elimination of Thermal 

{ratification in 

eservoirs and the 

esulting Benefits

ITH SPECIAL EMPHASIS ON STUDY OF 

\KE WOHLFORD, CALIFORNIA

GORDON E. KOBERG and MAURICE E. FORD, JR.

NTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

'LOLOGICAL SURVEY WATER-SUPPLY PAPER 1809-M

-epared in collaboration with

 rcondido Mutual Water Company, 

.icondido, California

.1TED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract__ ______________________________________________________ Ml

Introduction. _____________________________________________________ 1

Distribution of temperature with depth._____________________________ 3

Effect of surface heating and cooling_____________________________ 3

Penetration of solar radiation___________________________________ 4

Effect of inflow and outflow_____________________________________ 4

Effect of wind__. ______________________________________________ 5

Predicting the distribution of temperature in a reservoir.___________ 6

The stability of thermally stratified water.___________________________ 6

Techniques to eliminate thermal stratification.________________________ 8

Floating aerator.______________________________________________ 8

Aerohydraulic technique______________________________________ 8

Plastic hose air-distribution system._____________________________ 9

Test of air-bubbling system at Lake Wohlford_______________________ 10

Description of Lake Wohlford___________________________________ 10

Quality-of-water problems______________________________________ 11

Description of air-bubbling system_____________________________ 13

Mixing of stratified water_____________________________________ 13

Water quality results._________________________________________ 20

Evaporation results.___________________________________________ 22

Conclusions_______________________________________________________ 25

References cited.__________________________________________________ 27

ILLUSTRATIONS

FIGURE 1. Temperature and velocity profiles as measured at Iceberg Page 

Canyon, Lake Mead, Ariz_____________________________ M5

2. A comparison of seasonal changes in stability expressed in 

terms of energy per unit area of surface for Lake Cachuma, 

Lower Otay Reservoir, Lake Hodges, and Lake Wohlford 

in California.________________________________________ 7

3. Map of Lake Wohlford____________________________ 11

4. Temperature profiles of Lake Wohlford__________________ 11

5. A schematic of anchor system to support plastic tubing._ __ 14

6. Average of 17 temperature profiles for Lake Wohlford_____ 15

7. Relationship between the surface area of the boil formed by 

the air-bubbling system and the number of hours of compressor operation for Lake Wohlford-___________________ 17

8. Velocity and temperature profiles as observed at raft station

in 1962_______________________________________ 18

9. Velocity and temperature profiles as observed at thermal

survey station No. 8 in 1962________-______---__--_--_- 18

in
IV CONTENTS

Page

FIGURE 10. The decrease in stability versus the number of hours of compressor operation_-.______-___--_----__------_---___-__ M19

11. Seasonal variation in the concentration of dissolved oxygen

and pH as determined from water samples.______________ 21

12. A comparison of the monthly mean temperature of the water 

surface for the period 1959-61 with the 1962 monthlj 

average for Lake Wohlford-___________________________ 23

13. A comparison of the 1962 monthly evaporation rates as determined by the mass-transfer method with monthly mean 

evaporation rates for the period 1959-61 for Lake Wohlford.__________-_________________-___-_---_----- 24

14. A comparison of the accumulated evaporation in acre-feet 

by months for 1962 with the monthly mean evaporation for 

the period 1959-61 for Lake Wohlford_-__--____--_---- 26

TABLE

Page 

TABLE 1. Dissolved-oxygen and pH analyses of water taken from Lake

Wohlford in 1960 on indicated dates______--__------_-_-_- M12
CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

ELIMINATION OF THERMAL STRATIFICATION IN

RESERVOIRS AND THE RESULTING BENEFITS,

WITH SPECIAL EMPHASIS ON STUDY OF

LAKE WOHLFORD, CALIFORNIA

By GORDON E. KOBERG x and MAURICE E. FORD, JR.2

ABSTRACT

Thermal stratification in lakes and reservoirs increases the concentration of 

certain undesirable substances in the hypolimnion with an 'accompanying decrease in the dissolved-oxygen concentration. Thermal stratification considered 

in this report is due to temperature variation with depth which controls the 

respective density of the water. The temperature distribution within a lake is a 

function of diffusion 'and conduction, and such external agencies as solar radiation, wind, inflow, and outflow.

The use of air bubbling to artificially induce mixing in a lake has been successful in several investigations reported in the literature. A detailed study 

of this technique in Lake Wohlford in collaboration with the Escondido Mutual 

Water Co. of Escondido, Calif., is presented.

The results of the Lake Wohlford study indicate that the air-bubbling system 

is economically feasible to remove undesirable taste and odors from the water 

used for domestic purposes and to increase the dissolved-oxygen concentration 

in the hypolimnion of the lake. The results also indicate that the elimination of 

thermal stratification can reduce evaporation. The elimination of thermal 

stratification in Lake Wohlford during May, June, and July reduced the evaporation 15 percent. Although the evaporation was increased 9 percent in September, October, and November, the net reduction for the 6 months was about 

6 percent.

The Lake Wohlford study and other tests cited have not presented enough 

data to define criteria for design of an air-bubbling system for a thermally 

stratified reservoir of a given capacity. Further studies are needed on larger 

reservoirs before design criteria can be established.

INTRODUCTION

Stratification of the water occurs in many reservoirs and lakes because of the temperature distribution with depth. The degree of 

stratification will vary according to the season and the depth of water 

in the reservoir. The summer months generally have the greatest

1 U.S. Geological Survey.

2 Escondido Mutual Water Company.

Ml
M2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

stratification whereas the winter months have the least. The stratification is best defined by the temperature distribution. Limnologists 

have divided the water by depth into three layers: (a) Epilimnion, 

the layer of isothermal water from the surface to the level where the 

temperature of the water begins to change rapidly with depth, (b) 

thermocline, the layer of water with rapidly changing temperature, 

and (c) hypolimnion, the layer of isothermal water between the thermocline and the bottom of the lake.

The quality of water stored in each of the three layers varies with 

the season of the year. McEwen (1941) stated that, in general, a 

gradient of the concentration of any dissolved substance gives rise 

to forces tending by diffusion to equalize the concentration. Likewise, 

a temperature gradient tends to be eliminated by the resulting conduction of heat. However, such processes tending to produce a uniform distribution of heat and dissolved substances in still water are 

very slow. Therefore, the quality of water stored in each layer depends on the natural mixing motion within the layer and how this 

mixing motion is transferred from one layer to another. This paper 

presents the results of a detailed study, made in collaboration with 

the Escondido Mutual Water Co. at Escondido, Calif., of mixing 

action induced into Lake Wbhlford by an air-bubbling system.

In many reservoirs the water has seasonal changes of taste and 

odor because the mixing motions are limited to the epilimnion by the 

temperature distribution of the water, a condition which poses a problem if the reservoir is used for domestic water supply. To overcome 

this problem water is usually withdrawn from the epilimnion, which is 

generally of the best quality. The concentration of substances in the 

epilimnion are equalized by the wind and the process of photosynthesis. Some reservoirs are restricted to several levels at which water 

can be withdrawn, and consequently when large demands are placed 

on the stored water, some of the obnoxious water will be withdrawn. 

Furthermore, during periods when the water in the reservoir overturns, the water from the hypolimnion is distributed throughout the 

reservoir, and the quality of the water in the epilimnion is lowered.

Reservoir operators who cope with problems of taste and odor have 

experimented with various methods of changing the temperature distribution to induce mixing into the thermocline and hypolimnion. 

In most of these methods air is forced into the bottom of the reservoir 

and, as the air rises in the form of bubbles, a mixing action is induced 

into the water. This method of mixing has been the most successful, 

and various techniques of forcing air into the water are described 

in this paper.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M3

The elimination of thermal stratification will change the seasonal 

rates of evaporation. Reservoirs with a large storage of water in the 

hypolimnion will have an appreciable reduction in evaporation during 

the late spring and summer months and a corresponding increase in 

evaporation during the fall and early winter months. However, the 

operation of a reservoir may be such that there is a net annual reduction in evaporation, as shown by measurements of evaporation at Lake 

Wohlford.

DISTRIBUTION OF TEMPERATURE WITH DEPTH

EFFECT OF SURFACE HEATING AND COOLING

The rate of heat exchange between the water in a reservoir and 

the atmosphere is dependent upon the temperature of the water at 

the surface and such external agencies as evaporation, conduction, 

and radiation. The rate of heat exchange between the surface and 

internal water is dependent upon the processes of conduction and diffusion and is controlled mainly by the temperature of the water at the 

surface.

The surface of the water is constantly undergoing changes in temperature because of the external agencies and the internal processes. 

These changes in temperature of the water surface are accompanied 

by corresponding changes in density. So long as the density of the 

water at the surface is less than that of the water below it, the water 

at the surface will remain in place. If the density becomes greater, 

however, particles of the water at the surface will descend until they 

reach a layer that is equal to their density.

The settling of particles of water due to an increase in density will 

cause a compensating upward displacement of warmer water. These 

particles of warmer water will move upward until they reach a layer 

of equal density. The vertical movement of particles of water that 

is caused by changes in density is considered to be a significant factor 

in the distribution of temperature with depth in a reservoir.

The diffusion process of warmer water rising to the surface will 

continue as long as the temperature of the water is above 4°C, the temperature at which the density of water is maximum. When the temperature of the water is below 4°C, the colder water will float on the 

surface, and the warmer water will sink toward the bottom.

The conduction of heat from the surface to the water below depends 

on the temperature gradient and the coefficient of thermal conductivity, 

which is approximately 0.0014 calorie per second per square centimeter times degrees Centigrade per centimeter for the normal range 

of water temperature. As the transfer of heat by conduction is a slow 

process in water, the effect of this process on the distribution of tern-
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M5

1949) illustrate this phenomenon best. Whenever water was released 

from the lower gate, approximately 200 to 300 feet below the surface, 

the temperature of the released water remained almost constant 

throughout the year, even though there was a 20°C range in temperature at the surface. If the release is great enough to change the 

storage in the reservoir appreciably, the distribution of temperature in 

the reservoir is changed for the layer at which water was taken from 

storage.

EFFECT OF WIND

The effect of wind in the distribution of temperature in the reservoir 

is considered an important factor in the diffusion process of vertical 

mixing. The frictional drag of winds upon the water surface results 

in a current directed with the winds. This upper layer in turn exerts 

a frictional drag upon the one underneath and so on to lower levels. 

Thus, particles of water are set in motion, and any tendency of the 

particles to move in the vertical directions is influenced by the different 

densities.

Horizontal currents, as induced by wind, cause the water to pile to 

the leeward side of the reservoir and to lower correspondingly at the

10

TEMPERATURE, IN DEGREES CENTIGRADE 

20 0 10 20 30

100

300

400

Inflow temp 

18.4° C

J,

/

Temp

May 24, 1944

/ 

'"Velocity

4

Feb

Velocity

Temp

.20, 1945

Inflow temp 

9.4° C

Velocity

\ \

0.20 0.40 0 

VELOCITY, IN FEET PER SECOND

0.20 0.40

FIGURE 1. Temperature and velocity profiles as measured at Iceberg Canyon, 

Lake Mead, Ariz., on indicated dates. Data from Lake Mead Density Currents Investigations (National Research Council, 1949). 

762-906 O 65 2
M6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

windward side. When the density is equal throughout, a return gradient current due to this difference in pressure is generated in the deep 

strata, extending to a shearing zone near middepth which divides the 

water mass into two currents flowing in opposite directions. The 

horizontal currents are connected by one directed downward at the leeward side and upward at the windward side; thus a complete circuit 

is formed, and eventually the whole mass is thoroughly mixed.

The complete circulation of water in a reservoir as induced by the 

wind occurs only during the winter months or whenever the reservoir 

has an equal density throughout. For these periods of equal density, 

the lake water is overturned by the wind, and as the hypolimnion 

water is brought to the surface, a change in quality is easily detected. 

The complete overturn is usually noted in early and late winter for 

northern lakes. For southern lakes, one overturn can be detected in 

the late fall, and then the lake usually remains isothermal until the 

warming cycle is again resumed in the spring. During the other seasons, a circulation of this type is resisted by the different densities of 

water which restrict the circulation to the epilimnion and leave the 

thermocline and hypolimnion in a condition approaching stagnation. 

Also, the differences in viscosities corresponding to the water temperature contribute to the resistance to mixing.

PREDICTING THE DISTRIBUTION OF TEMPERATURE IN A

RESERVOIR

A mathematical expression was formulated by McEwen (1929) to 

predict the distribution of temperature with depths as a result of external agencies and internal processes. The expression is very complex 

and is based on many assumptions. McEwen (1929) also considered 

the chemical properties of the water so that the expression may be 

applied to the oceans.

As most lakes and reservoirs in a particular geographical area follow 

the same seasonal pattern of temperature distribution, engineers can 

predict with confidence the temperature distribution for a proposed 

reservoir from temperature observations of nearby reservoirs and lakes 

that have sufficient depth. Therefore, the use of such an expression as 

formulated by McEwen (1929) had very little practical use.

THE STABILITY OF THERMALLY STRATIFIED WATER

Water in a reservoir, which is thermally stratified and resists mixing 

and thereby causes the concentration of undesirable substances, justifies the formulation of a precise definition and a method of calculating 

this definition. Schmidt (1915) used the amount of work that must 

be expended to change the initial stratification into a stratification
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M7

of equal density throughout; this energy, which he called the stability, 

is the work required to raise the whole mass of water through a distance 

equal to the difference in height of the centers of gravity in the two 

conditions. The calculation of this energy requires observation of 

the temperature distribution from the surface to the bottom and of 

the relation of the horizontal sectional area of the reservoir with 

respect to depth.

To compute stability as defined by Schmidt, the reservoir is divided 

into layers of 1 to several meters thick, from the surface to the bottom 

with a known respective volume of water. The summation of each 

volume times its distance from the surface to its center of gravity 

which is divided by the total volume determines the center of gravity 

in reference to the surface. After the center of gravity is determined, 

the algebraic sum (plus above the center of gravity and minus below) 

of each volume times its distance from the center of gravity times 

its density determines the work required to raise the water for the two 

conditions. McEwen (1941) developed a tabular method for determining the stability which he described in detail.

The seasonal changes in the computed stability are illustrated in 

figure 2 for four lakes of different sizes. For these lakes, stability, or 

the work required to create isothermal conditions, is divided by the 

surface area for comparative purposes and is expressed as kilogrammeter per square meter.

140 p:

=1 <

* o

, o

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

FIGURE 2. A comparison of seasonal changes in stability expressed in terms of 

energy per unit area of surface for Lake Cachuma (200,000 acre-ft), Lower 

Otay Reservoir (57,000 acre-ft,) Lake Hodges (38,000 acre-ft), and Lake Wohlford (2,500 acre-ft) in California. Data for Lower Otay Reservoir and 

Lake Hodges (McEwen, 1941, fig. 5).
M8 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

TECHNIQUES TO ELIMINATE THERMAL 

STRATIFICATION

Three of the techniques that have been described in the literature 

use air bubbling as the means of inducing a mixing motion in the 

thermocline and hypolimnion. Each of these techniques uses a different approach to force air into the water.

FLOATING AERATOR

Reddick (1957) described a floating aerator which he designed for 

use on Indian Brook Reservoir at Ossining, N.Y. This reservoir has 

a maximum depth of 28 feet and a capacity of 316 acre-feet. The 

aerator, which is 40 feet long, floats on the surface at the deepest part 

of the lake. By use of a 3-inch steel pipe, which is submerged 71/£ 

feet below the surface, 160 cubic feet per minute (cfm) of free air is 

forced through nozzles on 1-foot centers. The bubble size, as visually 

evaluated, ranged from y2 - to 8-inch diameters.

After the aerator was installed, Reddick (1957) reported that the 

temperature of the water at the start of continuous operation ranged 

from 14° to 24° C and the dissolved oxygen ranged from less than 1 

part per million (ppm) to 8 ppm. After a week of operation the 

dissolved oxygen was constant with depth at approximately 6.5 ppm, 

or approximately 75 percent saturation. After 1 month of operation 

the dissolved oxygen was still constant with depth and had increased 

to 90 percent saturation.

The floating aerator, as designed by Reddick, required that the 

air compressor be located on the shore and that air be conveyed to the 

aerator by two 2-inch plastic hose lines, 400 feet in length. These 

lines floated on the surface and could present a problem of maintenance. The floating aerator could be adapted to larger and deeper 

reservoirs, but maintenance problems may be such that a submerged 

aerator is preferable.

AEROHYDRAULIC TECHNIQUE

The aeroliydraulic technique utilizes a device manufactured by 

Aero-Hydraulics Corp. of Montreal, Canada, that is patented and 

commercially available. This device, called an Aero-Hydraulic Gun, 

consists of a polyethylene stack pipe of various lengths in diameters 

of either 12 or 18 inches. The design of an air distributor at the bottom end permits the formation of a large bubble which acts as a piston 

in its movement up through the pipe. The manufacturer claims that 

an 18-inch pipe with a vertical length of 25 feet will move water at 

the rate of 12 acre-feet per day with a power input of less than 1 

horsepower.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M9

The manufacturer has reported, in a pamphlet, the results of two 

field tests of the Aero-Hydraulic Gun. The first field test was made 

at Belham Tarn Lake, Westmorland, England. This lake has a capacity of approximately 300 acre-feet and a maximum depth of 44 

feet. Five 12-inch guns 17 feet long were installed in the deepest 

part of the lake. The combined output of the guns was restricted 

to 11 acre-feet per day.

At the start of the test, the temperature of the water ranged from 

9° to 15°C, and the dissolved oxygen ranged from less than 1 to 9 ppm. 

After 2 weeks of operations, the temperature of the water ranged 

from 12° to 14° C, and dissolved oxygen ranged from 6 to 8 ppm.

The second test was made at Iriniscarra Reservoir, County Cork, 

Ireland. The capacity of this reservoir is 38,400 acre-feet with a 

maximum depth of 103 feet. Sis? 12-inch guns, 44 feet long, were 

installed so that the bottoms of the ^ims were 86 feet below the surface. 

The combined output of the guns was 61 acre-feet per day.

The purpose of the test at Inniscarra Reservoir was not to eliminate 

thermal stratification completely but only to eliminate it sufficiently 

to raise the dissolved-oxygen content to more than the minimum safe 

limit of 5 ppm for fish in the hypolimnion. However, this test was 

not completely successful as the dissolved oxygen was less than the 

safe limit for depths greater than 75 feet during the summer months.

The aerohydraulic technique may be more efficient than other 

methods in moving water from the hypolimnion to the epilimnion; 

however, if the density of the water does not change appreciably, the 

water will immediately sink back to the hypolimnion. For this reason 

it is difficult to determine whether the aerohydraulic technique is 

more efficient than other methods in mixing water.

PLASTIC HOSE AIB-DlSTBIBUTION SYSTEM

The plastic industry has formulated a special plastic tubing with 

a lead keel encapsulated to the bottom of the tubing. Fabricated 

into the %-inch tubing are tiny, barely visible check valves spaced 

iy2 inches apart.

Meyer (1962) described the ini

tubing for a 2.8-acre oxidation lagoon in an unincorporated area of

DuPage County, 111. In this in stallation 7,500 feet of the tubing

tallation of the special fabricated

was laid on the bottom of the lagoon, 4.5 feet below the surface. The 

amount of free air delivered to the distribution system was 150 cfm. 

The cost of the system amounted to approximately $6,500.

The purpose of the system as reported by Meyer (1962) was to 

increase the biological load capacity of the lagoon by a factor of four. 

It is not known at this time if the system has increased the load factor
M10 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

by 4, but the results have been encouraging. The dissolved-oxygen 

content of the lagoon averaged 10 ppm for the winter of 1962. During this period subzero weather was prevalent, and 10 percent of the 

lagoon was kept ice free by the air-bubbling system.

Another field test (Heath, 1961) using the perforated plastic tubing 

was made at a small idyllic lake called Langsjon in Sweden. This lake 

was completely devoid of dissolved oxygen and incapable of supporting plant or fish life. In this field test 1,640 feet of perforated plastic 

was weighted at intervals and laid on the bottom of the lake. A 10- 

horsepower compressor was used to supply air to the distribution system. After the system had been in operation 3 weeks, the dissolved 

oxygen had risen to 57 percent of saturation, and a growth of new 

plant life was well established. At the time the system was placed in 

operation, a temperature difference of 12 °C existed between the surface and the bottom of the lake; and after 3 weeks of operation, the 

lake was completely isothermal.

In the field tests of the perforated plastic tubing, an elaborate distribution system has been used. In the field test at Lake Wohlford 

the system for distributing the air in the lake was only 60 feet long. 

Although, except for Inniscarra Reservoir, Lake Wohlford is much 

larger than any of the lakes used for previous field tests, the system 

was very successful in eliminating thermal stratification, and for this 

reason more detailed information is presented.

TEST OF AIR-BUBBLING SYSTEM AT LAKE 

WOHLFORD

DESCRIPTION OF LAKE WOIELFOBD

Lake Wohlford is a canyon-type reservoir formed by an earthfill 

dam on Eseondido Creek 7 miles northeast of Escoiidido, Calif. The 

lake has a capacity of 7,000 acre-feet at full pool with a corresponding 

surface area of 222 acres. During the test period of 1962, the contents 

of the reservoir averaged approximately 2,500 acre-feet wth a surface 

area of 130 acres. Figure 3 shows a map of the lake during the test 

period.

The climate at Lake Wohlford is such that the temperature of the 

water always exceeds 4°C. According to Welch (1952), lakes and 

reservoirs with this characteristic are classified as tropical. The 

annual temperature cycle is illustrated in figure 4, which shows temperature profiles for various seasons of the year.

Natural inflow into Lake Wohlford is contributed by Eseondido 

Creek; however, as there is very little flow in the creek, the main source 

of supply is a diversion from San Luis Hey River by a canal with a 

maximum capacity of 70 cubic feet per second (cfs). The outlet for
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS Mil

releasing water from the lake consists of a concrete tower with three 

gates for releasing water into the 48finch pipe. The outlet gates are 

25 feet apart, the bottom gate being alt the lowest level of the lake.

QUALITY-OF-WAT^R PROBLEMS

Water that is released from Lake Wohlford is used primarily for 

irrigation, about 20 percent being u^ed for domestic purposes. The

Compressor and 

distribution system

Outlet tower

FIGURE 3. Lake Wohlford, Calif., showing location of instrumentation and airbubbling system.

10

20

30

40

50

Jan. 5

Mar. 5

Oct.

July 15

12 14 16 18 20 22 24 26 

TEMPERATURE, IN DEGREES CENTIGRADE

28

FIGURE 4. Temperature profiles of Lake ^Vohlford, Calif., for the various seasons

in 1060.
Ml2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

treatment of the water consists of a disinfection by chlorine when the 

water is released and occasional application of copper sulfate to the 

water in the lake.

The seasonal changes in climate at Lake Wohlf ord are accompanied 

by changes in the quality of the water. These changes are attributed 

to the life cycle of the various organisms, such as plankton and algae, 

which tend to proliferate at temperature levels peculiar to each genus 

and then to die and sink to the bottom of the lake when the life cycle 

is completed. The accumulation of organic matter on the lake bottom 

has increased the demand for oxygen in the decaying process; consequently, by 1959 the oxygen had been depleted from Lake Wohlford 

to the extent that most varieties of fish died.

In 1960, samples of water at the surface and near the bottom were 

taken near the dam at intervals of approximately a month. The 

samples were analyzed by the Escondido Mutual Water Co. for dissolved-oxygen concentration and pH. Table 1 shows the results of 

the samples on indicated dates and how the dissolved-oxygen concentration at the bottom decreased from 10 ppm in January to 1 ppm in 

August. The decrease of dissolved oxygen at the surface was not as 

great as the bottom, but may have been approaching the safe limit for 

fish production.

TABLE 1. Dissolved-oxygen and pH analyses of water taken from Lake Wohlford

in 1960 on indicated dates

Date

Jan. 5 ___ _ __ .__ _____ ___

Feb. 1 __ _ _ __ _________

Apr. !___ ___. ________________

May 31_______________________

Aug. 2 __ ____________________

pH

Surface

8.4 

8.4 

8.3 

8.6 

9. 0

Bottom

8.4 

8.2 

7.4 

7. 5 

7.6

Dissolved oxygen (ppm)

Surface

10 

10 

9

7 

6

Bottom

10

7 

4 

4 

1

Table 1 shows that pH increased in the surface water from January 

to August and decreased in the bottom water. These seasonal changes 

in pH as exhibited by Lake Wohlford appear to be normal, as observations made in other lakes show this same characteristic. Weiss and 

Oglesby (1962) observed this same phenomenon in the University 

Lake near Chapel Hill, N.C. They attributed the changes in pH at 

the surface to the photosynthetic activity of chlorophyll-bearing 

micro-organisms that use carbon dioxide, and the decrease in pH at 

the bottom as the result of an accumulation of carbon dioxide from 

respiratory activity.

Samples of water collected at the bottom of Lake Wohlford in the 

middle of April 1960 indicated the presence of hydrogen sulfide. The
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M13

presence of hydrogen sulfide indiqates a depletion of oxygen and a 

corresponding increase in the anaerobic organisms which can survive 

in the absence of oxygen. The concentration of hydrogen sulfide in 

the hypolimnion caused a very disagreeable taste and odor in the 

water whenever it was released through the lowest gate. The reason 

for releasing water through the lowest gate during the summer months 

was to gradually reduce the water stored in the hypolimnion to a 

minimum so that when the reservoir water overturned in late fall, 

the taste and odor problems would be minimized. This type of reasoning may be questionable, as some people would prefer to obtain 

the best quality of water as long! as possible and then endure the 

disagreeable taste and odor when thfey occur.

DESCRIPTION OF AI^l-BUBBLING SYSTEM

In April 1961 experiments were undertaken to determine the feasibility of using air to induce mixing from the epilimnion to the 

hypolimnion by bubbling air from the bottom of Lake Wohlford. 

A temporary line was lowered into the lake through which compressed 

air was forced by a portable compressor. In this experiment, thermal 

stratification was eliminated throughout the lake, which, in turn, improved the quality of the water.

As a result of the April experiment, a permanent air-bubbling system was installed. This system consisted of a 210-cfm (free-air) 

compressor powered by a 50-horsepower electric motor. Air is conducted from the compressor through a 2-inch galvanized pipe extending into the lake a sufficient distance to cool the air. Attached to the 

end of the pipe is a l^-inch polyvinylchloride plastic pipe that conducts the air to the point where }t is forced into the bottom water. 

At this point, 60 feet of the plastic pipe is perforated with 90 holes, 

9/64 inch in diameter, and spirally located to equalize the thrust of the 

escaping air. The plastic pipe is [suspended 5 feet above the bottom 

of the lake by an anchor as shown in figure 5. The location of the 

air-bubbling system in relation to Lake Wohlford is shown in figure 3.

The general plan of operation for the air-bubbling system was to 

start the compressor in the morning and let it remain in operation 

for approximately 9 hours each day. Of course, changes were made 

in the plan to meet the requirements of the special lake studies. The 

general plan was based on the hypothesis that, the bottom water, as 

it was brought to the surface, would be heated by solar energy and 

then would remain at the surface.

MIXING OF STRATIFIED WATER

The operation of the air-bubbling system in 1962 was delayed until 

April 18, when the lake exhibited a definite thermal stratification.
M14 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

At that time, the storage in the lake was 2,500 acre-feet, and the maximum depth was 50 feet. On the 17th of April, 17 profiles of temperature were taken at stations representing equal surface areas. The 

measurements of temperature were made with a standard resistance 

thermometer. These profiles were taken approximately daily thereafter to show the progress of the air-bubbling system in mixing the 

lake water. The 17 profiles were averaged for each day and are 

shown in figure 6. The temperature profile on April 25 indicates that 

the lake was nearly isothermal. For the period April 18-25, the 

compressor was operated 9 hours each day until April 24, when the 

compressor was left in operation for 24 hours.

Styrofoam cube

Eyebolt 

Plastic tubing

Clamp

Cable

Concrete anchor

FIGUBE 5. Anchor system to support plastic pipe 

5 feet above bottom of lake.
10

20

- 30

I

40

50

60

APRIL 17 18 19 20 23 25

7

10 14 18 10 14 18 10 14 18 10 14 18 10

TEMPERATURE IN DEGREES CENTIGRADE

14 18 10 14 18 22

FIGURE 6. Average of 17 temperature profiles for Lake Wohlford, Calif., which show the changes in the thermocline during

the initial operation in 1962 of the air-bubbling system.
Ml6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

The air-bubbling system was stopped from July 15 to July 24 in 

order that a thermocline could re-form in the lake. When the compressor was started again, detailed studies were made of the temperature of the boil which was formed by the air rising to the surface, 

the velocity profiles of the water in the lake, and the temperature 

profiles at the location of the velocity measurements.

The temperature studies of the boil formed by the air-bubbling system showed that the temperature of the water in the boil was constant 

throughout the upward movement of the boil. The surface area of 

the boil could be easily detected by the sharp contrast in the temperature between the outer edge of the boil and the unmixed lake water. 

During the period July 24-26, when the air-bubbling system was 

continuously operated, the surface area of the boil was approximately 

determined at daily intervals from the temperature contrasts at the 

edge of the boil. The changes in the surface area of the boil in relation to the number of hours of operation of the compressor are shown 

in figure 7. The rate of free-air injection was 210 cfm. Figure 7 

shows that the boil does not reach its maximum size until the compressor is operated for approximately 30 hours.

The velocity measurements were made with an aluminum vane which 

was designed by Shulman and Bryson (1961). This vane was supported at a particular depth by a string and a small polyethylene bottle 

which floated at the surface. The design of the vane was such that 

the area of vane was 20 times the area of the supporting bottle to 

minimize the surface velocities and wind effect. The movement of the 

bottle was followed by a transit on shore and a stadia in a boat. Whenever the boat was at the same location as the bottle, stadia distance 

and time readings were taken. These readings were plotted on a map 

of the surface area of the lake, and the velocities were determined from 

these plots. The velocity profiles are shown in figure 8 as observed 

for the raft and in figure 9 for the thermal survey station No. 8 during 

the period July 24-26. The temperature profiles at these stations and 

the temperature of the boil are also shown.

During the velocity studies it was difficult to separate the effect of 

the boil from that of the wind. Also, most of the velocity profiles indicate that all the water is going in one direction, which, of course, is 

impossible; but attempts to determine the location of the return flow 

were unsuccessful. The return flow probably occurs along the shores 

and shallow water, and the system of vanes and bottles was probably 

not satisfactory in the attempt to locate these flows. The velocity profiles as shown in figures 8 and 9 were influenced by the wind, but the 

changes in velocity distribution are considered to be caused by the 

temperature of the boil, as the wind was nearly constant during the 

study period.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M17

0.8

0.6

± 0.5

0.1

10 20 30 40 50 

COMPRESSOR OPERATION, IN HOURS

60

FIGUBE 7. Relationship between the surface area of the boil 

formed by the air-bubbling system and the number of hours 

of compressor operation for Lake Wohlford, Calif.

Figures 8 and 9 indicate that the velocities were influenced by the 

temperature of the boil in relation to the temperature profile. As the 

boil begins to form, its temperature is near that of the hypolimnion. 

At the same time, the velocity profiles indicate an increase in the 

velocity in this layer away from the boil area. As the mixing continues, the temperature of the boil increases, and the water becomes 

isothermal toward the surface. Also, as the water becomes isothermal, 

the velocities increase toward the surface with a corresponding decrease in the velocity of the water in the hypolimnion. This process 

will continue until the lake is isothermal.
M18 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

TEMPERATURE, IN DEGREES CENTIGRADE 

22 23 24 25 26 22 23 24 25 26 22 23 24 25 26 27

10

15

^ 20

30

35

Downwind

7

Wemp

/elocity

[7

July 24

,-Boil temp

7

July July 26

40 

-0.1 0 0.1 0.2 0.3 -0.1 0 0.1 0.2 0.3 -0.1 0 0.1 0.2 0.3 0.4

VELOCITY, IN FEET PER SECOND

FIQUBE 8. Velocity and temperature profiles as observed at the raft station in 

1962 for Lake Wonlford, Calif.

TEMPERATURE, IN DEGREES CENTIGRADE 

22 23 24 25 26 22 23 24 25 26 22 23 24 25 26 27

Dpwnwir 

I

Veloc

10

20

25

30

-Temp

July 24 Juy 25 uly 26

^

|~Boilternp

35 

-0.1 0 0.1 0.2 0.3 -0.1 0 0.1 0.2 0.3 -0.1 0 0.1 0.2 0.3 0.4

VELOCITY, IN FEET PER SECOND

FIQUBE 9. Velocity and temperature profiles as observed at thermal survey station No. 8 in 1962 for Lake Wohlf ord, Calif.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M19

The changes in the velocity distribution are considered to be caused 

by an excess of water around the perimeter of the boil, and this water 

has a temperature almost equal to that in the boil. The circulation in 

the boil cannot support this water, and consequently this water will 

descend in the lake until it reaches a layer of equal density. An accumulation of this water near the boil area will cause the water to move 

away in a horizontal direction and increase the velocity of the water 

at this depth. Therefore, this phenomenon would indicate that the 

rate of mixing in the lake is dependent on the size of the boil area. 

The rate of mixing, in turn, affects the ratio of the accumulated volume 

of excess water near the boil area to the volume of water in the lake 

of the same density. As this ratio decreases, the rate of mixing 

decreases.

The stability of the water in the lake has been computed daily for 

the period April 18-25 and July 24-26. Figure 10 shows the decrease 

in stability as the hours of operation of the compressor accumulate. 

The curve in figure 10 indicates that stability decreases much more

10 20 30 40 50 60 

OPERATION OF COMPRESSOR, IN HOURS

70

CO

FIGUKE 10. The decrease in stability in relation to the number of hours of 

compressor operation on indicated dates in 1962 for Lake Wohlford, Calif.
M20 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

rapidly when the lake is thermally stratified than when it is nearly 

isothermal. This curve confirms the previous reasoning that if the 

ratio of the volume of excess water to the volume of lake water of the 

same density decreases, the rate of mixing decreases.

WATER QUALITY RESULTS

During 1962, the Escondido Mutual Water Co. obtained weekly 

samples of water near the dam. These weekly samples were collected 

near the bottom and surface of the lake and were analyzed in the laboratory for dissolved-oxygen concentration and pH by the Hellige 

color comparator and applicable reagents. The results of these 

samples are shown in figure 11.

Figure 11, showing the seasonal variations in the concentration of 

dissolved oxygen near the surface and bottom of Lake Wohlford, indicates that the air-bubbling system maintained the dissolved-oxygen 

concentration in the hypolimnion above 5 ppm from April 18 to September 5 except for one period in August. During the rest of the year, 

the natural mixing motion of the wind maintained the concentration 

above 5 ppm. The concentration at the surface indicates that the 

water is occasionally supersaturated during the summer months. As 

this concentration seems to be extremely high, analyses of the water 

samples may be questionable, and a better method of determining the 

concentration of dissolved oxygen may be needed.

The pH determination shown in figure 11 indicates that the surface 

remained fairly constant during the year but that the bottom exhibited 

the same seasonal variation as in table 1. From this comparison, the 

air-bubbling system does not seem to affect the pH of the water in the 

lake.

During 1960, the standards of the U.S. Public Health Service for 

drinking water required a maximum dosage rate of chlorine of 12 ppm. 

In 1962, the maximum dosage rate was reduced to 7 ppm. The decrease in the required dosage rate is attributed to the air-bubbling 

system. Laubush (1958) stated that theoretically one part of H2S 

will oxidize with 8y% parts of C12 . The high dosage rate of 1960 was 

then required because the large concentration of H2S in the hypolimnion oxidized approximately half the chlorine before free available 

residual chlorine was obtained. In 1962, the concentration of H2 S 

in the hypolimnion was reduced by the air-bubbling system, and the 

reduction in concentration of H2S in turn reduced the required 

chlorine dosage rate.

The difference in the taste and odor of the water delivered to the 

city of Escondido in 1960 and 1962 is difficult to analyze quantitatively. 

However, the citizens of Escondido must have noted a definite change
r

I I I I I I I I I I I I I I I I I i i i i i i i i i i i

3d siavd

Nl 'N30AXO QBAHOSSia

3 2

S5

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a 

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M22 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

in the taste and odor of the water because complaints were received 

by the water company in 1963 when thermal stratification began to 

form in the lake and the dissolved-oxygen concentration in the hypolimnion was reduced.

EVAPORATION RESULTS

The measurement of evaporation from Lake Wohlford in 1962 was 

made by both the energy-budget and mass-transfer methods. The 

use of the energy-budget method in this study was mainly to determine the coefficient for the mass-transfer method which is utilized in 

the computations to evaluate the changes in evaporation rates. For 

a complete description of the two methods and necessary instrumentation, the reader is referred to the paper by Harbeck and others 

(1958).

The mass transfer equation is expressed as

E=Nu (e0 ea ), 

in which

E=evaporation in inches per day,

N=coefficient as determined by the energy-budget method,

u= wind speed in knots,

e0 =saturation vapor pressure corresponding to the temperature

of the water surface in millibars, and 

ea =vapor pressure of the air in millibars.

The measurements of wind speed (anemometer) and temperature of 

the water surface (thermograph) were made at the raft station and 

the measurement of vapor pressure (thermocouple psychrometer) at 

the radiation station (figure 5). The instruments were installed in 

November 1961. Previous to this time, the only measurement that 

could be used in the analysis to determine the changes in evaporation 

rates were observations of temperature of the water surface taken by 

the Escondido Mutual Water Co. at approximately 4-day intervals.

In order to determine the changes in the temperature of the water 

surface because of the air-bubbling system, the temperature observations of water surface were averaged to obtain monthly means for the 

period 1959-61. The mean temperatures were then- compared with 

the 1962 average monthly temperatures as determined by the thermograph at the raft. Figure 12 shows the comparison of temperatures 

between the period 1959-61 and the year 1962.

Figure 12 indicates that the air-bubbling system in 1962 reduced 

the surface temperature of Lake Wohlford in May, June, and July 

and increased it in October, November, and December. This type of 

deviation in temperature was expected. During the period May-July, 

the air-bubbling system was mixing the cold water normally found in
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M23

the hypolimnion with the warm water in the epilimnion. The mixing 

action decreased the temperature of the water surface during this 

period with an accompanying decrease in the evaporation rate. Because of the decrease in the evaporation rate and the gain in energy 

from incoming radiation and conduction, more energy was stored in 

the lake until the surface reached its normal temperature in August. 

The mean surface temperature for September 1959-61 indicated that 

the lake had started its cooling cycle; but the drop in surface temperature for September 1962 was less than that of 1959-61. The temperatures of the surface during the cooling cycle of 1962 remained higher 

in order to dissipate the additional energy.

Changes in the temperature of the water surface attributable to 

the air-bubbling system were accompanied by changes in the evaporation rates. To estimate the rate changes based on the sparse data 

available prior to 1962, the humidity and wind data observed in 

1962 were assumed to be the mean for the period 1959-61. Using

28

24

20

16

- 12

LJ 

(£.

\ 

(£. 8

Average 1959-61

FIGURE 12. A comparison of the monthly mean temperature of the water surface for the period 1959-61 

with the 1962 monthly average for Lake Wohlford, 

Calif.
M24 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

these data, monthly evaporation rates have been computed by the 

mass-transfer method for 1962 and an estimated monthly mean evaporation rate for the period 1959-61. The results of these computations 

are shown in figure 13.

Figure 13 indicates that evaporation was appreciably reduced in 

June and July of 1962 with a corresponding increase in October and 

November. The decrease in evaporation for the 2 summer months

FIGURE 13. A comparison of the 1962 monthly evaporation rates as determined by the mass-transfer method 

with monthly mean evaporation rates for the period 

1959-61 for Lake Wohlford, Oalif.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M25

was 2.9 inches, whereas the increase was only 1.3 inches for the two 

fall months. Because of the sparse data available prior to 1962, it is 

difficult to explain why the increase in evaporation during the fall 

months did not equal the decrease in the summer months.

The operation of Lake Wohlford is such that change in evaporation rates results in a greater saving in volume of water than if the 

surface area of the lake had remained constant through the year. 

During the winter and spring months water is stored in the lake, and 

the stage at the end of the storage period is maintained at this level 

through the summer months for recreational purposes. In the fall 

months, the stage of the lake is reduced in anticipation of winter and 

spring runoff. With this type of operation, the volume of evaporation saved by the air-bubbling system, may be determined.

To determine the net saving in volume of water for the 1962 year, 

the average area of the water surface for each month of this year was 

considered to be the same as the monthly mean surface area for the 

period 1959-61. Using the evaporation rates as determined previously and the respective areas, the volume of evaporation was computed for each month of 1962 and the monthly means for the period 

1959-61. The accumulated evaporation by months for the year 1962 

and the period 1959-61 are shown in figure 14.

Figure 14 shows that the divergence between the two curves is 

greater during the summer months than during the fall months; during the September-November period they are almost parallel, a fact 

indicating that the evaporation for this period in 1962 is almost the 

same as the period 1959-61. The total divergence between the two 

curves at the end of December indicates the net reduction in evaporation to be 35 acre-feet, and this reduction is attributed to the operation 

of the air-bubbling system.

CONCLUSIONS

Thermal stratification in lakes and reservoirs retards the naturalmixing motions between the hypolimnion and the surface until the 

concentration of certain substances in the hypolimnion becomes undesirable. To overcome this problem, a few investigators tried 

bubbling air from the bottom of the reservoir to artificially mix the 

water and break up the stratification. In the investigations cited, the 

investigators were pleased with the results obtained. However, none 

of these investigators presented enough data to define certain criteria 

needed for the design of such a system for other lakes and reservoirs. 

The Lake Wohlford study has presented considerably more detail; 

but, because of the rapid-mixing motions, there are still not enough 

data to define an air-bubbling system for a thermally stratified reser-
M26 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

voir of a given capacity. During 1963, additional data were obtained 

at Lake Wohlford; however, as the operator must serve the users, it was 

difficult to let the reservoir stratify to a condition desirable for further 

study. Additional studies of this type should be undertaken on larger 

reservoirs, for it is very difficult to extrapolate data from a reservoir 

whose capacity is 2,500 acre-feet to a reservoir 100 or 1,000 times as 

large.

The Lake Wohlford study has shown the importance of the wind 

in mixing the water in the lake if no thermal stratification is present. 

The only need of the air-bubbling system at Lake Wohlford is when 

the lake begins to store energy in the spring and summer.

The mixing motions as induced into the hypolimnion and epilimnion by the air-bubbling system are still not thoroughly understood. 

One conclusion that the operator made in 1962 was that the compressor should be operated for periods longer than 9 hours as first 

planned. The reasoning for this conclusion was the time needed to

700

600

500

400

300

< 

cc 

O

Q.

ui 200

100

Average 1959-61 / /

Q O O $

FIGURE 14. A comparison of the accumulated evaporation 

in acre-feet by months for 1962 with the monthly mean 

evaporation for the period 1959-61 for Lake Wohlford, 

Calif.
ELIMINATION OF THERMAL STRATIFICATION IN RESERVOIRS M27

build up the boil and horizontal circulation to patterns in the lake. 

The-operating procedure now is to operate the air-bubbling system 

until thermal stratification is eliminated. The compressor is then 

stopped until thermal stratification begins to form and dissolved 

oxygen in the hypolimnion approaches the safe limit for fish.

The economics of operating the air-bubbling system are difficult 

to evaluate. The operator stated that the reduction in chlorine needed 

to meet health standards compensated for the cost of operating the 

compressor. The removal of tastes and odors from the water supply 

could have been accomplished by a water treatment plant, but the difference between the cost of a compressor and a water treatment plant 

is considerable. Also, the difference in maintenance cost between a 

compressor and a water treatment plant is substantial. The improvement in the ^recreational benefits is very difficult to evaluate economically. For example, an air-bubbling system might forestall a fish kill 

such as occurred in 1959; if the game fish in the lake die, the lake is 

completely lost as a recreational facility for the remainder of the 

season. The savings in evaporation may not be considered significant 

when compared to the improvement in taste and odor and recreational 

facilities; but, because the problem of water shortage is increasing, 

an air-bubbling system may become economically feasible to reduce 

evaporation for some of the larger reservoirs.

REFERENCES CITED

Birge, E. A., 1916, The work of the wind in warming a lake: Wisconsin Acad. 

Sci. Trans., Arts and Letters, v. 18, pt. 2, p. 341-391.

Harbeck, G. E., Kohler, M. A., Koberg, G. E., and others, 1958, Water-loss investigations Lake Mead Studies: U.S. Geol. Survey Prof. Paper 298, 100 p.

Heath, W. A., 1961, Compressed air revives polluted Swedish lakes: Water and 

Sewage Works, v. 108, p. 200.

Laubush, E. J., 1958, Chlorination of water: Water and Sewage Works, v. 105, 

p. R234-R241.

Meyer, O. L., 1962, Aeration system increases lagoon capacity adds operating 

flexibility: Public Works, v. 93, no. 8, p. 79,80.

McEwen, G. F., 1929, A mathematical theory of the vertical distribution of 

temperature and salinity in water under the action of radiation, conduction, 

evaporation, and mixing due to the resulting convection. Derivation of a 

general theory, and illustrative numerical applications to a tank, a lake, and 

a region of the North Pacific Ocean: Scripps Inst. Oceanography Bull., v. 

2, no. 6, p. 197-306.

McEwen, G. F. 1941, Observations on temperature, hydrogen-ion concentration, 

and periods of stagnation and overturning in lakes and reservoirs of San 

Diego, California; Scripps Inst. Oceanography Bull., v. 4, no. 9, p. 219-260.

National Research Council, Interdivisional Committee on Density Currents, 

Subcommittee on Lake Mead, 1949, Lake Mead density currents investigations, 1937-40, v. 1, 2; 1941-16, v. 3: Washington, U.S. Bur. Reclamation, 3v., 

904 p.
M28 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

Reddick, R. M., 1957, Forced circulation of reservoir water yields multiple benefits at Ossining, N.Y.: Water and Sewage Works v. 104, no. 6, p. 231-237.

Schmidt, Wilhelm, 1915, Ueber den Energiegehalt der Seen, mit Beispielen voon 

Lunzer Untersee nach Messungen mit einem einfachen Temperaturlot: 

Internat. Rev. ges. Hydrobiologie u. Hydrographie, Hydrog. Supp. no. 1 to 

v. 6, p. 1-25.

Shulman, M. D., and Bryson, R. A., 1961, The vertical variation of wind driven 

currents in Lake Mendota: Limnology and Oceanography, v. 6, no. 3, p. 347- 

355.

Weiss, C. M., and Oglesby, R. T., 1962, Limnology of a North Carolina Reservoir 

and its use in water treatment: Am. Water Works Assoc. Jour., v. 54, no. 6, 

p. 671-683.

Welch, P. S., 1952, Limnology, 2d ed.: New York, McGraw-Hill, p. 538.

o