o 2 \ feet above msl, salt-wate r would thus be held to -100 feet m s l i n t h e a q u :i.fer . b. Reduce Et (evapotranspiration ) losses. This can bes t 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. F.ducate 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, preferrably 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" -7-
(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 discharge channels and works leading to those parts of the Southwest Florida Wa~er 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 production 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 electrical capacity. 4. In the shore-zone region which has been invaded by salt-water encroachment, almost unlimited supplies of brackish water are available. This water ranges from nearly as salty as ocean waters to only slightly more -8-
salty than normal ground water. Most of it extends a mile o r 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 sunnner 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 previously mentioned. Engineering studies will need to be made to evaluate just how much these alternatives will cost us. Then, with such knowledge, the taxpayers will be in a position to make the necessary choices. B. Mine the aquifer. The Floridan Aquifer and its associated overlying shallow system of watertable 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 -9-
our Gulf Coast either have been lost to salt-water encroachment or are in innninent 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 understandings 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.Go 12/27 /72 -10-
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