SWGW A COMPUTER PROGRAM FOR ESTIMATING 

GROUND-WATER DISCHARGE TO A STREAM USING 

STREAMFLOW DATA

By GREGORY C. MAYER and L. ELLIOTT JONES

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 96-4071

Atlanta, Georgia 

1996
U.S. DEPARTMENT OF THE INTERIOR 

Bruce Babbit, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

The use of trade names in this report are used far identification purposes only and 

does not imply endorsement by the U.S. Government

For additional information, including how to acquire 

the subject computer program, please write to:

District Chief

U.S. Geological Survey

3039 Amwiler Road, Suite 130

Peachtree Business Center

Atlanta, GA 30360-2824

Copies pf this report may be purchased from:

U.S. Geological Survey

Branch of Information Services

Denver Federal Center 

Box 25286, Mail Stop 517 

Denver, CO 80225-0286
CONTENTS

Abstract 1 

Introduction 2

Problem 2

Purpose and scope 4

Hydrograph separation 4 

Program SWGW 5

Design and features 5

Examples of SWGW analyses and comparisons 12

Statistical analysis of example results 17

Discussion 19 

Selected references 20

ILLUSTRATIONS

Figure 1. Map showing location of streamflow gaging stations and respective drainage areas

used in this report, and the Apalachicola-Chattahoochee-Flint and Alabama-CoosaTallapoosa River basins (ACF-ACT) study area 3

2. Streamflow hydrograph of the Flint River near Culloden, Georgia, (station 02347500) 

for water year 1941, showing the major rise and the major recession periods 6

3. RORA hydrograph separation and SWGW adjusted recession based on streamflow 

data from Big Hill Creek near Cherryvale, Kansas (station 07170700), for 

March 1974 8

4. Results of automated streamflow hydrograph separation using four adjustment options 

of SWGW:

(a) Chattahoochee River at West Point, Georgia (station 02339500), for water 

year 1952 10

(b) East Fork of Stones River at Woodbury, Tennessee (station 03426800), for 

water year 1975 11

TABLES

Table 1. Streamflow-gaging stations where separation results based on manual methods are

compared to results from automated recession-curve-displacement procedures, and 

characteristics of the respective drainage basins 12

2. Summary of estimates of baseflow based on manual and automated recession-curvedisplacement procedures for scenario (a), Kmr=Ket 13

3. Summary of estimates of baseflow based on manual and automated recession-curvedisplacement procedures for scenario (b), Kmf>Ket 15

4. Summary of statistics for sets of differences of SWGW computed estimates of 

baseflow from manually derived estimates of baseflow 18

in
IV
CONVERSION FACTORS AND ABBREVIATIONS AND ACRONYMS

CONVERSION FACTORS

Multiply to obtain

inch (in.) 

foot (ft) 

mile (mi)

square mile (mi2)

per second (ft3/s)

Length

25.4 

0.3048 

1.609

Area

2.59

Flow 

0.02832

millimeter 

meter 

kilometer

square kilometer

cubic meter per s

ABBREVIATIONS AND ACRONYMS

ACF Apalachiacola-Chattahoochee-Flint

ACT Alabama-Coosa-Tallapoosa

AD APS Automated Data Processing System, USGS internal

ASCII American Standard Code Information Interchange

G2 A USGS-developed computer graphics application

QCCHEK A subroutine of the computer program SWGW

RORA A computer program (Rutledge, 1993) based on methods 

developed by Rorabaugh (1964)

SWGW A computer program (this report) developed from RORA

TRANS A computer program (Rutledge, 1993) used to format input 

data for the programs RORA and SWGW

UNIX A computer operating system 

USGS U.S. Geological Survey
SWGW A COMPUTER PROGRAM FOR ESTIMATING 

GROUND-WATER DISCHARGE TO A STREAM USING

STREAMFLOW DATA

by GREGORY C. MAYER and L. ELLIOTT JONES

ABSTRACT

The computer program SWGW automates 

the application of the recession-curvedisplacement method for estimating groundwater discharge to streams (baseflow) from 

streamflow data. SWGW was developed from 

an existing U.S. Geological Survey computer 

program, RORA, and provides the user with 

four enhancements useful for the analysis of 

streamflow data. These enhancements (1) 

compute an adjustment factor to account for 

the apparent over-estimation of the recession 

curve by RORA; (2) produce data files for 

graphical output of hydrograph separation 

results; (3) incorporate two recession 

indices one for the major rise period and one 

for the major recession period; and (4) 

consider streamflow data on a water-year 

basis. An adjustment factor is computed for 

each day following a streamflow peak that the 

computed recession discharge exceeds 

measured streamflow discharge. The adjustment factor equals the measured streamflow 

discharge divided by the computed recession 

discharge. For each peak, minimum adjustment factors are determined and are used to 

provide the user four choices for adjustment of

the recession curve. The choices are (1) no 

adjustment (equivalent to RORA); (2) 

adjustment of the entire recession curve by the 

average of the minimum adjustment factors 

determined for each peak; (3) adjustment of 

the entire recession curve by the smallest of 

the minimum adjustment factors determined 

for each peak; or (4) adjustment of each 

segment of the recession curve by the 

minimum adjustment factor determined for the 

corresponding peak.

Computed results from RORA and SWGW 

are evaluated by comparing them to published 

estimates of baseflow derived manually using 

the recession-curve-displacement method. Differences between manually derived estimates 

and computed estimates from RORA and 

SWGW using different adjustment factors are 

computed for selected streams. A statistical 

analysis of the differences indicates that the 

closest comparison of the computed estimates 

from RORA and SWGW with the manually 

derived estimates is achieved by SWGW using 

two recession indices and an adjustment factor 

between the minimum and the average 

adjustment factors computed for each stream.
INTRODUCTION Problem

Increasing and competing demands for 

water in the Apalachicola-Chattahoochee-Flint 

and the Alabama-Coosa-Tallapoosa (ACFACT) River basins (fig. 1) have created a need 

for evaluation of surface- and ground-water 

resources. In 1992, the Governors of Alabama, 

Florida, and Georgia and the Assistant 

Secretary of the Army for Civil Works signed 

a Memorandum of Agreement establishing a 

partnership to address interstate and intrastate 

water-resource issues, and promote coordinated system-wide management of water 

resources. These study partners subsequently 

requested the U.S. Geological Survey (USGS) 

to provide the ground-water-supply element of 

the Comprehensive Study of the ACF-ACT 

River basins (U.S. Army Corps of Engineers, 

1991). This element addresses ground-water 

resources in each basin, particularly the 

ground-water contribution to major rivers. 

Based on hydrologic and physiographic 

boundaries, the two basins were further 

divided into eight subareas. Individual reports 

describing the ground-water resources of each 

of the subareas are planned by USGS staff in 

the Alabama and Georgia District Offices.

The surface- and ground-water resources 

of the ACF-ACT River basins are major 

components of a dynamic hydrologic system 

comprised of a network of aquifers, streams, 

reservoirs, control structures, floodplains, and 

estuaries. To manage this system effectively, 

hydrologic relations between the surface- and 

ground-water resources should be described 

and quantitatively evaluated. Streamflow 

hydrograph separation was identified as a 

useful methodology to estimate the groundwater contribution to streamflow. This contrbution is commonly termed baseflow. In this 

report, the term baseflow refers to groundwater discharge to a stream, which is the end 

result of the streamflow hydrograph separation 

procedures discussed.

One of the primary objectives of the ACFstudy is to determine ground-water 

discharge to streams (baseflow) under drought 

conditions. To support that objective, estimates 

of baseflow under average annual conditions 

are needed to provide a point of reference for 

the drought condition analysis. The FORTRAN computer program RORA (Rutledge, 

1993) separates streamflow hydrographs using 

the recession-curve-displacement procedure 

developed by Rorabaugh (1964). Only by 

using a computer-automated, hydrograph 

separation procedure like RORA could 

average baseflows be determined for a large 

number of streams. However, RORA, when 

applied to hydrographs typical of streams in 

the ACF-ACT study area, does not always 

provide completely satisfactory results. In 

particular, estimates of baseflow computed by 

RORA commonly exceed corresponding 

published estimates determined by manual 

hydrograph separation (Faye and Mayer, 1990; 

HooS, 1990). Differences range from about 4 

to 2(K) percent. Also, inspection of numerous 

streamflow hydrographs from the ACF-ACT 

stud)| area for various water years, plotted with 

the corresponding ground-water recession 

curves computed by RORA, reveals that the 

computed recession curves commonly exceed 

measured streamflow. Subsequently, RORA is 

used to compute recession curves over a wide 

range of streamflow conditions and locations 

within the ACF-ACT study area. Results from 

this evaluation are similar to comparisons 

made between manually derived and RORA 

computed estimates of baseflow, in that 

computed recession curves exceed streamflow 

for many locations, particularly during 

summer and early fall.
88° 86° 84° 82°

36° 

34° - ^

32' 

30° 

NORTH 

CAROLINA

SOUTH 

CAROLINA

Base modified from U.S. Geological Survey digital files

50 100 MILES

50 100 KILOMETERS

EXPLANATION 

X// A ACF-ACT study area

Drainage area 

A ga05 Gaging station

Figure 1 . Location of streamflow gaging stations and respective drainage areas used in this report, and the 

Apalachicola-Chattahoochee-Flint and Alabama-Coosa-Tallapoosa River basins (ACF-ACT) study area.
Although RORA employs an automated 

procedure to determine the displacement of the 

recession curve for selected streamflow peaks, 

it does not provide graphical output for visual 

inspection and evaluation of the computed 

results. Such inspection is particularly critical 

for a gaging station on a stream that has widely 

varying flow characteristics, and for a regional 

investigation that includes compar-isons of 

hydrograph separations from several gaging 

stations. Consequently, plots of streamflow 

hydrographs and computed recession curves 

are considered essential to evaluate computed 

results for various streams and streamflow 

conditions.

In the southeastern United States, 

increased riparian evapotranspiration during 

summer and early fall ncreases the rate of 

streamflow recession. The effect of increased 

evapotranspiration can be accounted for in a 

recession-curve-displacement analysis by 

decreasing the value of the recession index, K. 

This index is inversely proportional to the 

slope of the recession curve; thus, a lower 

value of ^corresponds to a greater slope of the 

recession curve, which would be expected 

during a period of high evapotranspiration. 

RORA allows specification of only a single, 

constant value of the recession index, which 

may not accurately represent conditions in the 

ACF-ACT study area during periods of 

riparian evapotranspiration. For the ACF-ACT 

study area, the ability to employ two recession 

indices, one for periods of lower riparian 

evapotranspiration and one for periods of 

higher riparian evapotranspiration, is considered necessary to appropriately apply the 

recession-curve-displacement procedure.

The ACF-ACT study requires water-yearperiod analyses. However, RORA determines 

a single, average annual value of baseflow for 

a user-defined, sequential, calendar-year 

period.

Purpose and Scope

This report describes modifications to the 

existing computer program RORA (Rutledge, 

1993). RORA uses the recession-curvedisplacement method originally described by

Rorabaugh (1964) to estimate the recharge for 

each 3eak in a streamflow hydrograph record. 

RORA is modified to accommodate specific 

requirements of the ACF-ACT ground-water 

study The resulting program, SWGW, is 

documented in this report. Several examples 

and comparisons of computed hydrograph 

separation results to published manual 

separations for streams within and near the 

ACF-ACT study area are provided to illustrate 

the application and utility of the modifications.

Hydrograph Separation

A streamflow hydrograph is a 

representation of the volumetric rate of 

strearftflow, usually plotted on a logarithmic 

scale, at a specific stream location for a 

specified period of time. It can be regarded as 

an integrated expression of the physical 

characteristics that govern the relations among 

rainfall, infiltration, evapotranspiration, runoff, 

and ground-water discharge for a particular 

drainage area. The rate of streamflow 

generally depends upon factors such as the 

magnitude and duration of rainfall, the 

seasonal distribution of rainfall, the vegetative 

coverj the infiltration capacity of the soil, the 

topography of the basin, and the characteristics 

of the ground-water flow system of the basin.

The streamflow hydrograph typical of 

rivers in the southeastern Piedmont and 

Coastal Plain is characterized by a sequence of 

runoff events and evapotranspiration periods 

that subdivide the hydrograph into several 

distinct intervals (Faye and Mayer, 1990, fig. 

2). During the initial part of a water year, 

generally October to December, stream 

discharge characteristically increases following the first substantial frost and the 

consequent reduction of evapotranspiration in 

late November or December. Throughout most 

of the winter and early spring, weather fronts 

producing rainfall cross the study area 

regularly and in relatively rapid succession. 

The runoff and aquifer recharge generated by

these storms initially caused streamflow to

increase rapidly and remain at a relatively high 

level through March or the early part of April. 

This oeriod of recharge and relatively high
streamflow is characteristically termed the 

major rise period (fig. 2). Following the 

cessation of winter rainfall and the onset of 

spring evapotranspiration, the streamflow 

normally undergoes a period of attenuated 

recession that periodically is interrupted by 

runoff generated by frequently intense, 

convective, summer rainstorms. A general 

recession in streamflow typically characterizes 

the hydrograph for most of the spring and 

summer months, and is termed the major 

recession period (fig. 2).

The objective of hydrograph separation 

usually is to estimate the quantity of ground 

water discharged from an aquifer or aquifer 

system to a stream. The range in scale of such 

investigations is large, from examinations of 

small site-specific watersheds (Daniel, 1976) 

to large regional studies (Faye and Mayer, 

1990). Similarly, the scope of studies 

employing hydrograph separation is broad, 

ranging from investigations of precise physical 

processes to evaluations of regional water 

resources.

A major disadvantage of most hydrograph 

separation analyses is the lack of independent 

field measurements available for direct 

comparison of results. Consequently, the 

results of most hydrograph separation analyses 

are difficult to confirm and often are presented 

as estimates of unknown quality and 

confidence, hydrograph separation analyses 

are perhaps most useful when accompanied by 

results of independent analyses of baseflow, 

even if the other results also are estimates.

PROGRAM SWGW

The hydrograph separation program 

SWGW is a modified version of RORA 

(Rutledge, 1993) developed to meet the 

requirements of the ACF-ACT project. 

Modifications address the period of analysis, 

graphical output, evapotranspiration effects, 

and the apparent high estimation of baseflow. 

Results of automated hydrograph separation 

analyses using both RORA and SWGW are 

compared to published results of manual 

applications of the recession-curvedisplacement method of Rorabaugh (1964) for 

several streams in the southeastern United 

States in and near the ACF-ACT study area. 

The discussion and descriptions of the results 

in this report are based on the assumption that 

the reader is familiar with the works of 

Rorabaugh (1964) and Rutledge (1993).

Design and Features

To calculate baseflow for an individual 

water year, the interactive routine in RORA 

that defines the time period of analysis is 

revised. Where RORA originally allows user 

specification of a beginning and an ending 

year, SWGW allows specification of a single 

water year. The beginning and ending years 

originally specified in RORA then are set to 

the two calendar years spanning the desired 

water year in SWGW, minimizing code 

revision. Performing the recession-curvedisplacement analysis over this two-year 

period allows calculation of the recession 

curve at the beginning of the water year, 

generally before the first major hydrograph 

peak for the water year has occurred. To avoid 

problems caused by missing data, SWGW 

assumes a complete, daily data record for the 

specified two-year period. The analysis 

terminates if any data are missing for that 

period. An option also is added to SWGW to 

analyze data for additional water years from 

the same gaging station, thus avoiding 

repeated specification of input parameters.

For each execution of SWGW at a 

particular streamflow gaging station for a 

specified water year, SWGW creates two, 

uniquely named, data output files. One, ending 

with the suffix '.gra', contains data for 

graphical output, and the other, ending with 

the suffix '.out', contains tabulated data and 

results. The two output file names begin with 

an eight-digit prefix, 'ssssyyrr', that is defined 

as follows:

'ssss', a four-digit station identifier; 

'yy', a two-digit water year; and 

'rr', a two-digit run code.
SECOND PER FEET CUBIC IN DISCHARGE, MEAN DAILY 

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The four-digit alphanumeric station 

identifier is defined by the user and also must 

be used to name the file of daily mean 

streamflow data from AD APS (the Automated 

DAta Processing System for retrieving data 

from a USGS data base). RORA and SWGW 

do not use the daily values file directly from 

ADAPS; the program TRANS (Rutledge, 

1993) must first be used to transform the 

streamflow data from ADAPS to the format 

used by RORA and SWGW. TRANS writes 

the transformed data to an input file used by 

RORA and SWGW, which has a five-digit 

name consisting of a 'z' and the four-digit 

station identifier. The station identifier also 

must be specified in another input file, 

'gaging', which contains information on each 

streamflow gaging station. The water year and 

run code are specified interactively during 

execution of SWGW. Thus, for the station 

designated 'tn09', the input file containing the 

transformed data is 'ztn09; and for water year 

1959, and the run specified as 'al', the output 

file containing the graphical data is named 

'tn0959al.gra'.

The graphics package, G2, chosen for 

displaying results of RORA and SWGW, is an 

internal product of the USGS (James L. 

Fulton, U.S. Geological Survey, written 

commun., 1993). The data in the graphical 

output file created by SWGW are in standard 

comma-separated ASCII format, and can be 

easily adapted to other graphics packages. The 

graphical output file (ending with '.gra') 

consists of two title lines followed by columns 

of each of the three data types to be graphed 

daily streamflow values, computed streamflow peaks, and endpoints of the computed 

streamflow recession curves. Plots of the 

separated hydrographs are produced by a 

UNIX script shell (called 'g2plot'), which 

creates an executable '.g2' program file from a 

template file by inserting the water year, the 

name of the '.gra' file, and the two titles. The 

UNIX script shell executes the G2 program, 

directing the output plot to a printer, and then 

deletes the '.g2' program file. The other output 

file (ending with '.out') is a modified version 

of the output file from RORA ('outror2'), and 

contains input parameters, variable definitions, 

tabulated results of the computations for each

peak considered, and values of baseflow to the 

stream for the major rise period and the major 

recession period of the water year.

Another modification to RORA employed 

by SWGW is the specification of two recession 

indices, one for the summer evapotranspiration 

period (Ket), and another for the rest of the 

year (Kmr). After specification of the two 

recession indices, the user is prompted for the 

months corresponding to the beginning and 

end of the summer evapotranspiration period 

(usually April, and September or October, 

respectively, for the ACF-ACT study area). 

The appropriate recession index then is 

assigned by SWGW to each streamflow peak, 

depending on the month of its occurrence.

As noted previously, inspection of the 

graphical output for numerous water-year 

analyses from several streamflow gaging 

stations indicates that the recession-curvedisplacement procedure employed by RORA 

often positions the computed ground-water 

recession curve above the streamflow hydrograph. Baseflow cannot be greater than the 

streamflow discharge and estimates of 

baseflow based on such an analysis would be 

unreasonable. A part of the RORA separated 

streamflow hydrograph for Big Hill Creek near 

Cherryvale, Kansas, is shown in figure 3. 

Several instances of the RORA computed 

ground-water recession curve (long dashed 

line) that exceed streamflow are evident. These 

data and separation results are presented to 

complement the descriptions of Rutledge 

(1993, fig. 6) and Bevans (1986, fig. 5).

QCCHEK, a subroutine written for 

SWGW, addresses the apparent overestimation of baseflow by RORA. For each 

computed streamflow peak, QCCHEK compares each daily discharge of the computed 

recession curve, Qr to daily streamflow 

discharge, Q, for the period from the peak of 

interest to the critical time, tc = 0.2144xAT 

(Rutledge, 1993) of the next peak (fig. 3). For 

the peak of interest in figure 3, Qr exceeds Q 

on three days. An adjustment factor, F=Q/Qr> 

is computed for each of these days. QCCHEK 

finds all occurrences of Qr greater than Q and 

displays the date of the peak, the date of the
10,000

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EXPLANATION

Streamflow hydrograph 

Computed streamflow peak

Peak of interest

 Computed recession curve (RORA) 

A Qr, daily computed recession discharge 

O Q, daily streamflow discharge

Adjusted recession curve (SWGW)

Adjustment factors (F) 

for peak of interest

3/19/74 28.3 24.0 0.85

3/20/74 26.3 21.0 0.80

3/21/74 24.5 21.0 0.86

Duration of subrouting QCCHEK 

comparisons for peak of interest

9 10 11 12 13 14 15 16 17

5 -

2 -

1 234567

Figure 3. RORA hydrograph separation and SWGW adjusted 

Hill Creek near Cherryvale, Kansas (station 07170700), for M;

18 19 20 21 22 23 24 25 26 27 28 29 30 31

recession based on streamflow data from Big 

rch 1974.
occurrence, and the value ofF. For the peak of 

interest, application of the smallest factor, 

F=0.80, repositions the recession curve (short 

dashed line) so that it does not exceed 

streamflow at any time during the QCCHEK 

period.

For each peak, QCCHEK selects the 

minimum F determined for that peak. After all 

peaks have been checked by QCCHEK, the 

average and smallest of the minimum Fs 

determined for all peaks are displayed. The 

user then is given the option of: (1) making no 

adjustment at all (F=1.0, results equivalent to 

RORA results); (2) adjusting all recession 

curves by the average of the minimum Fs; (3) 

adjusting all recession curves by the smallest 

of the minimum Fs; (4) adjusting each 

segment of the recession curve by the 

minimum F computed for the corresponding 

peak; or (5) adjusting all recession curves by 

some other user-specified F. Option 4 is used 

for the adjusted recession curve in figure 3.

The ground-water recession curves from 

SWGW represent approximately the latter 

one-half of the total baseflow related to a 

particular recharge occurrence, the first half 

occurring prior to critical time (Rorabaugh, 

1964; Glover, 1964). The graphical 

presentation of the SWGW recession curve 

also depicts approximately one-half of the total 

baseflow, and does not show baseflow that 

occurs between the peak of interest and critical 

time. The total volume of baseflow is 

determined by doubling the baseflow 

computed at the critical time of each peak 

(Rutledge, 1993, eqn. 7).

Results of automated streamflow 

hydrograph separation using the first four 

adjustment options of SWGW are shown in 

figures 4a and 4b. The streamflow data 

represent discharge at gaging stations on the 

Chattahoochee River at West Point, Georgia, 

for water-year 1952; and on the East Fork of 

Stones River at Woodbury, Tennessee, for 

water-year 1975. The drainage area of the 

station in Georgia is 3,550 square miles, and is 

in the Piedmont Province. The drainage area of 

the station in Tennessee is 39.1 square miles,

and is in the Highland Rim Province. These 

two stations are selected to illustrate effects of 

different runoff characteristics and hydrograph 

separation results using SWGW adjustment 

options.

The four plots of each gaging station and 

water year in figures 4a and 4b differ by the 

options used to specify the adjustment of the 

ground-water recession curve. For the first 

plot, corresponding to option 1, no adjustment 

is made (F=1.0), and the computed recession 

curve (dashed line) is equivalent to results 

obtained using RORA, depicting occasional 

computed ground-water recession curve 

greater than streamflow discharge. The second 

plot shows results of option 2, which adjusts 

the entire ground-water recession curve 

downward by the average of the minimum 

factors computed for each peak in the twocalendar-year period. As shown in figures 4a 

and 4b, this option may produce results 

depicting occasional computed ground-water 

recession discharge greater than streamflow 

discharge, but the overages are smaller and 

less frequent than in option 1. The third plot of 

each station hydrograph depicts the use of the 

minimum adjustment factor determined for the 

two-year period (option 3). This option results 

in positioning the ground-water recession 

curve below streamflow throughout the entire 

period, and produces a relatively conservative 

baseflow estimate. Options 2 and 3 do not 

change the shape of the ground-water 

recession curve, but simply displace the 

original curve (F=1.0) downward uniformly 

throughout the analysis period. Results of 

option 4 are shown in the fourth plot of figures 

4a and 4b. Using option 4, each individual 

segment of the ground-water recession curve, 

corresponding to each peak, is adjusted 

downward to less than streamflow if needed, 

based on the minimum F determined for that 

peak. Like option 3, option 4 results in 

positioning of the ground-water recession 

curve below streamflow throughout the entire 

period, but the resulting estimate of baseflow 

is less conservative than in option 3. Option 5 

is not used for figures 4a and 4b.
100,000

10,000 -

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100

Option 1. No adjustment 

(F=1.000) 

Otof=17.43in.

Streamflow hydrograph 

Computed streamflow peak 

Computed recession curve

Q 100,000

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Q_

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Option 2. Average F 

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Option 3. Minimum F 

(F=0.624) 

Qtof=10.88 in

100

100,000

10,000

1,000

100

Option 4. Variable F 

(by peak) 

Qtof=15.02in.

Oct. Nov. 

1951

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

1952

Figure 4a. Results of automated streamflow hydrograph sepa 

SWGW, Chattahoochee River at West Point, Georgia (station

^ation using four adjustment options of 

02339500), for water year 1952.

10
10,000

1,000

100

10 Streamflow hydrograph 

Computed streamflow peak 

Computed recession curve

Option 1. No adjustment 

(F= 1.000) 

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Option 3. Minimum F 

(F=0.533)

10,000 E

1,000 r

100 r

Option 4. Variable F 

(by peak)

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

1974

Figure 4b. Results of automated streamflow hydrograph separation using four adjustment options of 

SWGW, East Fork of Stones River at Woodbury, Tennessee (station 03426800), for water year 1975.

11
Adjustment of the ground-water recession 

curve is not intended to be arbitrary, but rather 

should be applied at the discretion and 

hydrologic judgment of the user, and with full 

consideration of the user's objectives. Options 

1 through 4 are used throughout the following 

examples to illustrate available user options. 

Options 1 and 3 provide the user a complete 

range of possible adjustment alternatives, and 

option 2 gives the user an idea of the central 

tendency of that range. The SWGW user also 

is given a fifth option of applying another userdefined adjustment factor, which can be 

chosen based on the range and average values 

provided.

Examples of SWGW Analyses and 

Comparisons

Estimates of baseflow were computed 

manually using the recession-curvedisplacement method for streamflow at gaging 

stations in the Piedmont and Coastal Plain of

Georgia (Faye and Mayer, 1990), and in 

variot s physiographic provinces in middle and 

easteri Tennessee (Hoos, 1990). Faye and 

Mayer (1990) evaluated eight water years at 

seven stations. Hoos (1990) estimated baseflow for three water years representing high-, 

average-, and low-flow conditions for each of 

ten gaging stations. For comparison of 

manually derived results to results from 

automated procedures, hydrograph separation 

results at four stations and three water years 

for each station are chosen from Hoos (1990), 

and results from all eight water years are 

selected from Faye and Mayer (1990). In all, 

separation results at 11 stations for a total of 20 

water years are compared. Two scenarios are 

considered: scenario (a) constant recession 

index, K (Kmr=Ket), and scenario (b) two 

values' of K (Kmr>Ket). The names of the 11 

gaging stations and characteristics of the

Table 1. Streamflow-gaging stations where separation results based on manual methods are 

compared to results from automated recession-curve-diSplacement procedures, and 

characteristics of the respective drainage basins

[Kmr Ket, recession indices for the major rise and major recession periods, respectively; , data not available, only 

one value of the recession index is used for these stations

St,afition 'D Station name (fig- 1)

gaOl

ga02

ga03

ga04

ga05

ga06

ga07

tn09

tn30

tn32

tn46

Chattahoochee River at West Point, Ga.

Flint River near Culloden, Ga.

Ogeechee River at Scarboro, Ga.

Ogeechee River near Louisville, Ga.

Ocmulgee River at Hawkinsville, Ga.

Ocmulgee River at Macon, Ga.

Oconee River at Milledgeville, Ga.

East Fork Stones River at Woodbury, Tn.

Lick Creek at Mohawk, Tn.

Doe River at Elizabethton, Tn.

Emory River at Oakdale, Tn.

Station number Phy

02339500

02347500

02202000

02200500

02215000

02213000

02223000

03426800

03467000

03485500

03540500

graphic 

ince

Piedmont

Piedmont

Coastal P

Coastal P

Piedmont

Coastal P

Piedmont

ain

ain

and

ain

Piedmont

Highland Rim

Valley an^l Ridge

Blue Ridj

Cumberla

Plateau

;e

nd

Drainage 

area, in 

square miles

3,550

1,850

1,940

800

3,800

2,240

2,950

39.1

220

137

764

Kmr 

in days

122

85

80

94

150

117

62

95

65

107

31

Kel, ET period, 

in days in months

80 Apr.-Sept.

54 Apr.-Sept.

 

 

 

 

 

54 Apr.-Sept.

56 Apr.-Sept.

69 Apr.-Oct.

16 Apr.-Sept.

12
respective drainage basins are in table 1. The 

location and drainage basins of the 11 stations 

are shown in figure 1.

For scenario (a), a single value of the 

recession index is used for both the major rise 

period and the major recession period 

(Kmr=Ket). The recession index values are 

from Faye and Mayer (1990) and Hoos (1990). 

Four runs of SWGW are made for each of the 

20 water years using different adjustment 

options 1 through 4. The first set of runs, 

designated 'al', employs no adjustment 

(option 1, F=1.0), and produces results 

identical to those from RORA for the period of 

the chosen water year. The second and third

sets of runs, 'a2' and 'a3', employ the average 

(option 2) and minimum (option 3) computed 

values of F, respectively, applied to each Qc in 

the specified water year. The final set of runs, 

'a4', adjusts each Qc in the specified water 

year by the minimum F computed for the 

corresponding peak (option 4).

Published, manually derived baseflow 

estimates and corresponding results from 

SWGW for scenario (a) are listed in table 2. 

In addition to estimates of total baseflow, 

estimates of baseflow components attributable 

to both the major rise and major recession 

periods also are listed in table 2 for both the 

manual and the automated procedures. Percent

Table 2. Summary of estimates of baseflow based on manual and automated recession-curve 

displacement procedures for scenario (a), Kmr=Ket

[Qmr Qet-> estimates of components of baseflow, in inches, attributable to the major rise period and the major 

recession period, respectively; Qtot (in bold), sum of Qmr and Qet, in inches; F, factor used to adjust recession curve; 

Diff., difference, in percent, of the SWGW computed estimate from the manually derived estimate]

Station Water Manually derived estimates

ID year O O O

Vmr Vet \itot

Runs, factors, SWGW computed estimates, and differences

Run Diff. Qe, Qtot

gaOl 1941

gaOl 1952

ga02 1941

ga03 1941

ga04 1941

ga05 1931

ga06 1905

ga07 1941

5.2 1.5 6.7 al

a2

a3

a4

9.6 0.7 10.3 al

a2

a3

a4

4.0 0.4 4.4 al

a2

a3

a4

4.0 3.2 7.2 al

a2

a3

a4

4.6 1.0 5.6 al

a2

a3

a4

3.9 0.9 4.8 al

a2

a3

a4

3.4 0.5 3.9 al

a2

a3

a4

3.7 0.4 4.1 al

a2

a3

a4

1.00

0.92

0.74

variable

1.00

0.94

0.64

variable

1.00

0.86

0.46

variable

1.00

0.81

0.36

variable

1.00

0.79

0.48

variable

1.00

0.79

0.44

variable

1.00

0.88

0.62

variable

1.00

0.95

0.78

variable

6.9

6.3

5.0

7.0

16

15

10

12

5.1

4.4

2.3

5.0

4.9

3.9

1.8

4.9

6.3

5.0

6.0

6.3

6.8

5.3

3.0

5.4

4.3

3.8

2.7

4.1

5.4

5.2

4.3

5.4

32

21

-2.9

35

64

55

4.2

23

27

9.7

-42

24

22

-1.7

-56

23

37

8.7

-35

38

74

37

-24

40

26

11

-22

20

47

40

15

45

1.8

1.7

1.4

1.6

1.7

1.6

1.1

3.5

1.1

1.0

0.52

0.56

2.7

2.2

0.96

1.6

2.7

2.2

1.3

1.4

1.6

1.2

0.68

1.0

1.9

1.7

1.2

1.6

1.7

1.6

1.3

1.4

23

13

-10

3.1

140

130

55

390

180

140

29

40

-17

-33

-70

-49

170

120

29

40

72

35

-25

13

280

240

140

220

310

290

220

260

8.7

8.0

6.4

8.6

17

16

11

15

6.2

5.4

2.9

5.5

7.5

6.1

2.7

6.5

9.0

7.2

4.3

7.7

8.3

6.6

3.6

6.5

6.2

5.4

3.8

5.7

7.1

6.8

5.6

6.8

30

19

-4.5

28

69

60

7.6

48

41

22

-35

25

4.5

-16

-62

-9.3

61

28

-24

38

74

37

-24

35

59

40

-1.5

46

73

65

36

66

13
Table 2. Summary of estimates of baseflow based on rr 

displacement procedures for scenario (a), Kmr=Ker-Co

[Qmr> Qef> estimates of components of baseflow, in inches, attril 

recession period, respectively; Qtot (in bold), sum of Qmr and Qet, i 

Diff., difference, in percent, of the SWGW computed estimate from

Station Water

ID year

tn09 1975

tn09 1971

tn09 1981

tn30 1950

tn30 1965

tn30 1948

tn32 1975

tn32 1959

tn32 1969

tn46 1973

tn46 1970

tn46 1969

Manually derived estimates

(2mr (let Qtot

12.1 1.6 13.7

7.7 1.2 8.9

3.0 0.9 3.9

5.8 1.8 7.2

4.4 0.8 5.2

1.8 0.8 2.6

12.3 3.8 16.1

9.4 1.3 10.7

8.4 1.7 10.1

10.3 -0.1 10.2

8.2 0.7 8.9

3.4 0.3 3.7

Run

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

al

a2

a3

a4

Runs, facto

F (

1.00 1

0.86 1

anual and automated recession-curve 

ntinued

mutable to the major rise period and the major 

n inches; F, factor used to adjust recession curve; 

the manually derived estimate]

rs, SWGW computed estimates, and differences

Imr

8

6

0.55 9.9

variable 14

1.00 12

0.86 10

0.55 6.7

variable f>.4

1.00

0.85

0.57

variable

3.9

3.3

2.2

3.7

1.00 fe.l

0.86 6.9

0.40 fe.2

variable 6.2

1.00 7.0

0.94

0.55

variable

1.00

0.93

i.5

5.8

>.2

1.8

U

0.72 3.4

variable 4.4

1.00 1^

0.91 It

0.70 13

variable 1 "

1.00 <

0.89 J

0.63 <

variable <

1.00 1

0.90 <

0.67

variable <

1.00 1"

0.79 1.

0.41 (

variable 1 .

1.00 i

0.71 (

0.20

variable

1.00

0.75

0.20

T

>.5

5.5

i.O

>.3

>.6

M

>.7

..8

1.9

..3

.8

r .O

'.3

i.5

.5

variable 6.5

Diff50

30

-18

12

59

36

-13

22

31

11

-26

23

39

19

-44

6.5

59

48

-13

40

160

150

91

140

53

39

7.5

35

1.1

-9.9

-36

-1.3

26

14

-16

16

62

28

-34

42

8.2

-23

-78

-14

120

62

-56

92

Qe,

0.87

0.75

0.47

1.7

1.6

1.4

0.90

2.1

1.1

0.96

0.64

0.87

2.8

2.4

1.1

2.3

1.0

0.97

0.57

1.1

0.84

0.78

0.61

0.88

4.5

4.1

3.2

5.7

6.4

5.7

4.0

5.3

3.2

2.9

2.1

3.2

7.2

5.7

3.0

5.6

4.4

3.1

0.9

2.3

3.9

2.9

0.8

2.2

Diff.

-46

-53

-70

4.9

38

18

-25

75

26

6.9

-28

-3.7

58

35

-37

26

30

22

-29

42

4.4

-2.4

-24

10

19

8.0

-16

51

390

340

210

310

88

70

25

87

_ l

_ 1

_ 1

_ 1

530

350

28

230

1,200

870

170

640

Qm

19

16

10

15

14

12

7.6

11

5.1

4.3

2.9

4.6

11

9.4

4.4

8.4

8.0

7.5

4.4

7.3

5.6

5.2

4.0

5.3

23

21

16

22

16

14

10

15

14

12

9.2

13

24

19

9.8

20

13

9.4

2.7

9.3

11

8.4

2.3

8.7

Diff.

39

20

-24

11

56

34

-15

29

30

10

-26

17

44

23

-42

11

54

44

-16

40

120

100

56

100

45

32

1.8

39

48

32

-6.8

36

37

24

-8.7

28

130

85

-4.2

98

49

5.8

-70

4.9

200

130

-38

140

'Residual difference not calculated due to apparent negative value of manual estimate of Q

14
differences, Diff.=\W*(Qcomp-Qman}/Qman, of 

the SWGW computed estimates, Qrnmn. from 

the manually derived estimates, Qman, 

listed in table 2.

comp->

also are

Scenario (b) is considered using a 

recession index, Ket, specified for the major 

recession period, that is smaller than the 

recession index, Kmn for the major rise period. 

Estimates of Ket (table 1) are made for the four 

stations from Hoos (1990) and for stations 

gaO 1 and ga02 from Faye and Mayer (1990) by 

graphical inspection of numerous streamflow

hydrographs, providing a total of 15 water 

years at 6 stations. The same four sets of runs 

of SWGW are completed as described for 

scenario (a), and are similarly designated 'bl', 

'b2', 'b3', and 'b4'. Run 'bl' differs from 

RORA only by the specification of two values 

of K. Published estimates of the major rise and 

major recession components of baseflow and 

total baseflow are listed in table 3 with the 

corresponding results from SWGW. Percent 

differences, Diff=\00*(Qcomp-QmanyQman, of

Table 3. Summary of estimates of baseflow based on manual and automated recession-curve 

displacement procedures for scenario (b), Kmt>Ket

[Qmr Qet-> estimates of components of baseflow, in inches, attributable to the major rise period and the major 

recession period, respectively; Qtot (in bold), sum of Qmr and Qet, in inches; F, factor used to adjust recession curve; 

Diff., difference, in percent, of the SWGW computed estimate from the manually derived estimate]

Manually derived estimates

Station Watpr

ID year

Qmr Qet Qtot R"n

gaOl 1941 5.2 1.5 6.7 bl

b2

b3

b4

gaOl 1952 9.6 0.7 10.3 bl

b2

b3

b4

ga02 1941 4.0 0.4 4.4 bl

b2

b3

b4

tn09 1975 12.1 1.6 13.7 bl

b2

b3

b4

tn09 1971 7.7 1.2 8.9 bl

b2

b3

b4

tn09 1981 3.0 0.9 3.9 bl

b2

b3

b4

Runs, factors, SWGW computed estimates, and differences

F

1.00

0.90

0.62

variable

1.00

0.94

0.64

variable

1.00

0.92

0.62

variable

1.00

0.88

0.55

variable

1.00

0.87

0.56

variable

1.00

0.90

0.57

variable

Qmr

6.8

6.2

4.2

7.1

16

15

10

12

5.1

4.6

3.2

5.0

18

16

9.9

14

12.3

10.7

6.9

9.7

4.0

3.6

2.2

3.7

Diff.

31

19

-19

36

64

54

4.1

22

27

16

-21

24

50

33

-18

14

59

39

-10

26

32

19

-25

24

Qe,

2.5

2.3

1.6

2.4

3.5

3.3

2.2

4.7

1.6

1.5

1.0

1.2

2.0

1.7

1.1

2.3

2.8

2.5

1.6

2.9

1.6

1.4

0.88

1.4

Dijf.

68

52

4.3

60

400

370

220

570

300

270

150

200

23

8.7

-33

44

140

110

34

150

72

55

-2.6

50

Qtot

9.4

8.4

5.8

9.5

19

18

12

16

6.7

6.1

4.2

6.2

20

18

11

16

15

13

8.5

13

5.5

5.0

3.1

5.1

Diff.

40

26

-14

42

87

75

19

59

52

39

-5.2

40

47

30

-20

17

70

48

-4.5

42

41

27

-20

30

15
Table 3. Summary of estimates of baseflow based on manual and automated recession-curvedisplacement procedures for scenario (b), Km[>/^/ Continued

[Qmn Qet-> estimates of components of baseflow, in inches, attributable to the major rise period and the major

recession period, respectively; Qtot (in bold), sum of Qmr and Qet, in inches; /% factor used to adjust recession curve;

Diff., difference, in percent, of the SWGW computed estimate from the manually derived estimate]

 . w Manually derived estimates

ID year

Vmr ^et ^lol

tn30 1950 5.8 1.8 7.6

tn30 1965 4.4 0.8 5.2

tn30 1948 1.8 0.8 2.6

tn32 1975 12.3 3.8 16.1

tn32 1959 9.4 1.3 10.7

tn32 1969 8.4 1.7 10.1

tn46 1973 10.3 -0.1 10.2

tn46 1970 8.2 0.7 8.9

tn46 1969 3.4 0.3 3.7

Run

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

bl

b2

b3

b4

Runs, factors

F

1.00

0.84

0.40

variable

1.00

0.94

0.55

variable

1.00

0.93

0.72

variable

1.00

0.92

0.76

variable

1.00

0.92

0.72

variable

1.00

0.91

0.71

variable

1.00

0.83

0.54

variable

1.00

0.78

0.19

variable

1.00

0.80

0.19

variable

Q,

8

6

, SWGW computed estimates, and differences

nr

1

8

3^2

6,2

7JO

6

3,

61

4.

4.

3.

4.

19

18

14

17

6

8

2

8

4

4

4

9.£

18

6.9

9.4

11

9.7

7.6

9.7

19

16

10

16

10

8.

2.

8.

7.

6.

1

)

I

I

I

1.5

6.V

Diff.

39

17

-44

6.5

59

49

-13

40

160

150

92

150

54

43

16

36

2.1

43

-27

-0.3

27

15

-9.7

16

82

51

-2.6

54

26

-1.1

-76

0.0

130

84

-57

98

Qe,

2.9

2.5

1.2

2.4

1.2

1.2

0.68

1.3

0.95

0.88

0.69

0.98

5.8

5.4

4.4

6.6

7.2

5.4

5.2

6.4

3.8

3.5

2.7

3.9

9.2

7.6

4.9

7.4

5.0

3.9

0.94

3.3

4.9

3.9

0.92

3.1

Diff.

63

37

-34

31

55

46

-15

66

18

11

-14

22

53

42

16

74

460

42

300

390

130

100

61

130

_ i

_ i

_ i

_ i

610

450

34

380

1,500

1,200

210

940

Qto,

11

9.3

4.4

8.5

8.2

7.7

4.5

7.5

5.7

5.3

4.1

5.4

25

23

19

23

17

23

12

16

14

13

10

14

28

23

15

23

15

12

2.9

12

13

10

2.4

9.8

Diff.

45

22

-42

12

58

48

-14

44

120

100

59

110

54

42

16

45

57

42

13

48

43

30

2.2

34

170

130

47

130

72

35

-68

30

240

170

-36

170

Residual difference not calculated due to apparent negative value of bianual estimate of Qet.

16
the SWGW computed estimates, Qcomp, from 

the manually derived estimates, Qman, also are 

listed in table 3.

Statistical Analysis of Example Results

A statistical analysis of the differences of 

the SWGW computed estimates from the 

manually derived estimates of total baseflow is 

used to evaluate the performance of each of the 

four described options for adjusting the 

displacement of the recession curve. To 

evaluate results for gaging stations in Georgia 

and Tennessee independently, the differences 

are grouped into several data sets. For each of 

the two scenarios, (a) and (b), three data sets of 

differences are formed: (1) stations in Georgia 

(data sets 'Georgia-a' and 'Georgia-b'); (2) 

stations in Tennessee (data sets 'Tennessee-a' 

and 'Tennessee-b'); and (3) all stations (data 

sets 'total-a' and 'total-b'). Because estimates 

of Ket are available only for three of the eight 

water years at stations in Georgia used in 

scenario (a), data sets 'Georgia-b' and 'total-b' 

consist of five fewer data pairs than the 

corresponding data sets developed by scenario 

(a). To provide an appropriate comparison 

between the two scenarios, an additional data 

set (called 'total-a ') is formed using only 

residual differences from scenario (a) for the 

Georgia stations represented in data set 'totalb'. The minimum and maximum differences, 

and the mean, standard deviation, and 

skewness of the differences in each data set are 

summarized in table 4.

For data set 'total-a', estimates of total 

baseflow computed by SWGW are greater than 

those based on manual methods by an average 

of 63 percent for runs 'al'; 40 percent for runs 

'a2'; and 41 percent for runs 'a4'. SWGW 

computed estimates are less than manually 

derived estimates by an average of 15 percent 

for runs 'a3'. The mean differences computed 

for runs 'al', which are equivalent to the 

estimates from RORA, are furthest from the 

published manually derived results for 18 of 

the 20 water years selected for comparison. 

The differences for runs using adjusted values 

of Qc, runs 'a2', 'a3', and 'a4', are not only 

smaller on average than the differences from

runs 'al', but also are less scattered and more 

normally distributed. For runs 'al', the 

standard deviation and skewness of the 

differences are 44 percent and 1.9, 

respectively. Corresponding values are 34 

percent and 1.1 for runs 'a2', 29 percent and 

0.4 for runs 'a3', and 35 percent and 1.2 for 

runs 'a4'. The value of F that would cause the 

annual baseflow estimates computed by 

SWGW to match exactly the corresponding 

manually derived estimates lay between the 

average F and the minimum F for 15 of the 20 

water years considered.

The differences of data sets 'Georgia-a' 

and 'Tennessee-a' for scenario (a) display 

results similar to the data set 'total-a' (table 4). 

Runs 'al' of each data set produced the highest 

mean residual, while runs 'a3' produced the 

lowest. The value of F that would cause the 

baseflow computed by SWGW to match 

exactly the manually derived estimate for the 

Georgia and Tennessee stations lay between 

the average F and the minimum F for 5 of the 

8, and 10 of the 12 water years, respectively.

For the 15 water years considered for 

scenario (b), data set 'total-b', SWGW 

estimates of baseflow are greater, on average, 

than manually derived estimates by 80 percent 

for runs 'bl'; 58 percent for runs 'b2'; and 56 

percent for runs 'b4'; and are less than 

manually derived estimates by 4.4 percent for 

runs 'b3'. For 14 of the 15 water years 

considered, the estimates of baseflow for runs 

'bl', which most closely correspond to the 

estimates computed by RORA, are the furthest 

from the published manually derived results. 

Like scenario (a), the differences for the runs 

using adjusted values of Qc, runs 'b2', 'b3', 

and 'b4', are not only smaller on average than 

the differences from runs 'bl', but also are less 

scattered and more normally distributed. For 

runs 'bl', the standard deviation and skewness 

of the differences are 57 percent and 1.9, 

respectively. Corresponding values are 44 

percent and 1.6 for runs 'b2', 33 percent and 

0.2 for runs 'b3', and 43 percent and 1.5 for 

runs 'b4'. The value of F that would cause the 

annual baseflow estimates computed by 

SWGW to match exactly the manually derived

17
Table 4. Summary of statistics for sets of differences of S5WGW computed estimates of baseflow 

from manually derived estimates of baseflow

Sets of 

difference 

data

Georgia-a

Tennessee-a

Total-a

Total-a* 1

Georgia-b

Tennessee-b

Total-b

Number 

of Run 

station-years

8 al

a2

a3

a4

12 al

a2

a3

a4

20 al

a2

a3

a4

15 al

a2

a3

a4

3 bl

b2

b3

b4

12 bl

b2

b3

b4

15 bl

b2

b3

b4

F

l.O(RORA)

average

minimum

variable

l.O(RORA)

average

minimum

variable

l.O(RORA)

average

minimum

variable

l.O(RORA)

average

minimum

variable

1.0

average

minimum

variable

1.0

average

minimum

variable

1.0

average

minimum

variable

I

Minimum Ma:

Scenario (a) Km =Ket

)ifference, in percent

cimum

4.5 74

-16 65

-62 36

-9.3 66

30 ! 200

5.8 130

-70 56

4.9 140

4.5 200

-16

-70

-9.3

130

56

140

30 200

5.8 130

-70 56

4.9 140

Scenario (b) Kmr>Ket

40 87

26

-14

40

41

22

-68

12

40

22

-68

12

75

19

59

240

170

59

170

240

170

59

170

Mean

51

32

-14

34

71

45

-16

46

63

40

-15

41

66

43

-15

44

59

47

0.0

47

85

61

-5.5

59

80

58

-4.4

56

Standard 

deviation

25

25

30

22

53

38

30

43

44

34

29

35

48

35

28

38

24

25

17

11

63

48

36

48

57

44

33

43

Skewness

-0.9

-0.5

0.0

-0.7

1.6

1.1

0.7

i.l

1.9

1.1

0.4

1.2

1.9

1.3

0.6

1.3

0.5

0.5

0.5

0.7

1.6

1.4

0.2

1.2

1.9

1.6

0.2

1.5

'Stations considered under scenario (a) that are also considered under scenario (b).

18
estimate lay between the average F and the 

minimum F for 9 of the 15 water years 

considered. Comparing these results with 

those computed using the same water-year 

data under scenario (a), data set 'total-a ', the 

means and standard deviations are higher 

under scenario (b) for all four sets of runs.

Of the total sets of runs from both 

scenarios, the best set is 'b3' in data set 'totalb' of scenario (b), which has a mean difference 

of -4.4 percent, a standard deviation of 33 

percent, and a skewness of 0.2. This set of runs 

has a mean value closest to the manually 

derived estimates and is the most normally 

distributed. Inspection of table 4 shows that of 

each set of water years analyzed, run '3' 

commonly has the mean difference closest to 

zero, and also the most normal distribution. 

These comparisons indicate that for streams in 

the ACF-ACT study area, the use of two 

recession indices and an adjustment factor (F) 

between the minimum and average values 

produces estimates of baseflow most 

consistent with estimates based on manual 

analysis of the same data.

DISCUSSION

This work describes results of streamflow 

hydrograph separation using recession-curvedisplacement procedures performed on 

streamflow data from selected gaging stations 

in Georgia and Tennessee. The results indicate 

that adjustment of RORA computed recession 

curves and baseflow may be appropriate for 

the ACF-ACT study area. Results also indicate 

that evapotranspiration effects on groundwater discharge to streams may be accounted 

for in the ACF-ACT study area by the use of 

appropriate baseflow recession indices. 

Features of SWGW, including water-year 

analysis period, graphic output of computed 

ground-water recession curves, and optional 

adjustments of RORA results, substantially 

enhance analysis of ground-water/surfacewater relations.

The effect of a number of hydrologic 

processes and basin characteristics probably 

are significant to the results of this or any

recession-curve-displacement procedure, and 

have not been thoroughly investigated. 

Rainfall, runoff, and recharge processes are 

complex and dynamic, especially in large 

watersheds. For example, in a large watershed, 

the temporal and spatial distribution of a 

rainfall event may influence the temporal and 

spatial patterns of consequent streamflow, and 

affect the displacement of the recession curve. 

Similarly, the distribution of aquifer hydraulic 

characteristics and ground-water flow 

processes may not coincide with watershed 

boundaries, and the development of regional 

and intermediate ground-water flow systems 

may further complicate the relation between 

surface water and ground water within a 

particular watershed. Other factors may affect 

the results of a recession-curve-displacement 

procedure. These include, but are not limited 

to, basin size, basin topography, land use, soil 

type, aquifer properties, climate, and 

streamflow characteristics.

In addition to the uncertain effects of 

hydrologic processes on the scale of the 

drainage basin discussed in the previous 

paragraph, there also is uncertainty in the 

present understanding of small-scale processes 

such as the movement of water from the 

ground through a streambed into a flowing 

stream. The physical mechanisms of the 

recession-curve-displacement method basically assume that a stream is similar to a 

horizontal well, in which water in the well 

bore is in direct contact with the surrounding 

saturated soil medium. Streambed sediments, 

however, may have substantially different 

hydrologic properties than the underlying soil 

medium, complicating the physical process of 

ground-water discharge to a stream. Without 

additional work to gain a thorough understanding of this and related processes, field 

verification of the estimates of baseflow 

obtained through any hydrograph separation 

procedure will be difficult.

Additional work is needed to refine 

recession-curve-displacement and related 

method-ologies, and to evaluate the confidence 

of analytical results. The relation of surfacerunoff processes to ground-water discharge

19
during and immediately following a rainfallrunoff event is of particular interest and 

complexity. The methods used in SWGW and 

in RORA are somewhat to highly subjective 

regarding the separation of runoff components 

during periods of rising and peak streamflow. 

This discretization of streamflow into 

components of surface water and ground water 

is extraordinarily difficult to approach because 

of the lack of a theoretical description of the 

physical processes. Investigation of this 

phenomenon is difficult and rarely has been 

undertaken.

SELECTED REFERENCES

Bevans, H.E., 1986, Estimating streamaquifer interactions in coal areas of 

eastern Kansas by using streamflow 

records, in Subitzky, Seymore, ed., 

Selected papers in the Hydrologic 

Sciences: U.S. Geological Survey WaterSupply Paper 2290, p. 51-64.

Daniel, J.F., 1976, Estimating groundwater 

evapotranspiration from streamflow 

records: Water Resources Research, v. 

12, no. 3, p. 360-364.

Faye, R.E., and Mayer, G.C., 1990, Groundwater flow and stream-aquifer relations 

in the northern Coastal Plain of Georgia 

and adjacent parts of Alabama and South 

Carolina: U.S. Geological Survey WaterResources Investigations Report 88- 

4143,83 p.

Glover, R.E., 1964, Ground-water movement: 

U.S. Bureau of Reclamation Engineering 

Monograph Series, no. 31, p. 30-39.

Hoos, A.B., 1990, Recharge rates and aquifer 

hydraulic characteristics for selected 

drainage basins in middle and east 

Tennessee: U.S. Geological Survey 

Water-Resources Investigations Report 

90-4015, 34 p.

Rorabaugh, M.I., 1964, Estimating changes in 

bank storage and ground-water contribution to streamflow: International 

Association of Scientific Hydrology, 

Publication 63, p. 432-441.

Rutledge, A.T., 1993, Computer programs for 

:scribing the recession of ground-water 

discharge and for estimating mean 

ground-water recharge and discharge 

fifom streamflow records: U.S. 

Geological Survey Water-Resources 

Investigations Report 93-4121, 45 p., one 

3*1/2 inch diskette.

U.S. Army Corps of Engineers, 1991, Draft 

plan of comprehensive study, AlabamaCoosa-Tallapoosa and ApalachicolaGhattahoochee-Flint River basins: 

Ivtobile District, U.S. Army Corps of 

Engineers, prepared by The Compreh^nsive Study Technical Coordination 

Group, July 1991,48 p.

20