HYDROLOGY OF THE REELFOOT LAKE BASIN, OBION 

AND LAKE COUNTIES, NORTHWESTERN TENNESSEE 

U.S. GEOLOGICAL SURVEY 

Water Resources Investigations Report 85-4097 

PREPARED IN COOPERATION WITH THE 

TENNESSEE WILDLIFE RESOURCES AGENCY 
Cover--Model-simulated water-level contqur map and 3-D projection of 

potentiometric surface for Reelfoot Lake study area for September 

1984 water levels. Water levels in feet above sea,level. 
HYDROLOGY OF THE REELFOOT LAKE BASIN, OBION 

AND LAKE OXJNTIES, NORTHWESTERN TENNESSEE 

By Clarence H. Robbins 

U.S. GEOLOGICAL SURVEY 

Water Resources Investigations Report 85-4097 

Prepared in cooperation with the 

TENNESSEE WILDLIFE RESOURCES AGENCY 

Nashville, flennessee 

1985 
UNITED STATES DEPARTMENT OF THE INTERIOR 

DONALD PAUL RODEL, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Reck, Director 

For additional information apies of this report can be 

write to: purchased from: 

District Chief 

U.S. Geological Survey 

A-413 Federal Building 

U.S. Courthouse 

kshville, Tennessee 37203 

OperrFile Services Section 

Western Distribution Branch 

U.S. Geological Survey 

Box 25425, Federal Center 

lakewood, Colorado 80225 

(.Te.lephone: .(303) 236-7476) 
CONTENTS 

Abstract ........................................................ 

Introduction .................................................... 

' Purpose .................................................... 

Scope ...................................................... 

Surface-water hydrology ......................................... 

lake storage and depth characteristics ..................... 

Rainfall ................................................... 

Evaporation ................................................ 

Surfacewater inflow and outflow ........................... 

Estimated monthly water budget for Reelfoot Lake ................ 

Wcertainties in estimating the monthly 

water budget for Reelfoot Lake ........................... 

Groundwater hydrology .......................................... 

Ground-water model of the Reelfoot Lake study area ......... 

Model construction .................................... 

Model simulation for NovemberDecember, 1984 .......... 

Model simulation for August-September, 1984 ........... 

Model sensitivity ..................................... 

Reliability of model calibration ...................... 

Model synthesis of ground-water levels as affected 

by proposed lake-level manipulations ................ 

Summary ......................................................... 

Selected references ............................................. 

ILLUSTRATIONS 

Figure 1. Graph showing stageduration curve for 

Reelfoot Lake November 1970 to September 1983 ....... 

2-4. Graphs showing: 

2. Variation of the model-calculated ground-water 

inflow to Reelfoot lake and root mean square 

error as affected by changes in recharge rate . . 

3. Variation of the model-calculated groundwater 

inflow to Reelfoot Lake and root mean square 

error as affected by changes in the aquifer 

transmissivity ................................. 

4. Variation of the model-calculated ground-water 

inflow to Reelfoot Lake and root mean square 

error as affected by changes in the constantflux rate ...................................... 

Plates 

(In pocket) 

Plates l-9. Map showing: 

1. location of Reelfoot Lake, streamflow 

gaging stations, rainfall stations, lake-stage 

gages, observation wells, and physiographic 

features 

Page 

1 

1 

2 

2 

3 

3 

6 

6 

8 

11 

14 

16 

17 

17 

18 

19 

20 

21 

25 

26 

29 

5 

22 

23 

24 

.ii 
ILLUSTRATIONS 

Plates 1-9. Map showing:--Continued 

2. Generalized water-level contours and direction 

of ground-water flow for September, 1984 

3. Generalized water-level contours and direction. 

of ground-water flow for December, 1984 

4. Finite-difference grid and model boundaries 

for the Reelfoot Lake study area for the 

November-December simulation period 

5. Finitedifference grid and model boundaries 

for the Reelfoot Lake study area for the 

August-September simulation period 

6. Differences between model-calculated and observed 

water levels for November-December 1984 

simulation period 

7. Differences between model-calculated and observed 

water levels for August-September 1984 

simulation period 

8. Model-calculated grounhwater level declines for 

August-September period for proposed lake-level 

lowering of 5.8 feet below normal pool. Based 

on a pool elevation of 280.5 feet above sea level 

9. Model-calculated ground-water level increases for 

November-December period for proposed lake-level 

increase of 1.0 feet above normal pool. Based 

on a pool elevation of 282.8 feet above sea level 

Page 

TABLES 

Table 1. Surface area, contents, mean depth, and depth 

characteristics of Reelfoot Lake........................ 4 

2. Comparison of the total monthly rainfall 

over Reelfoot Lake for the period May-December 

1984 to the 30year standard normal (1951-80) 

monthly rainfall at the National Weather Service 

station at Samburg, Tennessee . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

3. Pan evaporation for National Weather Service stations 

at Martin and Jackson, Tennessee, and lakesurface evaporation for Reelfoot Lake................... 9 

4. Monthly runoff for surface-water inflow 

and outflow stations, estimates 

for ungaged area, and mean monthly runoff 

from a long-term continuousrecord station.............. 10 

5. Estimated monthly water budget for Reelfoot Lake 

for the period May 1 to December 31, 1984............... 12 

6. Estimated monthly water budget for Reelfoot Lake 

for refill period beginning November 1 in a 

normal climatic year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

7. Hydrologic components used in the monthly watepbudget 

calculations and percent error associated with 

the measurement or interpretation of each component..... 15 

iii 
CONVERSION FACTORS 

For readers who may prefer to use the International System of l.kAts 

(SI) rather than the inch-pound units used herein, the conversion factors are 

listed below: 

Multiply 

acre 

acre-foot (acre-ft) 

foot (ft) 

cubic foot per second (ft3/s) 

foot squared per day (ft2/d) 

inch (in.) 

inch per year (in/yr) 

square mile (mi2) 

mile (mi) 

EY 

0.4047 

1233 

0.3048 

0.02832 

0.0929 

25.40 

25.40 

2.590 

1.609 

To obtain 

hectare (ha) 

cubic meter (m3) 

meter Cm) 

cubic meter per second (m3/s) 

meter squared per day (m2/d) 

millimeter (mm) 

millimeter per year (mm/yr) 

square kilometer (km2) 

.- Y 

kilometer (km) 

iv 
HYDROLOGY OF THE REELFOOT LAKE BASIN, OBION 

,AND LAKE COUNTIES, NORTHWESTERN TENNESSEE 

Clarence H. Robbins 

ABSTRACT 

Hydrologic data and analyses and ground-water modeling results are 

provided to aid in the evaluation of proposed lake management strategies for 

Reelfoot Lake in northwestern Tennessee. Surface-water and ground-water data 

collected from May 1 to December 31, 1984, were used to evaluate the Reelfoot 

Lake hydrologic system. Long-term streamflow, rainfall, evaporation, and lakelevel data were used to describe the normal hydrologic conditions at Reelfoot 

Lake. 

Estimated monthly water budgets, assuming normal climatic and hydrologic 

conditions, were derived for Reelfoot Lake in order to determine the time 

required to refill the lake beginning on November 1, following a proposed 

mid-summer drawdown of 5.8 feet below normal pool. Additionally, estimates of 

the time required to fill the lake to 1.0 foot above normal pool were made. 

Results of these determinations indicate that a normal pool elevation of 282.20 

feet above sea level would probably be achieved by mid-January, and a pool 

elevation of 283.24 feet above sea level by the end of January. 

A calibrated two-dimensional ground-water flow model was used to simulate 

groundrater levels following the proposed lake drawdown and refill. Simulation results are presented as ground-water level difference maps which indicate 

the potential difference in ground-water elevations for the two proposed lakelevel extremes. Low lakelevel simulation results indicate ground-water levels 

may decline approximately 5.7 feet below normal in the areas immediately adjacent to the lake. The high lake-level simulation results indicate ground-water 

-levels may increase approximately 1.0 foot above normal in the areas immediately adjacent to the lake. 

INTRODUCTION 

Reelfoot Lake is located in northwestern Tennessee in Lake and Obion 

Counties (plate 1). It is the largest natural lake in Tennessee, with approximately 15,500 acres at a normal pool elevation of 282.2 feet above sea level. 

The Reelfoot Lake drainage basin covers 240 mi2, including a small part in 

Kentucky. The lake lies within the Mississippi embayment section of the Gulf 

Coastal Plain. Topographically, the area is characterized by several prominent 

physiographic features: Reelfoot Lake, Mississippi River and flood plain, Tiptonville Dome, a bluff line which bisects the basin along a northeast-southwest 

axis, and the uplands east of the bluffs (plate 1). 

Sedimentation from the bluffs, uplands, and flood plain is contributing to 

an accelerated deterioration of the lake and its wetland areas. The potential 
loss of Reelfoot Lake with its economic and wildlife resources has prompted the 

State to consider several management strategies designed to help restore the 

lake and its environs. 

The Tennessee Wildlife Resources Agency ('IWRA) has been designated by the 

State legislature as the lead agency for management of Reelfoot lake. TWRA has 

proposed a lake management strategy composed of a mid-summer drawdown of 5.8 

feet below normal pool (276.4 feet above sea level), followed by a temporary 

late fall refill to 1.0 foot above normal pool (283.2 feet above sea level). 

According to TWRA, the purpose of the drawdown would be to expose large areas 

of unconsolidated colloidal sediments and to allow these areas to compact and 

dry for a period of 120 days. The purpose of the temporary above normal pool 

refill would be to create a larger fish habitat area to aid in repopulation of 

the lake. 

Before this management strategy can be implemented, the State has deter 

mined that several potential environmental and social impacts must be evaluated. WRA was requested by a State legislative task force to prepare an environmental assessment of the impacts of this proposed management strategy on 

Reelfoot Lake and the surrounding area. The U.S. Geological Survey was requested by TWRA to make the necessary study pertaining to basin hydrology. 

Results of this study will be integrated by TWRA into an environmental assessment for Reelfoot Lake. 

PURPOSE 

The purpose of this report is to provide hydrologic data and analyses and 

ground-water modeling results that can be used in the evaluation of the lake 

management proposal. The purpose of the study is to describe: (1) hydrologic 

conditions during the study period, (2) normal hydrologic conditions at Reelfoot Lake, and (3) the potential effects of implementing the proposed lake 

management strategy on the surface-water and ground-water regimes when compared 

to normal hydrologic conditions. 

SCOPE 

This report describes the surface- and ground-water hydrology in the Reelfoot Lake basin based on available data and under the hydrologic conditions 

that existed during the period of data collection (May-December 1984). The 

report is intended to furnish the following information related to proposed 

lake-management strategy: 

1. The expected rate of refill to normal pool beginning November 1 under 

normal hydrologic and climatic conditions. 

2. The ground-water flow patterns in the Reelfoot Lake area. 

3. The expected effects of the Mississippi River on ground-water levels 

and surface runoff at high and low lake levels. 

4. The expected effects of above-normal lake levels on water levels in 

the alluvial watertable aquifer. 

:2 
5. The expected effects of below-normal lake levels on water levels in 

the alluvial water-table aquifer. 

6. The outflow discharge rates necessary for maintaining lake levels at 

the drawdown level during the period June 1 to October 31, under normal climatic and hydrologic conditions. 

l'he data used in this study were collected as part of a separate cooperative program between the U.S. Geological Survey, the Tennessee Department of 

I%ealth and Environment (Division of Water Management) and the Tennessee Wildlife Resources Agency. In addition to the data collected by the Geological 

Survey, rainfall and evaporation data were furnished by the National Weather 

Service in Memphis, Tenn. All surface-water and ground-water elevations in 

this report are referred to in feet above sea level. 

SURFACE-WATER HYDROLOGY 

LAKE STORAGE AND DEPTH CHARACTERISTICS 

Bathymetric contour maps of Reelfoot Lake (U.S. Army Corps of Engineers, 

1956) were updated using data from depth sounding surveys made in 1983 by the 

Water Quality and Watershed Research Laboratory of the Agricultural Research 

. Service, Durant, Oklahoma, and in 1984 by the Tennessee Wildlife Resources 

Agency. The updated bathymetric contour maps were digitized to obtain surface 

area, capacity, mean depth, and depth characteristics as shown in table 1. 

Interpolating between the 282- and 283-foot water-surface elevations in 

table 1, Reelfoot lake has a surface area of approximately 15,500 acres, a 

volume of approximately 80,300 acre-ft, and a mean depth of approximately 5.2 

feet at the normal pool elevation of 282.2 feet. Approximately 43 percent of 

the total lake area has a depth of 3.0 feet or less at this stage. At the proposed mid-summer drawdown elevation of 276.4 feet, Reelfoot lake would have a 

surface area of approximately 5,340 acres, a volume of approximately 19,700 

acre-ft, and a mean depth of approximately 3.7 feet. Approximately 68 percent 

of the total lake area would have a depth of 3.0 feet or less at this stage. 

The temporary late fall pool elevation of 283.2 feet would give Reelfoot lake 

a surface area of approximately 19,500 acres, a volume of approximately 99,700 

acre-ft, and a mean depth of approximately 5.2-feet. Approximately 44 percent 

of the total lake area would have a depth of 3.0 feet or less at this stage. 

A water-surface elevation staff gage was maintained by the Geological 

Survey at the Reelfoot Lake spillway from July 23, 1940, to December 31, 1970. 

Continuous water-surface elevation data have been collected since December 2, 

1970, at a gaging station about 0.75 mile west of the spillway. The maximum 

water-surface elevation of record at Reelfoot lake, 287.22 feet based on surveyed high-water marks, occurred in January 1937. The minimum watersurface 

elevation of record, 279.59 feet, occurred on November 20-21, 1953. 

Results of a stage-duration analysis based on the period of continuous 

record from December 1970 through September 1983 are shown in figure 1. The 

data base used for this analysis consisted of instantaneous lake stage readings 

3 
I 

I 

mm 

4 
cy. OD 

cy 

5 

cv (Y 

u 

8’ 

I 

. 

13i31 kS 3AdW 133iNl’ 

(NOllVA313 33vduns-td31vM) 30VlS 3)IVl 

5 
at midnight for the period of record. The analysis indicates that a watersurface elevation of 282.3 feet has been equaled or exceeded 50 percent of the 

time during the period of record. A watersurface elevation of 283.0 feet has 

been equaled or exceeded 5 percent of the time, and a water-surface elevation 

of 280.7 feet has been equaled or exceeded 99.98 percent of the time since 

December 1970. 

RAINFALL 

Total monthly rainfall over Reelfoot Lake from May 1 to December 31, 1984, 

was computed using the Thiessen method (Linsley and others, 1975, p. 78-83). 

Rainfall data needed for these computations were obtained from two Geological 

Survey stations at Reelfoot Lake and from the National Weather Service station 

at Samburg (plate 1). The total monthly rainfall was then compared to the 30- 

year standard normal (1951-80) monthly rainfall at the National Weather Service 

station at Samburg to determine the departure of the total monthly rainfall 

during the study period from rainfall for the 30year standard normal period 

(table 2). 

On an average annual basis, the minimum monthly rainfall normally occurs 

in October (2.55 inches) and the maximum monthly rainfall normally occurs in 

-March (5.05 inches). During the 8-month study period, a minimum monthly rainfall of 1.16 inches occurred in July, 1984, and a maximum monthly rainfall of 

9.89 inches occurred in October 1984. The July rainfall was 2.79 inches below 

normal and the October rainfall was 7.34 inches above normal (table 2). 

EVAPORATION 

Monthly pan evaporation data were obtained from the National Weather 

Service stations at Martin, Tenn. (approximately 28 miles east southeast of 

Reelfoot Leke), and Jackson, Tenn. (approximately 62 miles south southeast of 

Reelfoot Lake) for the period 1977-84. The Martin station is not operational 

during the months of Ibvember through March each year. heref ore, it was necessary to estimate pan evaporation at Martin for these months using the Jackson 

station assuming the interstation correlation is constant year round. The 

coefficient of determination (r2) which is the ratio of the sum of squares 

for this analysis was 0.97, and the equation-describing the relation is as 

follows: 

M = 1.015 - 0.56 (1) 

where M is mean monthly pan evaporation at Martin, Tenn., and 

J is mean monthly pan evaporation at Jackson, Tenn. 

Monthly free water-surfacq evaporation for the May 1 to December 31, 1984 

period was estimated using the Martin monthly pan evaporation data and mean 

monthly estimates, and a pan coefficient of 0.76 (U.S. Department of Commerce, 

1982). Although evaporation from a lake surface may differ significantly from 

free water-surface evaporation during a given month because of changes in heat 

storage in the lake, it was assumed for the purposes of this study that free 

water-surface evaporation and lake-surface evaporation were equivalent. The 

6 
d !z 

oz 

z 

d 0 

pr: 2 

7 
estimates of monthly lake-surface evaporation, and mean monthly pan evaporation 

for the National Weather Service stations at Martin and Jackson are listed in 

table 3. It should be noted the estimates of lake-surface evaporation (based 

on estimates of free water-surface evaporation) are probably biased by the 

hysteresis effect of heat storage in the lake. 

On an average annual basis, pan evaporation at Martin and lake-surface 

evaporation at Reelfoot Lake are lowest in January (0.69 inch and 0.52 inch, 

respectively) and highest in June (7.51 inches and 5.71 inches, respectively). 

During the study period, a minimum monthly mean pan evaporation at Martin (1.15 

inches) and a minimum monthly lake-surface evaporation at Reelfoot Lake (0.87 

inches) occurred in December. A maximum monthly pan evaporation at Martin 

(7.66 inches) and a maximum lake-surface evaporation at Reelfoot Lake (5.82 

inches) occurred in June during the study period. 

SURFACE-WATER INFLOW AND OUTFLOW 

Reelfoot Lake has three major tributaries, Reelfoot Creek, Indian Creek, 

and Running Slough, and one major outflow, Running Reelfoot Bayou (plate 1). 

Surface flow into and out of Reelfoot Lake was continuously monitored at gaging 

stations on each of these streams, except Indian Creek (070267951, during the 

study period, May 1 to December 31, 1984, as part of a separate cooperatively 

funded project. Indian Creek was monitored September 1 to December 31, 1984. 

The monthly flows for these stations were computed in terms of volume per unit 

time and then divided by the drainage area of each gaging station to obtain 

monthly surface runoff in inches. Monthly surface runoff into Reelfoot Lake 

at these gaging stations ranged from 9.14 inches for Running Slough at Ledford, 

Ky. (07026640), in May, to 0.0 inch for Running Slough and North Reelfoot Creek 

(07026370) in August and September (table 4). Surface outflow from Reelfoot 

Lake is regulated by a lorhead spillway with gates, and the discharge is usually a function of surface inflow. Outflow ranged from 5.22 inches in May to 

0.01 inch in September at the gaging station 1.5 river miles downstream from 

the spillway. 

Of the 240 mi2 drainage area, approximately 24.2 mi2 (10 percent) are 

covered by the lake at normal pool. Approximately 111 mi2 (46 percent) of 

the total drainage area had flows which were not measured from May to September 

1984, and from September through December 1984, approximately 103 mi2 (43 

percent) had flows which were not measured. Approximately 38.1 mi2 (37 percent) of the ungaged area are in the Mississippi River flood plain and are 

affected by water seeping from the Mississippi River during the months of 

December through May. During this period, the water-surface elevation of the 

Mississippi River is normally 10 to 20 feet higher, on the average, than the 

watersurface elevation of Reelfoot Lake and its tributaries draining the 

Mississippi River flood plain. 

The seepage water sustains flow in the flood plain tributary streams at a 

higher volume than that normally attributed to rainfall during the December 

through May period. For example, total surface runoff for Running Slough at 

ledford, KY., for May 1984 was 9.14 inches, while the total rainfall for the 

month was only 7.45 inches Streamflow data from the other surface inflow 

. 8 

h 
. 
m 

00 

F; 

,o 

; 

0 

d 

0 

d 

m 

m 

r; 

;;t 

; . 

.lO 
stations indicate that approximately 60 percent of the total rainfall in May 

became surface runoff. Assuming that the same percentage of total rainfall 

applies to the gage on Running Slough, approximately 4.5 inches of the 9.14 

inches of runoff in May was produced by rainfall. The remaining 4.6 inches of 

runoff is assumed to be seepage from the Mississippi River. 

Surface inflow from the remaining 64.9 ml '2 (63 percent).of the ungaged 

area is similar to that at the three gaging stations in the Mississippi bluff 

and upland areas (plate 1). Based upon the area1 extent of the two runoff-area 

types (the Mississippi River flood plain and the Mississippi bluff and uplands), monthly runoff was estimated for the ungaged area using a weighted 

average unit runoff coefficient. Results of these determinations, along with 

the measured total monthly runoff for the study period from the gaged areas 

are listed in table 4. 

In addition to streamflow data collected during the study period, the Geological Survey maintained a continuous-record streamflow station on Reelfoot 

Creek near Samburg, Tenn. (07026500),. from January 1, 1951, to September 30, 

1973. Monthly mean runoff was computed for this station for the period of 

record (table 4). The minimum monthly mean runoff of 0.21 inch occurred in 

October, and the maximum monthly mean runoff of 2.43 inches occurred in March. 

Months in which these extremes occurred are the same months in which the 

rainfall extremes occurred during the 30year standard normal period at the 

Samburg, Tenn., rainfall station (table 2). 

ESTIMATED MONTHLY WATER BUDGET FOR REELFOOT LAKE 

A monthly water budget for Reelfoot Lake was estimated for the study 

period May 1 to December 31, 1984. Qdrologic components included in the 

water budget were: (1) net change in lake storage, (2) surface-water inflow, 

(3) surface-water outflow, (4) rainfall, (5) lake-surface evaporation, and (6) 

ground-water inflow and outflow. 

Evapotranspiration losses were not evaluated, and the amount of groundwater inflow or outflow at Reelfoot lake was estimated as the residual of the 

monthly water budget. The ground-water inflow or outflow was estimated by 

substituting the monthly values of the hydrologic budget components indicated 

in the previous sections of this report into th.e following equation: 

S = SI+P-SO-E+GW (2) 

where S is net change in lake storage, in acre-feet, 

SI is total surface flow into the lake, in acre-feet, 

P is total rainfall into the lake, in acre-feet, 

SO is total surface flow out of the lake, in acre-feet, 

E is total lakesurface evaporation, in acre-feet, and 

+ GW is net ground-water flow into (+> or out of (-1 the lake, in 

acre-feet. 

Result6 of the computations using equation 2 and the values used for each 

hydrologic budget component are listed in table 5. Uncertainties associated 

with the monthly water-budget computations are discussed in a later section of 

this report. 

11 
Table 5 .--Estimated monthly water budget for Reelfoot Lake 

for the period May 1 to December 31, 1984 

Hydrologic Volume of water, in acre-feet 

budget 

component May June July August September October November December 

Net change -100 -10,600 -12,600 -7,700 -700 +27,000 +1,700 *24,700 

in lake 

storage 

Total rain- 11,260 

fall into 

lake. 

Total sur- 64,700 

face flow 

into lake. 

Total sur- 68,800 

face flow 

out of lake. 

Total lake- 7,360 

surface 

evaporation. 

Net ground- +lOO 

water flow 

into (+) or 

out of (-1 

the lake. 

2,440 1,340 1,600 4,040 11,350 5,560 10,640 

2,820 1,170 0 250 19,500 15,200 43,900 

4,870 4,350 3,560 3,560 17,300 38,900 

7,490 

-3,500 

6,390 4,060 2,410 2,920 1,270 

-4,370 -1,680 

130 

3,330 

-1,530 +2,120 +1,160 +10,330 

Calculations were made to determine the length of time required to refill 

the lake during a normal climatic year to normal pool (282.2 feet) from a water 

surface elevation of 276.4 feet beginning on November 1. The following assumptions were made for these calculations: 

(1) Normal monthly rainfall (table 2). 

(2) Normal monthly lake-surface evaporation (table 3). 

(3) Normal monthly surfacewater inflow (runoff) into the lake. 

(4) Negligible evapotranspiration losses. 

(5) No surface-water outflow. 

(6) The amount of water needed to refill the lake is the volumetric difference in storage between the starting and ending water-surface 

elevations from table 1 (60,600 acre-ft). 

(7) A net groundwater inflow of 5,000 acre-ft (based on average of 

October, November, and December ground-water inflow in table 5). 

12 
Mean monthly runoff from the loneterm continuous-record station (table 4) 

was used to compute normal monthly surface-water inflow to the lake from the 

bluff and upland areas for the refill period. In order to account for the 

effects of seepage from the Mississippi River on runoff volumes in the flood 

pla'in tributaries, an adjusted normal monthly surface-water inflow was computed 

for the flood plain area. 

A ratio of the 30year standard normal monthly rainfall to the study 

period monthly rainfall (table 2) was multiplied by the study period monthly 

runoff at Running Slough at Ledford, KY. (table 4), to obtain a normal monthly 

runoff for Running Slough. Normal monthly runoff for Running Slough was used 

to compute the normal monthly surface-water inflow to the lake from the flood 

plain area. Normal monthly inflow volumes for the bluff, upland, and flood 

plain areas were summed to give a total normal monthly inflow to the lake, 

item 3 above, for the refill period. 

This procedure required compilation of additional discharge and rainfall 

data for Running Slough for January 1985. The volume of runoff for Running 

Slough for January 1985 is 2.37 inches and the Thiessen corrected total rainfall is 1.69 inches. Results of the lake refill computations and the values 

used in equation 2 for each hydrologic budget component are listed in table 6. 

Table 6.-- Estimated monthly water budget for Reelfoot lake for refill 

period beginning November 1 in a normal climatic year 

Hydrologic budget 

component 

Total rainfall 

into lake. 

Volume of water, in acre-feet 

November l-30 December 1-31 January l-31 

1,940 2,870 4,040 

Total surface flow 

into lake. 

9,240 18,500 32,100 

Net ground-water 

inflow. 

5,000 5,000 5,000 

Total lake-surface 

evaporation. 

970 574 520 

Net change in 

lake storage. 

15,210 25,800 40,620 

Cumulative net 

change in lake 

storage. 

15,210 41,010 81,630 

Water-surface 

elevation, in feet 

above sea level 

by end of month. 

278.45 2g0.85 283.24 

13 
According to the calculations as shown in table 6, if no water is released 

through the spillway starting on November 1, Reelfoot Lake will fill to approximately 1 foot above normal pool by January 31 in a normal climatic year. 

' Depending upon climatic conditions, more or less time may be required to 

refill Reelfoot hake to a desired pool elevation in any given year. The controlling hydrologic factor in the monthly water budget is total rainfall. 

Rainfall determines: (1) the volume of water added directly to the lake surface; (2) the antecedent soil moisture conditions which control, to a large 

extent, the proportional volume of rainfall that becomes surface runoff; and 

(3) the volume of water available to groundqater recharge. Rainfall also 

influences river stage which affects the volume of seepage from the Mississippi River. 

UNCERTAINTIES IN ESTIMATING THE MONTHLY 

WATER BUDGET FOR REELFOOT LAKE 

Errors or uncertainties in measuring and estimating the hydrologic components interacting with a lake can have a significant impact on the calculation 

of the water budget of the lake. The errors can be large if one or more components are calculated as the residual term, and the errors in the measured 

.components are not considered in the interpretation of that residual term 

(Winter, 1981). 

The errors associated with the hydrologic components in a water budget 

can be broadly classified into those of measurement and regionalization (interpretation) (Winter, 1981). Measurement errors result from measuring a quantity 

at a point using imperfect instruments and inadequate sampling design and data 

collection procedures. Regionalization errors result from estimating quantities in a time space continuum through extrapolation from point data. Roth 

types of errors, in turn, are influenced by one’s understanding of the controlling physical principles; that is, the instrumentation, equations, and 

techniques used to regionally extend the point data (Winter, 1981). 

Water budgets, without estimates of errors, can be misleading and can give 

a false impression about how well the budgets are known. No water budget is 

without error when compared to the exact natural phonomena and interdependency 

of the controlling hydrologic components. Therefore, an error analysis of the 

hydrologic components in the monthly water budget for Reelfoot Lake was performed in order to put the monthly water budget in perspective. This error 

analysis allows the use and limitations of the information contained in the 

water budget to be assessed by the reader and allows the budget to be used 

realistically. 

Ground-water flow into or out of Reelfoot Lake was calculated as the 

residual of the water-budget equation; therefore, monthly groundwater flow 

estimates contain errors (uncertainties) resulting from each of the hydrologic 

components used in the water-budget equation along with the error associated 

with neglecting evapotranspiration losses. The magnitude of the error associated with measurement or interpretation of each hydrologic component was determined by methods described by Winter (1981) and is shown in table 7. 

14 
Table 7. --hydrologic components used in the monthly water-budget calculations 

and percent error associated with the measurement or interpretation of each 

component 

I-@drologic factor 

Total Ne t 

Total Total lake- groundNet change surface- surface- surface water 

in lake water water Total evapor- flow 

storage inflow outflow rainfall ation (residual) 

Estimate of 20 15 5 15 50 4W 

potential 

error, in 

percent. 

Average 2,130 2,770 878 905 2,200 1,310 

monthly 

quantity 

of water 

represented 

by the error, 

in acre-feet. 

* Varies monthly depending upon the magnitude of each hydrologic component's 

contribution to the total water budget, and decreases as longer time 

periods are considered. The error represents an average of the maximum 

potential positive and negative accumulated errors using the percentage 

figures given for each component in the table. 

In order to relate these estimates of error to the calculation of groundwater flow at Reelfoot Lake, numerous possible combinations of the magnitude 

and sign of the errors are possible. In computing the ground-water flow for 

this study, it was assumed that the errors shown in table 7 are independent 

and that there is some compensation of measurement and interpretation errors 

that are too high with those that are too low. Additionally, it was assumed 

that the monthly values of the hydrologic budget components in tables 5 and 6 

reflect this compensation and represent the best possible values using accepted 

hydrologic methodology and the given data base. 

A ground-water flow simulation model was developed to check independently 

the residual term of equation 2 (net ground-water flow into or out of Reelfoot 

lake) and to estimate the effects of the proposed lakeqanagement strategies 

on ground-water levels surrounding the lake. The model and the results of the 

analysis are discussed in the following sections of this report. 

15 
GROUND-WATER HYDROLOGY 

The following generalized discussion is from Strausberg and Schreurs 

(1958) except where noted. 

f The three uppermost geohydrologic units in the vicinity of Reelfoot Lake, 

in descending order, consist of approximately 100 to 200 feet of Mississippi 

River alluvium (water-table aquifer), approximately 250 feet of clay and fine 

sand (confining unit), and approximately 600 feet of highly permeable sand 

(Memphis Sand). Proposed lake-level manipulations are assumed to effect only 

the ground-water levels in the alluvial water-table aquifer. 

The alluvium consists of a sequence of sedimentary deposits which grade 

irregularly upward from gravel and coarse sand into progressively finer grained 

deposits of sand, silt, and clay. Average thickness of the alluvium is about 

140 feet. The alluvium may be divided into a lower permeable sand and gravel 

unit and an upper, less permeable unit, because of the general upward decrease 

in grain size. The ground-water system generally is under water-table conditions; however, localized artesian conditions may exist where the upper unit 

contains significant amounts of clay. 

Results of aquifer tests in the Mississippi River alluvium (Boswell and 

others, 1968) indicate that the transmissivity of the alluvium in the vicinity 

of Reelfoot Lake is approximately .45,500 ft2/d. Although transmissivity 

probably varies areally, no additional data are available, and the transmissivity is assumed to be constant throughout the study area. 

The principal source of recharge to the alluvial water-table aquifer is 

precipitation. According to a study by Zurawski (19781, between 17 and 20 per 

cent of the average annual precipitation becomes recharge to the alluvial aquifer. This percentage range yields approximately 8.1 to 9.6 in/yr of recharge 

on an average annual basis for the Reelfoot Lake basin and is assumed to be 

uniformly distributed over the basin. Seepage from the Mississippi River and 

its tributaries, surface runoff onto the alluvial plain from adjacent uplands, 

seepage from Reelfoot Lake, and underflow from the bluff area also contribute 

recharge to the alluvial aquifer. 

Gn a regional scale, ground water moves west toward the MiSSiSSippi River; 

however, when the water-surface elevation of the Mississippi River is higher 

than the adjacent water-table (generally during DecemberMay), the river contributes to ground-water recharge. The rate of groundrater flow to or from 

the river is dependent on the head gradient between the water table and the 

river assuming a uniform distribution of transmissivity. Ground water is also 

discharged to Reelfoot Lake, tributary streams, and as evapotranspiration by 

phreatophytes. 

A network of 31 observation wells was measured periodically throughout the 

study period. Fotentiometric surface maps of the study area (plates 2 and 3) 

were prepared for the low (September 1984) and high (December 1984) groundwater elevation periods based on data from the observation wells occurring at 

the end of each period. At the end of September 1984, ground-water flow was 

generally from the bluffs toward Reelfoot Lake and then south and west from 

a 
the lake toward the Mississippi River (plate 2). Additionally, water levels at 

the north end of the lake indicate a slight gradient (approximately 0.1 foot 

per mile) from the lake toward the Mississippi River. At the end of December 

1984, groundqater flow was generally from the bluffs and Mississippi River 

toward Reelfoot Lake (plate 3). Ground water flowed from the southern end of 

the lake toward the Mississippi River during this period also. At the north 

end of the lake in December, the flow direction was reversed from that in September because of head gradient changes. Roth periods indicate a ground-water 

divide exists between the lake and the Mississippi River (plates 2 and 3). 

Additionally, a groundwater mound is indicated between the southern end of the 

lake and the model boundary. 

GROUND-WATER MODEL OF THE REELFOOT LAKE STUDY AREA 

A computer program developed to simulate groundwater flow (McDonald and 

Harbaugh, 1984) was used to model the Reelfoot Lake study area. The program 

uses finite-difference techniques to solve the ground-water flow equation for 

two- or three-dimensional, steady or nonsteady flow in an isotropic or anisotropic, heterogeneous medium. The model was used in order to simulate and 

evaluate two-dimensional flow between Reelfoot Lake and the Mississippi River. 

.Vertical-flow components, which may be present locally where silt and clay 

confine the aquifer, are not simulated in the two-dimensional analysis. 

Neglecting vertical-flow components is assumed to have a negligible effect on 

the overall model analysis. The aquifer was assumed to be homogeneous and 

isotropic. 

The calibration scheme for modeling was to match the general hydrologic 

conditions and observed ground-water levels occurring at the end of two 

periods: August to September 1984 and November to December 1984. The physical 

properties of the aquifer, such as geometry and transmissivity, were held constant for each period. Seasonal variations between modeling periods were simulated by changing the water levels and the area1 extent of the lake, its tributaries and outflow, and the Mississippi River. Steady-state conditions were 

assumed for modeling purposes , although hydrographs from the observation wells, 

Mississippi River, and Reelfoot Lak-e, indicate that water levels changed very 

slowly during the two time periods. On the average, water levels in the observation wells, the Mississippi River, and in Reelfoot Lake declined 0.7 feet, 

5.0 feet, and 1.0 feet, respectively, during the August-September period and 

increased 0.5 feet, 2.5 feet, and 1.0 feet, respectively, during the November+ 

December period. Values for transmissivity and recharge were initially chosen 

based on the previous literature cited. 

Model Construction 

Finite-difference techniques used in the model require that the groundwater system be divided into rectangular blocks. Average values for the 

aquifer characteristics are assigned to each grid block, and water-level 

values are assigned and calculated at the center, or node, of each block. A 

grid having equal-sized grid blocks was used for the study area. Each grid 

.17 
c 

block represents an area of 0.21 mi2, 2,400 feet on a side. The finitedifference grid used to simulate groundrater flow in the alluvial aquifer for 

November-December is shown in plate 4 and for August-September, in plate 5. 

The aquifer was simulated as a one layer system having an assumed uniform 

transmissivity of 45,500 ft2/d for each grid block. The clay and fine sand 

(confining unit) underlying the alluviumwas assumed to be the impermeable base 

of the model. The south boundary of the model was assumed to be a no-flow 

boundary because groundwater flow is generally parallel to the boundary 

(plates 2 and 3). 

Grid blocks that represent the Mississippi River (north and west boundaries, plates 4 and 5) were simulated as constant head nodes, and groundwater 

flow across this boundary was calculated by the model for both simulation 

periods. Each grid block that represents the river was assigned a water level 

based on the measured or calculated stage at that river mile occurring at the 

end of each simulation period. The large river meander along the west boundary (plates 4 and 5) was projected onto the grid as a straight line. The water 

levels for these grid blocks were adjusted to compensate for the change in 

water-surface elevation with distance. 

Grid blocks corresponding to Reelfoot Lake, its tributaries, and Running 

Reelfoot Bayou, which is the lake outflow (plates 4 and 5) were also modeled as 

constant head nodes in direct contact with the aquifer. Ground-water flow 

across these boundaries was calculated by the model for both simulation per- 

-iods. Measured surfacewater levels which occurred at the end of each simulation period were used as input for the constant head nodes. Comparisons of 

the lake hydrograph with hydrographs of wells located near the lake indicate a 

good hydraulic connection between the lake and the aquifer. Changes in lake 

level and water levels in the aquifer appear to be simultaneous. 

Grid blocks representing inflow from the bluff area along the east bound- 

- ary (plates 4 and 5) were modeled as constant-flux nodes. The rate of flow for 

each simulation period was estimated from flow net analyses along the bluff 

line in plates 2 and 3. An average flow rate of 0.24 ft3/s was used for 

each constant-flux node during the August-September period, and 0.98 ft3/s 

for the November-December period. 

Model Simulation for Novepber-December, 1984 

Steady-state groundwater conditions were assumed for the NovemberDecember period. Hydrographs from the observation wells indicate water levels 

changed very slowly during this time period and the systemwas assumed to be in 

equilibrium. A net area1 recharge to the aquifer of 2.93 inches was assigned 

to each active model node. Ihis recharge rate is approximately equal to 25 

percent of the total rainfall during the 2month simulation period. 

Model results were compared to measured water levels and potentiometric 

contour lines from plate 3 and to estimates of the ground-water inflow to 

Reelfoot Lake (table 5). The average head difference between model-calculated 

and measured ground-water level values is 0.8 foot, and head differences range 

I8 
from 8.6 feet to -4.4 feet. The calculated root mean square error and standard 

deviation of the differences between model-calculated and measured ground-water 

levels is 2.5 feet and 2.3 feet, respectively. 

' Overall, the model-calculated water-levels are similar to the observed 

water levels. Model-calculated water levels generally were higher than measured levels except between the south boundary and the lake (plate 6). The 

lower model-calculated levels in this area may indicate that assumed aquifer 

transmissivity in this area was too large, or that groundqater seepage to 

tributary streams was overestimated, or that recharge rates to this area were 

underestimated. The higher model-calculated water levels along the east 

boundary of the model (plate 6) may have resulted from using an over-estimated 

average constant-flux rate for each node along the full length of the boundary, 

rather than a constant-flux rate for each node that varied with the computed 

flux rates from the flow net analyses. Righer model-calculated water levels 

may also be due to inaccuracies in simulating this boundary with respect to its 

actual location in the study area. 

The model-calculated ground-water inflow to Reelfoot Lake for the 2month 

simulation period is 71.6 ft3/s (8,660 acre-ft), and the estimated water 

budget ground-water inflow (table 5) is 95.0 ft3/s (11,490 acre-ft). In this 

report, the ground-water flow in acre-feet is a conversion of an instantaneous 

flow rate to a 2-month flow rate. The 23.4 ft3/s (2,830 acre-ft) discrepancy 

may result from underestimation of the area1 recharge rate during the simulation 

period, overestimation of the transmissivity distribution, or underestimation 

of the flux from the east boundary (bluffs). The estimated water-budget groundwater inflow could also be overestimated because of uncertainties associated 

with calculating groundrater flow as the residual of the water-budget equation. 

The potential error associated with this calculation is approximately 40 percent. Therefore, the model%alculated value is within the accuracy range of the 

water-budget method. 

Model Simulation for August-September 1984 

Steady-state ground-water conditions were also assumed for the AugustSeptember period. Hydrographs from the observation wells indicate water levels 

changed very slowly during this time period and the system was assumed to be in 

equilibrium. This model simulation included the same transmissivity value used 

in the simulation of November-December conditions. Values for constant head 

nodes were changed to simulate water levels in the river, lake, and streams 

occurring at the end of September. location of some constant head nodes were 

changed to reflect the lesser area1 extent of the river, lake and tributaries 

(plate 5). 

The recharge rate was varied areally in order to simulate evapotranspiration by phreatophytes in the approximately 31.5 mi2 wetland area (estimated 

from topographic maps) along the west and north shores of the lake. Grid blocks 

representing the wetland area were simulated with no recharge, because evapotranspiration is assumed to equal recharge in the wetland area. The remaining 

model nodes were assigned an area1 recharge of 2.93 inches for this simulation 

period. Although this recharge volume is equal to approximately 49 percent of 

19 
the total rainfall during the L-month simulation period, it was required in 

order to obtain an acceptable model simulation of observed ground-water levels. 

This recharge volume may be realistic because a lowered water table and other . 

possible hydrologic factors may have resulted in le+6s rejected recharge, resulting in a larger percentage of rainfall contributing to recharge. 

Model results for this simulation were compared to the measured water 

levels and potentiometric contour lines from plate 2 and to estimates of the 

groundwater outflow from Reelfoot Lake (table 5). The average head difference 

between model-calculated and measured ground-water level values is -1.9 feet, 

and head differences ranged from 4.8 feet to -11.5 feet. The calculated root 

mean square error and standard deviation of the differences between modelcalculated and measured ground-water levels is 3.8 feet and 3.4 feet, respectively. 

Overall, the model-calculated water levels are similar to the observed 

water levels. Model-calculated groundwater levels generally were lower than 

measured levels except between the north boundary and the lake (plate 7). The 

higher model-calculated water levels in this area may be due to underestimation 

of the evapotranspiration rate, overestimation of recharge to this area, or 

underestimation of the transmissivity distribution. The lower model-calculated 

_ water levels between the west boundary and the lake (plate 7) may result from 

projecting the large river meander onto the model grid as a straight line, 

placing the meander closer to the lake than its true location, or from over 

estimation of the transmissivity distribution, or from underestimation of 

recharge to this area. 

The model-calculated ground-water outflow from Reelfoot Lake for the 2- 

month simulation period is 11.7 ft3/s (1,410 acre-ft). The estimated water 

budget 

s 

round-water outflow is 26.5 ft3/6 (3,210 acre-ft) (table 5). The 

14.8 ft /s (1,800 acreft) discrepancy may be due to an overestimation of 

evapotranspiration for the entire wetland area in the model simulation, an 

overestimation of transmissivity, an underestimation of the constant flux from 

the east boundary (bluffs), or an underestimation of recharge to the flood 

plain. The estimated water-budget ground-water outflow could also be ovep 

estimated because of uncertainties associated with calculating ground-water 

flow as the residual of the water-budget equation. The potential error associated with this calculation is approximately 40 percent. Therefore, the modelcalculated value is only slightly outside the accuracy range of the water 

budget method. 

Model Sensitivity 

Tests of model sensitivity to variations in input values were made as an 

integral part of the model-calibration process. The procedure was to hold all 

input values constant for ,the November-December simulation period except the 

one being analyzed and to vary that value through a range that included the 

uncertainty in the value. Variations in the root mean square error for water 

levels and model-calculated ground-water inflow to Reelfoot Lake from acceptable calibrated values were used to analyze model sensitivity. 

20 
Input values that were varied included transmissivity, recharge, and the 

constant-flux rate (figs. 2, 3, and 4). Model-calculated ground-water levels 

were most sensitive to variations in the recharge rate and transmissivity, and 

least sensitive to variations in the constant-flux rate (figs. 2, 3, and 4). 

Doubling the recharge rate increased the root mean square error by approximately 3.5 feet, and decreasing the recharge rate by half increased the root 

mean square error by 1 foot (fig. 2). Decreases in transmissivity increased 

the root mean square error by as much as 5.0 feet, and increases in transmissivity increased the root mean square error by as much as 1.5 feet (fig. 3). 

Variations in the constant-flux rate (fig. 4) had a negligible effect on the 

root mean square error because the constant-flux rate affects only a small part 

of the overall model area. 

Model-calculated ground-water inflow to Reelfoot Lake was most sensitive 

to variations in the recharge rate and about equally sensitive to variations 

in transmissivity and the constant-flux rate. Doubling the recharge rate 

increased the ground-water inflow by 99 percent (70.9 ft3/s), and decreasing 

the recharge rate by half decreased the groundwater inflow by 48 percent (34.4 

ft3/s) (fig. 3). Doubling the transmissivity decreased ground-water inflow 

from the Mississippi River by 25 percent (17.9 ft3/s), and decreasing the 

transmissivity by half increased the groundwater inflow from the Mississippi 

. River by 13 percent (9.3 ft3/s) (fig. 2). Ground-water inflow*to Reelfoot 

Lake is controlled by the head gradient between the aquifer and the lake. As 

transmissivity is increased, the heads are lowered and the gradient flattens 

and less flow is delivered to the lake. As transmissivity is decreased, the 

heads are raised and the gradient steepens and more flow is delivered to the 

lake. Doubling the constant-flux rate increased groundwater inflow to the 

lake by 30 percent (21.5 ft3/s), and decreasing the constant-flux rate by 

half decreased ground-water inflow by 15 percent (10.7 ft3/s)(fig. 4). 

Tests of model sensitivity for the August-September period show approximately 

the same relations except ground-water outflow from Reelfoot Lake is affected 

by variations in transmissivity. 

Reliability -of Model Calibration 

The model-calculated water levels for the two simulation periods generally 

are within + 5 feet of the observed water levels for these periods. Reliability of the model results cannot be evaluated solely on the basis of the similarity in water levels because the water levels are, to a large extent, controlled by river and tributary stream elevations and constant head values input 

to the model. Similarity of model-calculated water levels to measured levels 

does suggest, however, that the flow quantities and aquifer characteristics 

used in the model are within reason. The model-calculated ground-water inflow 

and outflow at Reelfoot Lake generally approximate the estimated ground-water 

inflow and outflow (table 5). Model results are thought to be as reliable as 

the estimates reported in table 5 (within approximately 40 percent). Galibration standards were set by the sensitivity analysis. A calibration was 

considered acceptable when the values used for transmissivity, recharge, and 

constant flux kept the root mean square error for water levels less than 4.0 

feet. A limiting value of 4.0 feet was chosen for the root mean square error 

because of the generalized nature of the model, use of uniform average transmissivity values, and uncertainty in some of the model parameters. 
\ 

. 

.22 
. 

i 

. 

0 

0 

I 

b 

‘23 
. 

24 
The relation between evapotranspiration and ground-water inflow and outflow at Reelfoot Lake is the least known factor in this study and, therefore, 

the most open to question. Evapotranspiration was assumed to affect the 

August-September simulation period only and was assumed to be equal to the 

recharge rate. The actual evapotranspiration rate probably is greater than 

the recharge rate for this season of the year, and the area1 -distribution of 

this rate is dependent on phreatophyte type and density. Use of average 

uniformly distributed recharge and evapotranspiration rates may be considered 

rough approximations at best. 

The value used in the model for aquifer transmissivity is the value 

reported by Roswell and others (1968) and is considered reliable. Aquifer 

transmissivity may vary areally around Reelfoot Lake; however, the use of a 

uniform transmissivity value did result in an acceptably calibrated model for 

the study area without varying other parameters areally. 

Model-calculated ground-water inflow to Reelfoot Lake indicates that the 

major source of ground-water inflow to the lake is from the bluff area during 

the winter months. The major mechanisms of discharge from Keelfoot Lake in 

the winter months are controlled surface outflow and ground-water outflow. 

Analysis of the model results suggests that the bluff area 'is also the 

major source of ground-water inflow to the lake during the summer months. The 

major mechanisms of discharge from Reelfoot Lake during the summer months are 

evaporation, transpiration, ground-water outflow to the Mississippi River, and 

controlled surfacewater outflow. 

Overall comparisons of the model-calculated water levels to observed water 

levels are acceptable. The model reasonably simulated ground-water conditions 

at Reelfoot Lake for two different ground-water and surface-water conditions, 

and the model may be considered as a reasonable representation of the groundwater system under equilibrium conditions. A transient simulation might more 

-closely represent the actual conditions in the aquifer because a longer timeperiod may be required for the flow system to reach steady-state than assumed, 

and a large quantity of water is potentially available from storage. 

MODEL SYNTHESIS OF GROUND-WATER LEVELS AS AFFECTED 

BY PROPOSED LA’KE-LEVEL MANIPULATIONS 

The calibrated model for August-September 1984 and November-December 1984 

was used to simulate ground-water levels resulting from the proposed lakelevel manipulations under the same hydrologic conditions as those in the two 

calibration periods. The simulation was accomplished by changing only the 

water-surface elevations in the grid blocks representing Reelfoot Lake and its 

tributaries in the wetlands. Previously simulated hydrologic properties were 

not changed. The water level in the lake was changed from 280.5 feet to 276.4 

feet (4.1 feet lower) for the August-September period, and from 282.8 feet to 

283.2 feet (0.4 foot higher) for the November-December period. Results of the 

synthesis are shown as ground-water level difference maps in plates 8 and 9. 

25 
Ground-water level decreases of approximately 4.0 feet in the area bordering the lake are indicated in plate 8. It is also noted that ground-water 

levels decreased less with increased distance from the lake. Ground-water 

level decreases shown in plate 8 may represent the maximum potential drawdown 

because as ground-water levels drop below the phreatophyte root zones, evapotranspiration losses may decrease. Ihis would result in less drawdown than 

that indicated by plate 8 in the wetland areas. It should be noted that the 

Mississippi River ultimately controls the amount of groundrater level decrease 

at great distances from the lake. Actual ground-water level decreases near the 

river will, therefore, be more dependent on river stage than lake stage. 

The lake level during the August-September simulation period was 280.5 

feet which is 1.7 feet lower than the normal pool elevation of 282.2 feet. In 

order to assess the potential impact of a lake drawdown of 5.8 feet from a 

normal pool elevation of 282.2 feet to a pool elevation of 276.4 feet, a constant of 1.7 feet must be added to each of the lines of equal value in plate 8. 

Model results for the 2qonth simulated drawdown period indicate a net 

ground-water inflow of approximately 1.4 ft3/s (167 acre-ft) to Reelfoot 

lake. Therefore, although initial surface-water outflow may be much greater, 

a sustained outflow of approximately 1.4 ft3/s would be needed in order to 

maintain the lower pool level. 

Ground-water level increases of approximately 0.4 foot in the area border 

ing the lake are indicated in plate 9 for the simulation of abovenormal lake 

level. It is also noted that ground-water levels increased less with increased 

distance from the lake. Effects of evapotranspiration for this seasonal period 

would probably be negligible because the phreatophytes are normally dormant 

during the winter. 

During the November-December simulation period, the water surface of Reelfoot Lake was at elevation 282.8 feet which is 0.6 foot higher than the normal 

pool elevation of 282.2 feet. In order to assess the potential impact of a 

lake level 1.0 foot above a normal pool elevation of 282.2 feet to a pool elevation of 283.2 feet, a constant of 0.6 foot must be added to each of the lines 

of equal value in plate 9. Ghanges in ground-water levels near.the river will 

be controlled by river stages more than by lake stages. 

SUmMARY 

Surface-water and ground-water data collected from May 1 to December 31, 

1984, under an ongoing separate cooperatively funded project were used to 

evaluate the hydrologic system of Reelfoot Lake. Long-term streamflow, rainfall, evaporation, and lake-level data were used to describe normal hydrologic 

conditions at Reelfoot Lake. These data were also used to assess the potential 

hydrologic effects of a mid-summer lake drawdown of 5.8 feet below normal pool 

(276.4 feet) and a late fall lake refill to 1.0 foot above normal pool (283.2 

feet) on ground-water levels surrounding Reelfoot Iake. 

26 
Estimated monthly water budgets, assuming normal climatic and hydrologic 

conditions, were derived for Reelfoot Lake in order to determine the time 

required to refill the lake to normal pool and then to 1.0 foot above normal 

pool beginning on November 1 of a normal year. Results of these determinations indicate that a normal pool elevation of 282.2 feet would probably be 

achieved by mid-January, and a pool elevation of 283.2.4 feet by the end of 

January. 

Ground-water level data from 31 observation wells were used to develop 

potentiometric contour maps for the lowest (September) and highest (December) 

ground-water levels during the study period. The potentiometric maps indicate 

ground-water flow generally is from the bluffs toward the Mississippi River 

and Reelfoot Lake. The direction of ground-water flow is controlled by head 

differences between the water-table aquifer, the Mississippi River, and Reelfoot Lake. Ground-water level data and surface-water records indicate the 

Mississippi River contributes to groundrater recharge and surface runoff from 

December to May when river stages are normally 10 to 20 feet higher than the 

adjacent water-table elevations and lake stages. 

Ground-water level data were also used to calibrate a two-dimensional 

ground-water flow model for two steady-state periods (August-September 1984 and 

November-December 1984). The calibrated model for August-September 1984 and 

November-December 1984 wa6 then used to simulate ground-water levels following 

a lake drawdown of 5.8 feet below normal pool and a lake refill to 1.0 foot 

above normal pool and to check independently the residual term (net groundwater flow into or out of Reelfoot Lake) of the waterbudget equation. 

Overall comparisons of the model results to observed groundwater conditions during the study period are acceptable. The model reasonably simulated 

groundwater conditions at Reelfoot Lake for two different ground-water and 

surface-water conditions, and the model may be considered as a reasonable 

representation of the groundwater system under equilibrium conditions. Modelcalculated water Levels for the two simulation periods generally are within + 5 

feet of the observed water levels for these periods. 

Model-calculated ground-water inflow and outflow at Reelfoot Lake gene 

rally approximate the estimated water-budget ground-water inflow and outflow. 

The model-calculated ground-water inflow for the NovemberDecember period is 

71.6 ft3/s, and the estimated waterbudget ground-water inflow is 95.0 

ft3/s. The model-calculated ground-water outflow for the August-September 

period is 11.7 ft3/s, and the estimated water-budget ground-water outflow is 

26.5 ft3/s. The larger discrepancy for the August-September period may be 

due to overestimation of evapotranspiration from the wetlands, overestimation 

of transmissivity, underestimation of the constant flux from the bluffs, or 

underestimation of recharge to the flood plain. The estimated water-budget 

ground-water outflow could also be overestimated because of uncertainties 

associated with calculating ground-water flow as the residual of the waterbudget equation. The potential error associated with this calculation is 

approximately 40 percent. 

Low lake-level simulation results indicate ground-water levels may decline 

approximately 5.7 feet below normal in the areas immediately adjacent to the 

‘27 
lake. The decline in ground-water levels will be less significant radially 

outward from the lake. High lake-level simulation results indicate groundwater levels may increase approximately 1.0 foot above normal in the areas 

immediately adjacent to the lake. The increase in groundrater levels also 

becomes less radially outward from the lake. 

The calibrated model for the hgust-September 1984 period was used to 

calculate the ground-water inflow and outflow at Reelfoot Lake during the 2- 

month simulated drawdown period. Model-calculated groundwater flow results 

indicate that the net ground-water flow during this period is approximately 

1.4 ft3/s (167 acreft) into Reelfoot Lake. Therefore, a sustained surfacewater outflow rate of approximately 1.4 ft 3 1s would be required to maintain 

the drawdown pool level. Rvapotranspiration, groundwater outflow, and controlled surface-water outflow are major discharge mechanisms during the summer 

months. 

28 
SELECTED REFERENCES 

Roswell, E. H., tishing, E. M., and bsman, R. L., 1968, eaternary Aquifers in 

the Mississippi embayment with a discussion of Quality of water by H. G. 

, Jeffrey: U.S. C;eological Survey Professional Paper 448-E, 15 p. 

Linsley, R. K., Jr., Kohler, M. A., and Paulhus, J. L. H., 1975, Hydrology for 

Engineers (2d ed.): New York, McGraw-Hill, 482 p. 

McDonald, M. G., and Harbaugh, A. W., 1984, A modular three-dimensional finitedifference groundwater flow model: U.S. Geological Survey Open-File 

Report 83-875, 528 p. 

Strausberg, S. I., and Schreurs, R. L., 1958, Ground-water resources of the 

Mississippi alluvial valley of Tennessee: U.S. Geological Survey openfile report, 49 p. 

U.S. Army Corps of Engineers, 1956, Sedimentation survey-Reelfoot Lake, File 

232/2(1-31, 3 p. 

U.S. Department of Commerce, 1982, Evaporation Atlas for the contiguous 48 

Lhited States: National Oceanic and Atmospheric Administration, 

Technical Report NWS 33, 26 p. . 

Winter, T. C., 1981, Uncertainties in estimating the water balance of lakes: 

Water Resources Hulletin, AWRA, v. 17, no. 1, 34 p. 

Zurawski, Ann, 1978, Summary appraisals of the Nations ground-water resourcesTennessee region: U.S. Geological Survey Professional Paper 813-L, 35 p. 

29