Citation
Control of lake levels in Orange County, Florida ( FGS: Information circular 47 )

Material Information

Title:
Control of lake levels in Orange County, Florida ( FGS: Information circular 47 )
Series Title:
FGS: Information circular
Creator:
Anderson, Warren
Lichtler, William F
Joyner, Boyd F
Geological Survey (U.S.)
Place of Publication:
Tallahassee
Publisher:
Florida Geological Survey
Publication Date:
Language:
English
Physical Description:
iii, 15 p. : illus., map. ; 23 cm.

Subjects

Subjects / Keywords:
Lakes -- Florida -- Orange County ( lcsh )
Hydrology -- Florida -- Orange County ( lcsh )
Orange County ( local )
City of Orlando ( local )
Lakes ( jstor )
Rain ( jstor )
Evaporation ( jstor )
Genre:
non-fiction ( marcgt )

Notes

General Note:
"Prepared by the United States Geological Survey in cooperation with the Florida Geological Survey and the Board of County Commissioners of Orange County."
Funding:
Digitized as a collaborative project with the Florida Geological Survey, Florida Department of Environmental Protection.
Statement of Responsibility:
by Warren Anderson, W. F. Lichtler, and B. F. Joyner.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
The author dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law and all related or neighboring legal rights he or she had in the work, to the extent allowable by law.
Resource Identifier:
022267463 ( aleph )
01310850 ( oclc )
AJA4768 ( notis )
a 65007606 ( lccn )

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Full Text
STATE OF FLORIDA
STATE BOARD OF CONSERVATION
DIVISION OF GEOLOGY
FLORIDA GEOLOGICAL SURVEY
Robert 0. Vernon, Director
INFORMATION CIRCULAR NO. 47
CONTROL OF LAKE LEVE LS IN ORANGE COUNTY, FLORIDA
By
Warren Anderson, W. F. Lichtler, and B. F. Joyner
Prepared by the
UNITED STATES GEOLOGICAL SURVEY
in cooperation with the
FLORIDA GEOLOGICAL SURVEY and the
BOARD OF COUNTY COMMISSIONERS OF ORANGE COUNTY
TALLAHASSEE
1965




AGRI
CULTURAL
IBRARY
Completed manuscript received December 8, 1964
Printed by the Florida Geological Survey-22
Tallahassee
1965




Introduction ............. .... ..................... 1
Factors that influence lake levels .............................. 2
Rainfall .......................... ........................ 2
Evaporation............................. ................. 2
Relationship of rainfall and evaporation to changes in lake
levels ...................................... .......... 4
Surface inflow ............................................ 4
Surface outflow ........................................... 4
Underground inflow and outflow ............................. 7
Fluctuations in lake levels .................................... 10
Control of lake stages......................................... 10
R ainfall ...................................... .......... 10
Evaporation .............................................. 10
Surface inflow ............................................ 10
Surface outflow ........................................... 11
Underground inflow and outflow ............................. 11
Effects of lake-stage control ....................... ............ 13
Summary..................................................... 14
ILLUSTRATIONS
Figure
1 Bar graph of rainfall at Orlando, Florida, showing monthly
averages, maximums and minimums ....................... 3
2 Bar graph of lake evaporation in Orange County, Florida,
showing monthly averages............................... 5
3 Bar graphs showing comparison of average monthly change
in stage of three lakes in Orange County, Florida (upper
graph), and average monthly difference in rainfall and
evaporation at Orlando, Florida (lower graph) .............. 6
4 Cross section of a lake perched above the water table ...... 8 5 Cross section of a water-table lake........................ 8
6 Cross section of an artesian lake ........................ 9
7 Map of Orange County, Florida, showing the range in fluctuation of selected lake levels and the decline in the piezometrio surface from September 1960 to May 1962 .............. 12
iii







CONTROL OF LAKE LEVELS IN ORANGE COUNTY, FLORIDA
By
Warren Anderson, W. F. Lichtler, and B. F. Joyner
INTRODUCTION
The many lakes in Orange County are a valuable natural resource. The lakes are of great value as recreational facilities, as settings for homesites, and as sources of water for the irrigation of more than 20,000 acres of citrus and truck crops on the mucklands adjacent to several of the lakes. Air temperatures near lakes are moderated by the lakes. The elevation and fluctuation of the water surface in lakes are of concern to all persons using the lakes for any purpose. The value of the lakes is reduced if their levels fluctuate excessively. Flooding caused by high lake levels damages homes, roads, and crops, and interferes with transportation. Low lake levels render boat houses and docks useless, cause loss of fish and wildlife, curtail aquatic recreation, increase the cost of irrigation, reduce the moderating effect of the lakes on the air temperature, and usually reduce the amount of recharge to underlying aquifers. These undesirable effects of extremes in lake levels could be reduced or even eliminated if the lake levels were controlled to reduce the range between extremes.
If a lake is used for more than one purpose, there is sometimes disagreement as to the optimum level of the lake. Before taking measures to control the level of a lake, the effect that those moasures will have on other areas and on municipal and industrial water supplies must be considered. Effective planning for the control of lake levels requires an understanding of the factors that affect lake levels, the methods that can be used to control them, and the effects of the control measures on the lake and its environment.
The purpose of this report is to provide information that will increase this understanding and thereby bring about better use of lakes in Orange County.




2 FLORIDA GEOLOGICAL SURVEY
FACTORS THAT INFLUENCE LAKE LEVELS
Lake levels are influenced by rainfall, evaporation, surface inflow, surface outflow, underground inflow, and underground outflow. All lake levels are influenced by rainfall and evaporation but the influence of one or more of the other factors may be absent.
RAINFALL
Rainfall, the source of water in Orange County lakes, is extremely variable. Variation in rainfall is the main reason lake levels fluctuate. The annual rainfall at Orlando, which averages 51.37 inches, has been as little as 33.76 inches and as much as 68.74 inches. Extremely low lake levels are usually the result of several successive dry years. The lowest lake levels of record occurred near the end of the drought period, 1954 to 1956, during which rainfall was deficient by 20 inches. On the other hand, high lake levels can be caused by excessive rainfall during a much shorter period such as August and September 1953, when rainfall was 10 inches above normal.
On the average, 57 percent of the annual rainfall in Orange County falls during the rainy season from June to September. Occasionally more rain falls during the dry than during the rainy season, as in 1958 when only a third of the year's rain fell during the rainy season. Figure 1 shows the maximum, minimum, and average monthly rainfalls at Orlando.
EVAPORATION
The average annual evaporation from lakes in Orange County was computed to be 51.07 inches, based on the normal pan evaporation reported by the U. S. Weather Bureau at Orlando, and adjusted by coefficients determined from evaporation studies at Lake Okeechobee between 1940 and 1946.1 This amount is only 0.3 inch less than the average annual rainfall at Orlando.
Evaporation is much less variable than rainfall. The annual evaporation at the Belle Glade station has departed from the norlU. S. Geological Survey, 1954, Water Loss Investigations: Lake Hefnel
Studies, Technical Report: U. S. Geological Survey Prof. Paper 269. p. 128.




INFORMATION CIRCULAR NO. 47 3
n IMA> IMUM zO
........ ... ... .. .
...MAXMUAVE.RAGl.
5
. .. .. .. . .. .
J F M A M J J A S 0 N D
Figure 1. Bar graph of rainfall at Orlando, Florida showing monthly
averages, maximums and minimums.




4 FLORIDA GEOLOGICAL SURVEY
mal by more than two inches in only three of the 36 years of record. The maximum difference was only seven and one half inches. Evaporation varies seasonally from a minimum rate in December and January to a maximum rate in May. Figure 2 shows the average monthly evaporation from lakes in Orange County.
RELATIONSHIP OF RAINFALL AND EVAPORATION TO CHANGES IN LAKE LEVELS
The direct exchange of water between lakes and the atmosphere is accomplished by rainfall and evaporation. Rainfall and evaporation determine the seasonal trend in lake levels. Figure 3 shows the differences in the average monthly rainfall and evaporation in comparison with the average monthly change in stage of three lakes in Orange County. When the difference is negative (rainfall less than evaporation), the lake levels fall, and when the difference is positive (rainfall more than evaporation), the lake levels rise. The reaction of some lake levels to an excess of rainfall over evaporation is delayed at the beginning of the rainy season by storage of water in the ground. The levels of lakes with surface outlets normally decline in October despite a slight excess in rainfall over evaporation because of the large surface outflow when the lakes are high.
SURFACE INFLOW
Surface inflow occurs when it rains so much the soil becomes saturated or when it. rains so hard the soil cannot absorb the rain fast enough to keep it from running off. The volume of inflow to a lake depends on the amount of rainfall rejected by the soil and the size of the area that. drains to the lake. The change in a lake level caused by a given volume of surface inflow depends on the area of the lake and the volume of water lost from the lake during the period of inflow. The inflow will cause the lake level to -rise if its volume is more than the volume lost, but the lake level will fall if the volume of inflow is less than the volume lost.
SURFACE OUTFLOW
Surface outflow occurs when the level of a lake rises above the lowest point on its rim. Many lakes in Orange County do not




INFORMATION CIRCULAR NO. 47 5
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Figure 2. Bar graph of lake evaporation in Orange County, Florida, showing monthly averages.




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Figure 3. Bar graphs showing comparison of average monthly change in stage of three lakes in Orange County, Florida (upper graph), and average monthly difference in rainfall and evaporation at Orlando, Florida (lower graph).




INFORMATION CIRCULAR NO. 47 7
have surface outflow because their levels never rise above their rims. Some lakes would have to rise more than 40 feet for surface outflow to occur.
Surface outflow normally increases as the level of a lake rises; however, sometimes the outflow is decreased by water backed up in the outlet channel even though the level of a lake is rising.
There is very little surface outflow from lakes in Orange County during extreme droughts.
UNDERGROUND INFLOW AND OUTFLOW
These two factors will be considered together because they are closely related. Indeed, it is possible for both processes to occur in the same lake at the same time. Figures 4, 5, and 6 show the relationships that lakes can have to aquifers. Three aquifers exist in Orange County; the water-table aquifer, the shallow artesian aquifer, and the Floridan or deep artesian aquifer. The water-table aquifer is the zone of saturated earth above the relatively impermeable layer that confines the artesian aquifers. The artesian aquifers are zones of saturation beneath impermeable layers that confine the water under greater than atmospheric pressure. The artesian pressure surface, which is called the piezometric surface, is the height to which water will rise in a tightly cased well that is drilled into the aquifer. The levels and fluctuations of the water table and the piezometric surface are more or less independent of each other, depending on the permeability of the confining bed.
Figure 4 depicts a lake that is perched above the water-table aquifer. There can be no underground inflow to this type of lake as its level is higher than the surrounding water table nor can there be much underground outflow from it because of its impermeable bottom.
Figure 5 shows a cross section of a water-table lake. This type of lake forms where the land surface dips below the water table. Under the conditions shown in the illustration, there is no exchange of water between the lake and the aquifer because the lake surface and the water table are at the same level. This condition seldom exists and most of the time the lake level is either below the water table, at which times water seeps into the lake,




SFLORIDA GEOLOGICAL SURVEY Land Surface
\Impervious
Water Table @""La er
Figure 4. Cross section of a lake perched above the water table.
Land Surface
,WaterTobl
Figure 5. Cross section of a water-table lake.




INFORMATION CIRCULAR NO. 47 9
Land Surface Water Table
p- iezometric N~. 'Surface
mN
1 ervious La or
-- Cover ous Limestone ---
Figure 6. Cross section of an artesian lake.
or the lake level is above the water table, at which times water seeps out of the lake. A cross section of an artesian lake is shown in figure 6. This type of lake is connected directly to the artesian aquifer and reflects the piezometric surface. Two lakes in Orange County Emerald Springs and an unnamed lake near Rock Springs are definitely known to be of this type and there may be others. Water from the water-table aquifer probably seeps into these lakes and recharges the artesian aquifer continuously. It is possible, however, for the opposite process to occur if the piezometric su rface becomes higher than the water table around the lake.




10 FLORIDA GEOLOGICAL SURVEY
FLUCTUATIONS IN LAKE LEVELS
All of the lakes in Orange County tend to fluctuate seasonally because the factors that affect lake levels seasonally rainfall and evaporation affect all of the lakes similarly. On the other hand, the range in the fluctuations of the lakes varies widely because the factors that control the range in fluctuation topography and geology vary widely from lake to lake. The observed range in stage of Lake Susannah is only 2.4 feet, but a range in stage of 20 feet has been observed for Lake Sherwood (fig. 7).
CONTROL OF LAKE STAGES
The logical approach to control of lake levels is through control of the factors that affect them. These factors are: rainfall, evaporation, surface inflow and outflow, and underground inflow and outflow.
RAINFALL
No way has been devised to prevent rain, so it is not possible to prevent the rise of lake levels in this way. Some success in causing rainfall by cloud seeding, under favorable cloud conditions, has been claimed. However, there is still doubt as to these claims and, because favorable conditions seldom exist in Orange County during droughts, rainmaking can be eliminated at this time as a practical means for preventing low lake levels.
EVAPORATION
No practical way of increasing the rate of evaporation has been devised, so this method of lowering lake stages can be eliminated. Some progress has been made in the use of chemical and plastic films to reduce evaporation but the techniques are still in the research stage.
SURFACE INFLOW
Control of lake levels through control of surface inflow has been practiced in Orange County for years. For example, water




INFORMATION CIRCULAR NO. 47 11
that would normally drain from Colonial Plaza into the south Orlando lakes is diverted to Lake Sue through a pipe line. Much water that would otherwise enter lakes from street drainage is diverted to the artesian aquifer through drainage wells. However, during extremely wet years, such as 1960, flow down these wells when added to the normal recharge builds up the piezometric surface so that the wells in some lakes refuse to take water and some wells even discharge water into the lakes.
SURFACE OUTFLOW
Surface outflow from a lake may be conveyed in open channels and culverts or pumped out through pipes. Open channels can be used to drain any of Orange County's lakes. However, it would be extremely expensive to dig open channels to many of the landlocked lakes because of the very deep cuts required and the long distances to discharge points on stream systems at elevations lower than the levels of the lakes. For instance, to drain Lake Sherwood through an open channel would require a channel more than 10 miles long with cuts up to 45 feet. The channel would, of course, provide drainage for those intervening lakes through which it could be routed, but might drain some of them dry.
Open channels require considerable maintenance, wide rightsof-way, are often unsightly, and they break up the continuity of the land and interfere with transportation.
A combination of open channels, culverts, and pumps can often be used to advantage.
UNDERGROUND INFLOW AND OUTFLOW
Artesian wells can be used to remove water from or to put water into a lake. If the lake level is higher than the piezometric surface, water will flow down the well; if the piezometric surface is higher than the lake level, water will flow out of the well. The natural direction of flow can, of course, be reversed by using pumps (however, pumping water down drainage wells is prohibited by State Board of Health regulations). Figure 7 gives the periods of record, maximum and minimum elevations, in feet above mean sea level, and the fluctuations, in feet, of selected lakes in Orange County. Also shown are contour lines representing the




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n'the pieometric surface from September" 1960 to May 1962.




INFORMATION CIRCULAR NO. 47 13
piezometric surface of the Floridan aquifer during high water in September 1960 and the decline in the elevation of the piezometric surface between September 1960 and May 1962.
The ability of a well to take or to yield water is dependent on the permeability of the aquifer into which it is drilled and the difference between the level of the lake and the piezometric surface. Wells are relatively cheap; they are unobtrusive; and they can be used either to supply water or to remove water from a lake. On the other hand, wells have less capacity than open channels and they may pollute the artesian aquifer when used for drainage. Also, many drainage wells become less effective during wet weather when they are needed most. This is because the piezometric surface rises more rapidly than lake levels during wet periods, causing a reduction in the difference between the levels of the lakes and the piezometric surface.
EFFECTS OF LAKE-STAGE CONTROL
The methods used to control lakes in Orange County nearly always fail to remedy the problem of lake-level fluctuation, but merely transfer the same problem to a lower elevation. This is because the control projects are usually directed at reducing high lake levels without regard to the effect on low lake levels. Increasing the outflow from a lake to prevent high levels without providing a means to prevent the loss of an equal amount of water or a means to replace the extra outflow causes the lake's level to become lower than normal during drought so that the range in fluctuation is virtually unchanged.
Most lakes in Orange County are water-table lakes whose levels are above the piezometric surface. In general, the water table conforms to the land surface and usually slopes downward toward the lakes, causing water to seep into the lakes from the surrounding water-table aquifer. When the level of a lake is artificially lowered by increasing surface outflow, the slope of the water table toward the lake is increased, causing an increase in the rate of seepage from the water-table aquifer into the lake. If the lower lake level is maintained, the increased outflow from the water-table aquifer will lower the water table until equilibrium is re-established between the outflow and inflow of the lake. Thus,




14 FLORIDA GEOLOGICAL SURVEY
lowering the level of a lake not only reduces the amount of water stored in the lake; it also reduces the amount of water stored in the water-table aquifer surrounding the lake.
In areas where the water table is higher than the piezometric surface, the rate that water percolates from the water-table aquifer to the artesian aquifer depends on the permeability of the intervening material and the height of the water table above the piezometric surface. When the levels of a lake and the surrounding water table are lowered, their heights above the piezometric surface are decreased and the rate of recharge to the artesian aquifer is decreased.
When drainage wells are used to lower lake levels, the effect is the same as that caused by surface removal except that recharge to the artesian aquifer is increased rather than reduced. The water is not lost, but stored in the artesian aquifer where no losses by evaporation occur. However, when water is put into the artesian aquifer by drainage wells, the slope of the piezometric surface is increased unnaturally, causing more than normal outflow from the aquifer. If ground water use increases to the point where the piezometric surface is permanently lowered, drainage of excess surface water to the Floridan aquifer may be beneficial, provided no polluted water is allowed to enter the aquifer.
SUMMARY
Lakes are one of the most valuable natural resources in Orange County. The usefulness of some of these lakes is reduced because their levels fluctuate excessively. These fluctuations occur because of differences in the rates that water enters and leaves the lakes. Water enters the lakes as a result of rainfall, surface inflow, and underground inflow. Water leaves the lakes as a result of evaporation, surface outflow, and underground outflow. Lake levels cannot be effectively controlled through control of rainfall and evapotranspiration. Lake levels can be controlled by altering the surface inflow and outflow and the underground inflow and outflow. Control of the levels of many of the lakes cannot be effected at all times by altering the underground outflow because the artesian aquifer will not accept this surplus water during wet periods.




INFORMATION CIRCULAR NO. 47 15
If the level of a lake is controlled by merely removing excess water during wet periods, the range in its fluctuations will be virtually unchanged, its average level will be lower, and its level will be lower than normal during droughts.
Lowering the level of a lake causes more water to seep into the lake from the surrounding water-table aquifer. Thus, not only is the amount of water stored in the lake reduced by lowering its level, but the amount stored in the adjoining water-table aquifer is also reduced.
Except when drainage wells are used to increase flow to the artesian aquifer, lowering lake levels and the water table reduces the rate of recharge to the artesian aquifer. Increasing recharge to the artesian aquifer causes the rate of water loss from the aquifer to increase.
Withdrawal of water from the artesian aquifer increases the rate of recharge to the aquifer and improves the effectiveness of drainage wells.




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PAGE 1

STATE OF FLORIDA STATE BOARD OF CONSERVATION DIVISION OF GEOLOGY FLORIDA GEOLOGICAL SURVEY Robert 0. Vernon, Director INFORMATION CIRCULAR NO. 47 CONTROL OF LAKE LEVE LS IN ORANGE COUNTY, FLORIDA By Warren Anderson, W. F. Lichtler, and B. F. Joyner Prepared by the UNITED STATES GEOLOGICAL SURVEY in cooperation with the FLORIDA GEOLOGICAL SURVEY and the BOARD OF COUNTY COMMISSIONERS OF ORANGE COUNTY TALLAHASSEE 1965

PAGE 2

4oo.( 4 4 L AGRI CULTURAL [1ORARY Completed manuaoript received December 8, 1964 Printed by the Florida Geological Survoy-22 Tallahassee 1965

PAGE 3

Introduction ............. .... .... .... ... ..... .......... 1 Factors that influence lake levels ............... ... ............ 2 R ainfall ........................ i .... ...... ........... .2 Evaporation .................... ..... .................. .2 Relationship of rainfall and evaporation to changes in lake levels ........................... .. ......... ......... 4 Surface inflow ................................. .... ..... .4 Surface outflow .......... ................................ 4 Underground inflow and outflow ..... ......... .... ....... 7 Fluctuations in lake levels ....... ............................. 10 Control of lake stages.......................................... 10 R ainfall ....................................... ........ .10 Evaporation .................................. ........... 10 Surface inflow........ .............. ....................... 10 Surface outflow ............................ ..... ........ .11 Underground inflow and outflow ........ ...... .............. .11 Effects of lake-stage control .......... ......... ... ............. 13 Summary ........................................ ........... 14 ILLUSTRATIONS Figure 1 Bar graph of rainfall at Orlando, Florida, showing monthly averages, maximums and minimums ............ ........... 3 2 Bar graph of lake evaporation in Orange County, Florida, showing monthly averages................................. 5 3 Bar graphs showing comparison of average monthly change in stage of three lakes in Orange County, Florida (upper graph), and average monthly difference in rainfall and evaporation at Orlando, Florida (lower graph) .............. 6 4 Cross section of a lake perched above the water table ...... 8 5 Cross section of a water-table lake ..................... .8 6 Cross section of an artesian lake ........................ 9 7 Map of Orange County, Florida, showing the range in fluctuation of selected lake levels and the decline in the piezometrio surface from September 1960 to May 1962 .............. 12 iii

PAGE 5

CONTROL OF LAKE LEVELS IN ORANGE COUNTY, FLORIDA By Warren Anderson, W. F. Lichtler, and B. F. Joyner INTRODUCTION The many lakes in Orange County are a valuable natural resource. The lakes are of great value as recreational facilities, as settings for homesites, and as sources of water for the irrigation of more than 20,000 acres of citrus and truck crops on the mucklands adjacent to several of the lakes. Air temperatures near lakes are moderated by the lakes. The elevation and fluctuation of the water surface in lakes are of concern to all persons using the lakes for any purpose. The value of the lakes is reduced if their levels fluctuate excessively. Flooding caused by high lake levels damages homes, roads, and crops, and interferes with transportation. Low lake levels render boat houses and docks useless, cause loss of fish and wildlife, curtail aquatic recreation, increase the cost of irrigation, reduce the moderating effect of the lakes on the air temperature, and usually reduce the amount of recharge to underlying aquifers. These undesirable effects of extremes in lake levels could be reduced or even eliminated if the lake levels were controlled to reduce the range between extremes. If a lake is used for more than one purpose, there is sometimes disagreement as to the optimum level of the lake. Before taking measures to control the level of a lake, the effect that those measures will have on other areas and on municipal and industrial water supplies must be considered. Effective planning for the control of lake levels requires an understanding of the factors that affect lake levels, the methods that can be used to control them, and the effects of the control measures on the lake and its environment. The purpose of this report is to provide information that will increase this understanding and thereby bring about better use of lakes in Orange County. 1

PAGE 6

2 FLORIDA GEOLOGICAL SURVEY FACTORS THAT INFLUENCE LAKE LEVELS Lake levels are influenced by rainfall, evaporation, surface inflow, surface outflow, underground inflow, and underground outflow. All lake levels are influenced by rainfall and evaporation but the influence of one or more of the other factors may be absent. RAINFALL Rainfall, the source of water in Orange County lakes, is extremely variable. Variation in rainfall is the main reason lake levels fluctuate. The annual rainfall at Orlando, which averages 51.37 inches, has been as little as 33.76 inches and as much as 68.74 inches. Extremely low lake levels are usually the result of several successive dry years. The lowest lake levels of record occurred near the end of the drought period, 1954 to 1956, during which rainfall was deficient by 20 inches. On the other hand, high lake levels can be caused by excessive rainfall during a much shorter period such as August and September 1953, when rainfall was 10 inches above normal. On the average, 57 percent of the annual rainfall in Orange County falls during the rainy season from June to September. Occasionally more rain falls during the dry than during the rainy season, as in 1958 when only a third of the year's rain fell during the rainy season. Figure 1 shows the maximum, minimum, and average monthly rainfalls at Orlando. EVAPORATION The average annual evaporation from lakes in Orange County was computed to be 51.07 inches, based on the normal pan evaporation reported by the U. S. Weather Bureau at Orlando, and adjusted by coefficients determined from evaporation studies at Lake Okeechobee between 1940 and 1946.1 This amount is only 0.3 inch less than the average annual rainfall at Orlando. Evaporation is much less variable than rainfall. The annual evaporation at the Belle Glade station has departed from the norlU. S. Geological Survey, 1954, Water Loss Investigations: Lake Hefnel Studies, Technical Report: U. S. Geological Survey Prof. Paper 269. p. 128.

PAGE 7

INFORMATION CIRCULAR NO. 47 3 15 -MAXIMUMA M... 0 Figure 1. Bar graph of rainfall at Orlando, Florida showing monthly averages, maximums and minimums. .. ... . 10... .. LL. ... ... .......... VERAG 5 ---MINIMUM .... .... ... .... 0 J F... M A M JJ A S 0 N Fiue1 a rp frifl tOlno lrd hwn otl avergesmaxiums andminiums

PAGE 8

4 FLORIDA GEOLOGICAL SURVEY mal by more than two inches in only three of the 36 years of record. The maximum difference was only seven and one half inches. Evaporation varies seasonally from a minimum rate in December and January to a maximum rate in May. Figure 2 shows the average monthly evaporation from lakes in Orange County. RELATIONSHIP OF RAINFALL AND EVAPORATION TO CHANGES IN LAKE LEVELS The direct exchange of water between lakes and the atmosphere is accomplished by rainfall and evaporation. Rainfall and evaporation determine the seasonal trend in lake levels. Figure 3 shows the differences in the average monthly rainfall and evaporation in comparison with the average monthly change in stage of three lakes in Orange County. When the difference is negative (rainfall less than evaporation), the lake levels fall, and when the difference is positive (rainfall more than evaporation), the lake levels rise. The reaction of some lake levels to an excess of rainfall over evaporation is delayed at the beginning of the rainy season by storage of water in the ground. The levels of lakes with surface outlets normally decline in October despite a slight excess in rainfall over evaporation because of the large surface outflow when the lakes are high. SURFACE INFLOW Surface inflow occurs when it rains so much the soil becomes saturated or when it. rains so hard the soil cannot absorb the rain fast enough to keep it from running off. The volume of inflow to a lake depends on the amount of rainfall rejected by the soil and the size of the area that. drains to the lake. The change in a lake level caused by a given volume of surface inflow depends on the area of the lake and the volume of water lost from the lake during the period of inflow. The inflow will cause the lake level to irise if its volume is more than the volume lost, but the lake level will fall if the volume of inflow is less than the volume lost. SURFACE OUTFLOW Surface outflow occurs when the level of a lake rises above the lowest point on its rim. Many lakes in Orange County do not

PAGE 9

INFORMATION CIRCULAR NO. 47 5 8 7 6 "') w5 0 z 0 a.3 2 .......... .......... ..:....... .' .X-.. ..-....... ... ......... -x .:..-:..:. ......:.:. :i '.'.:' ' !::':·; .·' :·: '''''':-: :':::" K :":$: N':' ...'... .........:! :.:.:::: ::::$::::s '.'i ::'::~::~~'&'i :-':·:Y-:-.: ..:...., ........ L :~XX .. . 0K J F M A M J J A SO N D Figure 2. Bar graph of lake evaporation in Orange County, Florida, showing monthly averages.

PAGE 10

0 .4 ... .r -..-. .-. ...... .. ... .. ....----.. .... .--03 02 wM A B M A B M A B A A 8 M A Af 0. AB" 0.0 i: ..*; B M A 8 M A 8 M A B M ii S^ S.1M -*M "1 -0 .4. _____' .........__:______________ ___ _ _____ 0 W -0.21 0:----.: " A -LAKE APOPKA B -LAKE BUTLER M-LAKE MAITLAND 0 S0.3 .> a.. < 0.20 W4 03 JAN. FEB. MAR. APR MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. < .3 JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. Figure 3. Bar graphs showing comparison of average monthly change in stage of three lakes in Orange County, Florida (upper graph), and average monthly difference in rainfall and evaporation at Orlando, Florida (lower graph).

PAGE 11

INFORMATION CIRCULAR NO. 47 7 have surface outflow because their levels never rise above their rims. Some lakes would have to rise more than 40 feet for surface outflow to occur. Surface outflow normally increases as the level of a lake rises; however, sometimes the outflow is decreased by water backed up in the outlet channel even though the level of a lake is rising. There is very little surface outflow from lakes in Orange County during extreme droughts. UNDERGROUND INFLOW AND OUTFLOW These two factors will be considered together because they are closely related. Indeed, it is possible for both processes to occur in the same lake at the same time. Figures 4, 5, and 6 show the relationships that lakes can have to aquifers. Three aquifers exist in Orange County; the water-table aquifer, the shallow artesian aquifer, and the Floridan or deep artesian aquifer. The water-table aquifer is the zone of saturated earth above the relatively impermeable layer that confines the artesian aquifers. The artesian aquifers are zones of saturation beneath impermeable layers that confine the water under greater than atmospheric pressure. The artesian pressure surface, which is called the piezometric surface, is the height to which water will rise in a tightly cased well that is drilled into the aquifer. The levels and fluctuations of the water table and the piezometric surface are more or less independent of each other, depending on the permeability of the confining bed. Figure 4 depicts a lake that is perched above the water-table aquifer. There can be no underground inflow to this type of lake as its level is higher than the surrounding water table nor can there be much underground outflow from it because of its impermeable bottom. Figure 5 shows a cross section of a water-table lake. This type of lake forms where the land surface dips below the water table. Under the conditions shown in the illustration, there is no exchange of water between the lake and the aquifer because the lake surface and the water table are at the same level. This condition seldom exists and most of the time the lake level is either below the water table, at which times water seeps into the lake,

PAGE 12

8 FLORIDA GEOLOGICAL SURVEY SLand Surface Impervious Water Table VLa er Figure 4. Cross section of a lake perched above the water table. Land Surface SWater Tble Figure 5. Cross section of a water-table lake.

PAGE 13

INFORMATION CIRCULAR NO. 47 9 Land Surface Water Table iezometric m -Coavernous Limestone-) " Figure 6. Cross section of an artesian lake. or the lake level is above the water table, at which times water seeps out of the lake. A cross section of an artesian lake is shown in figure 6. This type of lake is connected directly to the artesian aquifer and reflects the piezometric surface. Two lakes in Orange County -Emerald Springs and an unnamed lake near Rock Springs are definitely known to be of this type and there may be others. Water from the water-table aquifer probably seeps into these lakes and recharges the artesian aquifer continuously. It is possible, however, for the opposite process to occur if the piezometric surface becomes higher than the water table around the lake.

PAGE 14

10 FLORIDA GEOLOGICAL SURVEY FLUCTUATIONS IN LAKE LEVELS All of the lakes in Orange County tend to fluctuate seasonally because the factors that affect lake levels seasonally -rainfall and evaporation -affect all of the lakes similarly. On the other hand, the range in the fluctuations of the lakes varies widely because the factors that control the range in fluctuation -topography and geology -vary widely from lake to lake. The observed range in stage of Lake Susannah is only 2.4 feet, but a range in stage of 20 feet has been observed for Lake Sherwood (fig. 7). CONTROL OF LAKE STAGES The logical approach to control of lake levels is through control of the factors that affect them. These factors are: rainfall, evaporation, surface inflow and outflow, and underground inflow and outflow. RAINFALL No way has been devised to prevent rain, so it is not possible to prevent the rise of lake levels in this way. Some success in causing rainfall by cloud seeding, under favorable cloud conditions, has been claimed. However, there is still doubt as to these claims and, because favorable conditions seldom exist in Orange County during droughts, rainmaking can be eliminated at this time as a practical means for preventing low lake levels. EVAPORATION No practical way of increasing the rate of evaporation has been devised, so this method of lowering lake stages can be eliminated. Some progress has been made in the use of chemical and plastic films to reduce evaporation but the techniques are still in the research stage. SURFACE INFLOW Control of lake levels through control of surface inflow has been practiced in Orange County for years. For example, water

PAGE 15

INFORMATION CIRCULAR NO. 47 11 that would normally drain from Colonial Plaza into the south Orlando lakes is diverted to Lake Sue through a pipe line. Much water that would otherwise enter lakes from street drainage is diverted to the artesian aquifer through drainage wells. However, during extremely wet years, such as 1960, flow down these wells when added to the normal recharge builds up the piezometric surface so that the wells in some lakes refuse to take water and some wells even discharge water into the lakes. SURFACE OUTFLOW Surface outflow from a lake may be conveyed in open channels and culverts or pumped out through pipes. Open channels can be used to drain any of Orange County's lakes. However, it would be extremely expensive to dig open channels to many of the landlocked lakes because of the very deep cuts required and the long distances to discharge points on stream systems at elevations lower than the levels of the lakes. For instance, to drain Lake Sherwood through an open channel would require a channel more than 10 miles long with cuts up to 45 feet. The channel would, of course, provide drainage for those intervening lakes through which it could be routed, but might drain some of them dry. Open channels require considerable maintenance, wide rightsof-way, are often unsightly, and they break up the continuity of the land and interfere with transportation. A combination of open channels, culverts, and pumps can often be used to advantage. UNDERGROUND INFLOW AND OUTFLOW Artesian wells can be used to remove water from or to put water into a lake. If the lake level is higher than the piezometric surface, water will flow down the well; if the piezometric surface is higher than the lake level, water will flow out of the well. The natural direction of flow can, of course, be reversed by using pumps (however, pumping water down drainage wells is prohibited by State Board of Health regulations). Figure 7 gives the periods of record, maximum and minimum elevations, in feet above mean sea level, and the fluctuations, in feet, of selected lakes in Orange County. Also shown are contour lines representing the

PAGE 16

ORANGE COUNTY , ., --FLORIDA ., ' %,, , , Panels fts e -O ( I S199-61 EXPLANATION IW --u 70, * 6 559 -04 fT |.,ll.> Late Cortie 1c \ I' , .1940-55 954-61 1 949-56 I 959S-63J 90a000 , --34 " ..J/. 91,--! 1,,ji I 9 0 OI"I' ~ w, wato .. L.Il LohI 6 " ViCrE L a Ai h p,/ake Susanl. m ed menwe obered 19491 '-5>6' /: 9 -,9 94.6 1. 4.e I, lk.»t ,s ~ a feel e ' --19-S6 1943-4 9 , IS9-OS * C iW es Im e lsh d vei e e eo 19 -. 3 A I J 4I te d o so"o..r *< --i-SV Wb -U 1a -60 " s ---5 Laker, L e ' S. 1948-5 1 94-54 l043 4l n iiiwie1 d i .iftl, 0 tiA 0 ' -' " i] "' ' ' M e ' 4 1I ?A ', 5 4 ' i 6 70 4 4 ,'". .fel obs of no , ,4 -iur 1. Ir a n , howi he f in l ,e lips > Li Behe .i oi. c -'r. -... .,e b Ma 1 2' A_ , .., ,.* ...... _... ... _... S ... ... ...g. 0, -. Io OWO Ma S S/ t I -I I 0 .5 1 ! %.,11 I, ',I I I 10. 6 2 > 0 I 1 959-6 I 194-S I -, 195963 1/949-63 1 .94 0-49 l6r' 1 .94-Se l ,942-S0 194,-56 1941-63 /1 '94.-S6 I , .I "" ' Figure 7. Map of Orange County, Florida, showing the range in fluctuation of selected lake evels and the decline in'the piezometric surface from September 1960 to May 1962.

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INFORMATION CIRCULAR NO. 47 13 piezometric surface of the Floridan aquifer during high water in September 1960 and the decline in the elevation of the piezometric surface between September 1960 and May 1962. The ability of a well to take or to yield water is dependent on the permeability of the aquifer into which it is drilled and the difference between the level of the lake and the piezometric surface. Wells are relatively cheap; they are unobtrusive; and they can be used either to supply water or to remove water from a lake. On the other hand, wells have less capacity than open channels and they may pollute the artesian aquifer when used for drainage. Also, many drainage wells become less effective during wet weather when they are needed most. This is because the piezometric surface rises more rapidly than lake levels during wet periods, causing a reduction in the difference between the levels of the lakes and the piezometric surface. EFFECTS OF LAKE-STAGE CONTROL The methods used to control lakes in Orange County nearly always fail to remedy the problem of lake-level fluctuation, but merely transfer the same problem to a lower elevation. This is because the control projects are usually directed at reducing high lake levels without regard to the effect on low lake levels. Increasing the outflow from a lake to prevent high levels without providing a means to prevent the loss of an equal amount of water or a means to replace the extra outflow causes the lake's level to become lower than normal during drought so that the range in fluctuation is virtually unchanged. Most lakes in Orange County are water-table lakes whose levels are above the piezometric surface. In general, the water table conforms to the land surface and usually slopes downward toward the lakes, causing water to seep into the lakes from the surrounding water-table aquifer. When the level of a lake is artificially lowered by increasing surface outflow, the slope of the water table toward the lake is increased, causing an increase in the rate of seepage from the water-table aquifer into the lake. If the lower lake level is maintained, the increased outflow from the water-table aquifer will lower the water table until equilibrium is re-established between the outflow and inflow of the lake. Thus,

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14 FLORIDA GEOLOGICAL SURVEY lowering the level of a lake not only reduces the amount of water stored in the lake; it also reduces the amount of water stored in the water-table aquifer surrounding the lake. In areas where the water table is higher than the piezometric surface, the rate that water percolates from the water-table aquifer to the artesian aquifer depends on the permeability of the intervening material and the height of the water table above the piezometric surface. When the levels of a lake and the surrounding water table are lowered, their heights above the piezometric surface are decreased and the rate of recharge to the artesian aquifer is decreased. When drainage wells are used to lower lake levels, the effect is the same as that caused by surface removal except that recharge to the artesian aquifer is increased rather than reduced. The water is not lost, but stored in the artesian aquifer where no losses by evaporation occur. However, when water is put into the artesian aquifer by drainage wells, the slope of the piezometric surface is increased unnaturally, causing more than normal outflow from the aquifer. If ground water use increases to the point where the piezometric surface is permanently lowered, drainage of excess surface water to the Floridan aquifer may be beneficial, provided no polluted water is allowed to enter the aquifer. SUMMARY Lakes are one of the most valuable natural resources in Orange County. The usefulness of some of these lakes is reduced because their levels fluctuate excessively. These fluctuations occur because of differences in the rates that water enters and leaves the lakes. Water enters the lakes as a result of rainfall, surface inflow, and underground inflow. Water leaves the lakes as a result of evaporation, surface outflow, and underground outflow. Lake levels cannot be effectively controlled through control of rainfall and evapotranspiration. Lake levels can be controlled by altering the surface inflow and outflow and the underground inflow and outflow. Control of the levels of many of the lakes cannot be effected at all times by altering the underground outflow because the artesian aquifer will not accept this surplus water during wet periods.

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INFORMATION CIRCULAR NO. 47 15 If the level of a lake is controlled by merely removing excess water during wet periods, the range in its fluctuations will be virtually unchanged, its average level will be lower, and its level will be lower than normal during droughts. Lowering the level of a lake causes more water to seep into the lake from the surrounding water-table aquifer. Thus, not only is the amount of water stored in the lake reduced by lowering its level, but the amount stored in the adjoining water-table aquifer is also reduced. Except when drainage wells are used to increase flow to the artesian aquifer, lowering lake levels and the water table reduces the rate of recharge to the artesian aquifer. Increasing recharge to the artesian aquifer causes the rate of water loss from the aquifer to increase. Withdrawal of water from the artesian aquifer increases the rate of recharge to the aquifer and improves the effectiveness of drainage wells.

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