METHOD OF ESTIMATING NATURAL 

RECHARGE TO THE EDWARDS 

AQUIFER IN THE SAN ANTONIO 

AREA, TEXAS

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations 78-10

Prepared in cooperation with the Texas Department of Water 

Resources, the Edwards Underground Water District, and 

the city of San Antonio
BIBLIOGRAPHIC DATA 

SHEET

1. Report No. 2. 3. Recipient's Accession No.

4. Title and Subtitle

METHOD OF ESTIMATING NATURAL RECHARGE TO THE EDWARDS 

AQUIFER IN THE SAN ANTONIO AREA, TEXAS

5. Report Date

April 1978

7. Author(s)

Celso Puente

8. Performing Organization Rept

No- USGS/WRI-78-10

9. Performing Organization Name and Address

U.S. Geological Survey, Water Resources Division 

300 E. Eighth Street 

Austin, TX 78701

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

U.S. Geological Survey, Water Resources Division 

300 E. Eighth Street 

Austin, TX 78701

13. Type of Report & Period 

Covered

Final

14.

15. Supplementary Notes

Prepared in cooperation with the Texas Department of Water Resources, the Edwards 

Underground Water District. and the city of San Antonio_____________________

16. Abstracts

The method of estimating annual recharge is based on data collected from a network 

of stream-gaging stations and on assumptions related to applying the runoff characteristics from gaged areas to ungaged areas. The basic approach is a water-balance 

equation, in which recharge within a stream basin is the difference between measured 

streamflow above and below the infiltration area of the aquifer plus the estimated 

runoff in the zone that includes the infiltration area. Recharge in the Medina 

River basin also includes seepage losses from Medina Lake and Diversion Reservoir.

The principal errors in the estimates of annual recharge are related to errors in 

estimating runoff in ungaged areas, which represent about 30 percent of the infiltration area. The estimated long-term average annual recharge in each basin, however, is probably representative of the actual recharge because the averaging 

procedure tends to cancel out the major errors.

17. Key Words and Document Analysis. l7a. Descriptors

*Ground-water recharge, Ground-water basins, Aquifers, Aquifer characteristics, 

Surface-ground-water relationships, Water sources, Infiltration

17b. Identifiers/Open-Ended Terras

Limestone hydrology; Bexar County, Texas; Balcones Fault Zone

17c. COSATI Field/Group

18. Availability Statement

No restriction on distribution.

19. Security Class (This 

Report)

UNCLASSIFIED

20. Security Class (This 

Page

UNCLASSIFIED

21. No. of Pages 

38

22. Price

FORM NT1S-35 (REV. 10-73) ENDORSED BY ANSI AND UNESCO. THIS FORM MAY BE REPRODUCED USCOMM-DC B205-P74
METHOD OF ESTIMATING NATURAL 

RECHARGE TO THE EDWARDS 

AQUIFER IN THE SAN ANTONIO 

AREA,TEXAS

By Celso Puente

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations 78-10

Prepared in cooperation with the Texas Department of Water 

Resources, the Edwards Underground Water District, and 

the city of San Antonio

APRIL 1978

I
Reproduced by the Texas Department of 

Water Resources as a part of the continuing 

program of cooperation in water-resources 

investigations between the Department and 

the U.S. Geological Survey.

Copies of this report may be obtained from the 

U.S. Geological Survey

Federal Building 

300 East 8th Street 

Austin, TX 78701
UNITED STATES DEPARTMENT OF THE INTERIOR

Cecil D. Andrus, Secretary

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director

FOR ADDITIONAL INFORMATION WRITE TO:

U.S. GEOLOGICAL SURVEY 

FEDERAL BUILDING 

300 EAST 8TH STREET 

AUSTIN, TX 78701

II
CONTENTS

Page

Abstract---—----—----——____________________—__——______—___ i

Introduction— ———— -- — -----__-----_ — — — — — _--_ —--___---— 2

Geology and hydrology--—----------—----—____—____——_________ 2

Method of determining annual recharge---—----—_-—----—--_-—-- 5

Basic approach-- — -------------------- — — — — — — — ______ — _ 5

Estimated runoff between upper and lower gages--------———- 6

Adjustments for distribution of precipitation-------—-----— 16

Recharge in individual basins----- — ---- — — — — -- — -- — — — -- — -- 17

Nueces-West Nueces River basin-------—----—--—------—---_ 17

Frio-Dry Frio River basin and adjacent areas-------- — ------- 18

Sabinal River basin and adjacent areas-- — — — -- — -- — ------- 20

Area between the Sabinal River basin and the Medina

River basin-------------------------- — — -- — ---- — ---- — -- 21

Medina River basin-- — ---------------- — — — — — — — _________ 23

Area between the Medina River basin and the Cibolo Creek

basin--------- —— -_____-____- ________ —— _________________ 25

Cibolo Creek and Dry Comal Creek basins-- — — — — — -- — ------ 26

Cibolo Creek basin------------- — -- — — __________ — _____ 26

Dry Comal Creek basin------- — — — ________ — __ — — __ — __ 27

Guadalupe River basin-----------—----—-_----—___--__---___ 28

Blanco River basin and adjacent area----———----—--------- 28

Conclusions------—----------------—----——--—--—__----__—___ 30

Selected references--------------------------------- — — __________ 33

III
ILLUSTRATIONS

Page

Figure 1. Map showing drainage basins and data-collection

sites in the San Antonio area—-- — — — — — ____ — __— 3

2. Map showing geologic and hydrologic features in

the San Antonio area--———____________________—_ 4

3. Graphical separation of a hypothetical hydrograph 

for estimating the components of direct

precipitation-- — ---- — ---- — — — — -- — _____-__--____ 8 

4.-11. Graphs showing:

4. Relationship between the base flow of the Nueces

River at Laguna and ground-water storage contributing to base flow in the Nueces River basin-------- 9

5. Relationship between the base flow of the Frio

River at Concan and ground-water storage contributing to base flow in the Frio River basin---—---- 10

6. Relationship between the base flow of the Dry

Frio River near Reagan Wells and ground-water

storage contributing to base flow in the Dry

Frio River basin—------------ — — — — — — — _________ n

7. Relationship between the base flow of the Sabinal 

River near Sabinal and ground-water storage 

contributing to base flow in the Sabinal River 

basin--------- — ---------------- — — — — _____________ 12

8. Relationship between the base flow of Seco Creek 

at Miller Ranch near Utopia and ground-water 

storage contributing to base flow in the Seco 

Creek basin---- — ------------------ — ______________— 13

9. Relationship between the base flow of Hondo Creek 

near Tarpley and ground-water storage contributing to base flow in the Hondo Creek basin--—----- 14

10. Relationship between the base flow of the Blanco 

River at Wimberley and ground-water storage 

contributing to base flow in the Blanco River 

basin---------------------------- — ---------- — ______ 15

11. Relationship between the monthly average contents

of Medina Lake and recharge to the Edwards aquifer

from Medina Lake and Diversion Reservoir------------- 24

TABLE

Table 1. Estimated annual recharge to the Edwards aquifer

by basins, 1934-75 ————————————————— ——————— -- 31

IV
METHOD OF ESTIMATING NATURAL RECHARGE TO THE 

EDWARDS AQUIFER IN THE SAN ANTONIO AREA, TEXAS

By

Celso Puente 

U.S. Geological Survey

ABSTRACT

Estimates of annual recharge to the Edwards aquifer in the San Antonio 

area, Texas, have been made by several investigators as part of the longterm studies of the aquifer by the U.S. Geological Survey under agreements 

with several local cooperators.

The method of estimating annual recharge is based on data collected 

from a network of stream-gaging stations and on assumptions related to 

applying the runoff characteristics from gaged areas to ungaged areas. 

The basic approach is a water-balance equation, in which recharge within 

a stream basin is the difference between measured streamflow above and 

below the infiltration area of the aquifer plus the estimated runoff in 

the zone that includes the infiltration area. Recharge in the Medina River 

basin also includes seepage losses from Medina Lake and Diversion Reservoir.

Gaging stations are located above and below the infiltration area in 

five of the nine basins that have been delineated. Gaging stations above 

and below the infiltration area are located in parts of the other four 

basins. Parts of two basins have stations only at points below the infiltration area, and parts of the other two basins have no gaging sites at 

all.

The principal errors in the estimates of annual recharge are related 

to errors in estimating runoff in ungaged areas, which represent about 30 

percent of the infiltration area. The estimated long-term average annual 

recharge in each basin, however, is probably representative of the actual 

recharge because the averaging procedure tends to cancel out the major 

errors.

-1-
INTRODUCTION

The purpose of this report is to present the method used, with periodic modifications since about 1955, for estimating natural recharge to 

the Edwards aquifer, the primary source of water in the San Antonio area, 

Texas. The method is based on data collected from a network of streamgaging stations (fig. 1) and on assumptions related to applying the runoff 

characteristics from gaged areas to ungaged areas. Several investigators 

have used the method to estimate monthly and annual recharge for various 

periods since 1934 (Lowry, 1955; Petitt and George, 1956; Garza, 1962, 1966; 

Rettman, 1966-70; Puente, 1971-75). The collection of data used in these 

studies is part of a program of hydrologic investigations by the U.S. 

Geological Survey in cooperation with the Edwards Underground Water District, the Texas Department of Water Resources (formerly Texas Water Development Board), and the city of San Antonio.

Estimates of recharge are made for each individual basin within the 

study area by use of a computer program, in which input data consist of 

drainage areas, streamflow records, and precipitation records. Most of 

the area is within the major basins of the Nueces and Guadalupe Rivers and 

consists of the following smaller basins: (1) Nueces-West Nueces River 

basin; (2) Frio-Dry Frio River basin; (3) Sabinal River basin; (4) the area 

between the Sabinal and Medina River basins; (5) Medina River basin; 

(6) the area between the Medina River and Cibolo Creek basins; (7) the 

Cibolo Creek and Dry Comal Creek basins; (8) the Guadalupe River basin; 

and (9) the Blanco River basin and adjacent area (area between the Guadalupe River basin and the Blanco River basin).

GEOLOGY AND HYDROLOGY

The regional geology and hydrology of the Edwards aquifer in the San 

Antonio area has been studied by Petitt and George (1956), Garza (1962, 

1966), and Maclay and Small (1976). The drainage areas affecting recharge 

to the Edwards aquifer are within two physiographic provinces, the Edwards 

Plateau section of the Great Plains province and the West Gulf Coastal Plain 

section of the Coastal Plains province, which are separated by an intensively faulted area known as the Balcones Fault Zone.

The Edwards aquifer as defined in this report consists of part of the 

Edwards and associated limestones of Cretaceous age (fig. 2), with hydraulic 

boundaries consisting of (1) the limits of the infiltration area on the 

north, (2) a ground-water divide in Kinney County to the west, (3) a groundwater divide in Hays County to the east, and (4) a zone of inferior water, 

arbitrarily delineated by the "bad-water" line, at the southern limits of 

the artesian area. The infiltration area, as delineated in this report, 

consists of the area shown on figure 2. The infiltration area consists of 

(1) the delineated outcrop of the Edwards aquifer as shown for areas east 

of Uvalde County; and (2) the delineated outcrop of the aquifer, based on 

surface- and ground-water divides, that separate the Edwards aquifer and 

the Edwards Plateau aquifer in Uvalde and Kinney Counties. The Edwards 

aquifer in the San Antonio area is about 180 miles long and varies in width 

from about 5 to 30 miles (fig. 2).

-2-
100"30' 

____ __ _ j___ 

EDWARDS 

) 

. 

~· I I ------------- -

99°30' 

~I_M_BL_E_T7------~--l.------==-=-=:::J~-~u~s;.___~2~90 __ -t-- r KERR 

99° 

C,' ~ , ZAVALA \ ,FRIO 

I 

100°30' 

Base from U S. Geological Sur'Vey 

State base map, 1 · 500,000 

TEXAS 

OC TIO 

I 

Oc=:======='0i==:=====2t:O=======:i30 Ml ~ES 

AP 

FIGURE 1.-Drainage basins and data-collection sites in the San Antonio area 

-3-

98°30' 

BLANCO 

95•30' 

98° 

NO. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 19. 

,O. 

21. 

22. 

23. 

24. 

25. 

26. 

/ 

26 

0 

• 

e 

• 

• 

e 

• 

e 

0 

~ 

EXPLANATION 

Nuece1-Weat Nu eces Rlver recha r ge bas in 

Frio-Dry Frlo Rlver recharge hasln 

S1blnal Rlv r recharge basin 

Ar ab twe n Sabinal Rlver basi n and Medina Rlver ba s in 

Medin River recharg basin 

Area betw en Medin• River and Cibolo Creek 

Ci lo and Dry Comal Creek recharge basi n 

Guadalupe Riv r r echarge basln 

Blanco River recharge basin 

PRECIPITAT!Or STATiON 

STREAM-GAGING STATION 

-••- BASIN BOUNDARY 

- · · - SUBBASIN DIVIDE 

0 

A 

B 

C 

D 

E 

F 

G 

STA. 0. 

08190500 

08190000 

08192000 

08196000 

08195000 

08197500 

08198000 

08198500 

08201500 

08202700 

08200000 

08200700 

08179000 

08179100 

08178700 

08183900 

08185000 

08169000 

08167000 

08167600 

08167500 

08166800 

08168500 

08171000 

08171300 

08172400 

U GAGED AREAS 

Lona River 

Blanco Creek 

l lltle Blanco and Nolton Cr ks 

Ranchero Creek 

~ar~crs and Live Odk Cr eks 

B tw n flondo Cre k and Medina River basin 

S1nk, Purgatory, and Alli gator Cre ks 

STREN-1-GAGING STATIONS 

NAM E 

est Nueces River near Brackettvil 1 , Tex. 

Nuec s R1v r t Laguna, Tex. 

lueces Riv r below Uva lde , Tex. 

Ory Frio River near Regn Wells, Tex. 

Frio River at Cancan, Tex. 

Frfo R1ver below Ory Frio Ri v r n ar Uvalde, Tex. 

Sabinal River near Sabinal, T x. Sabinal River at Sabinal , Tex. 

Seco Cr ek a Hiller Ranch near U opia, Tex. 

Seco Cree at Rowe Ranch near D'H3nis, Tex. 

Hondo Creek n ar Tarpley , Tex. 

Hondo Creek a Ing Waterhol near Hondo, Tex. 

H dina River near P1pe Creek , Tex. 

Red Bluff Cree near Pipe Cr k, T x. Salado Cree (upper station) a San Antonio, Tex. 

Cibolo Cre near Boerne, T x. Cibolo Cre at Selma, Tex. 

Comal River at ew Braunfels, Tex. 

Guadalup River at Comfort, T x. Rebecca Cr ek near Spring Branch, T x. Guadalup R1v~r near Spri ng Bra nch, Tex. 

Guadalupe Rf er at Sattler, Tex. 

Guadalupe R1v r above Comal Riv rat N Braunfels, Tex. 

Blanco R1ver at Wimberley, Tex. 

Blanco Riv rat Kyl , Tex. 

lum Cree at Lockh r , Tex. 
00°30 

_ SUT_!~ _ _j_ _________ j___ 

j' ED ARDS o 1 )(? ~iver I '-

....___ .s ....r I 

I 

I 

I 

I 

30•--,.---

1 

I •. ( i ~ \ 

y · 

' ( l ·., WI 

~ ~-EV-

~, 

29°30'---+-------

) 

I 

I O I I 

C, ------·-+---- i IZAVALA 

100° 100° 

EDWARDS 

O ia gramat,c cross section 

Base from US. Geolog ica l Sur e 

S a e base map, I: 500,000 

Edwards and associated 

l imestones ( Edwards Plateau aquifer) 

Infiltration area 

9" o· 

Edwards and assoc iated 

l imestones(Edwards aquifer) 

FIGURE 2.-Geologic and hydrologic features in the San Antonio area 

-4-

20 30 M LES 

l 

98°30' 

BLANCO 98° 

□ ~ 

m 

-----

EXPLANATION 

FORMATIONS YOUNGER THAN EDWARDS 

AND ASSOCIATED LIMESTONES 

EDWARDS AND ASSOCIATED LIMESTONES 

FORMATIONS OLDER THAN EDWARDS AND 

ASSOCIATED LIMESTONES 

INFILTRATION AREA OF EDWARDS AQUIFER 

AS DELINEATED IN THIS REPORT 

DRAINAGE DIVIDE 

--

11

BAD-WATER11LINE (DOWNDIP LIMIT OF 

FRESHWATER) 

______ STREAMFLOW LOSSES 

- DIRECTION OF GROUND-WATER FLOW 

9 WATER LEVEL 

L LEAKAGE FROM LAKE 

Q SPRING 
The Edwards and associated limestones consist of 400 to 500 feet of 

fine-grained carbonate rocks between the base of the Del Rio Shale and 

the top of the Glen Rose Formation. Maclay (1973) describes the Balcones 

Fault Zone as the dominant structural feature in the area (fig. 2). The 

major faults occur as a series of closely spaced step faults that generally 

trend east and northeastward across the study area. Individual fault displacements vary from less than 10 feet to more than 500 feet.

Most of the drainage area that contributes recharge to the Edwards 

aquifer is on the Edwards Plateau (fig. 2). The base flow of the streams 

that drain the plateau is sustained mostly by springflow from a watertable aquifer (Edwards Plateau aquifer) underlying the plateau. This base 

flow and a part of the floodflows are lost by infiltration where the streams 

cross the infiltration area of the Edwards aquifer. Recharge to the aquifer is derived mainly from infiltration of streamflow plus direct infiltration from precipitation in the infiltration area. The exceptionally high 

capacity of the aquifer to receive and transmit water is due to the fractures 

and porous zones that are well developed in some stratigraphic units.

METHOD OF DETERMINING ANNUAL RECHARGE 

Basic Approach

The basic method of estimating annual recharge to the Edwards aquifer was developed by Lowry (1955), Petitt and George (1956), and refined 

by Garza (1962, 1966). Recharge in the infiltration area consists of the 

infiltration of streamflow plus direct infiltration of precipitation in 

the interstream areas. This basic approach of estimating recharge in each 

stream basin is a water-balance equation, in which recharge is the difference between the measured streamflow above and below the infiltration area 

plus the estimated runoff within a zone that includes part of the infiltration area. This zone is the drainage area between the two gaging stations 

in each stream basin.

Stream-gaging stations are located a short distance above the northern edge and below the southern edge of the infiltration area of the aquifer. The locations of the stream-gaging stations were selected on the 

basis of geologic and seepage studies of the streams involved.

A monthly time interval is used in estimating recharge to the Edwards 

aquifer, and it appears to be adequate to permit the analysis of individual 

storms. Although the effects of some storms may overlap into a succeeding 

month, the estimated recharge is distributed proportionately according to 

the occurrence of both floodflow and base flow during each month.

The estimate of recharge in each basin is made by use of a computer 

program that requires the input of streamflow data in cubic feet per seconddays (cfs-days), a unit used in the initial handling of discharge measurements. Monthly estimates are then converted to acre-feet (1 cfs-day =

-5-
1.9835 acre-feet). The basic equation for computing monthly recharge is 

as follows:

R = (Qu + SI - QL) (1.9835) 

where II is monthly recharge, in acre-feet;

QU is the volume of flow at the upper gage, in cfs-days;

SI is the estimated volume of runoff (including infiltration) resulting from precipitation in the intervening area, in cfs-days; 

and

QL is the volume of flow at the lower gage, in cfs-days.

Streamflow data are available for most of the basins within the infiltration area; however, some of the streams are ungaged because suitable 

sites for the installation of gages are not available. Approximately 30 

percent of the total infiltration area is ungaged. The estimated amounts 

of recharge in the ungaged areas are based on the assumption that the runoff characteristics are similar to the runoff characteristics of the adjacent gaged areas. The procedures are explained in the detailed analyses of 

the individual basins.

Estimated Runoff Between Upper and Lower Gages

The runoff resulting from direct precipitation in the drainage area 

between the upper and lower gaging stations can be a significant factor 

in the water-balance equation. Part of this runoff may percolate downward 

in the infiltration area, and part may eventually contribute to streamflow 

between the gages. Obviously, this total runoff cannot be gaged and cannot 

be separated into seepage or runoff components, but the inability to separate it does not affect the water-balance equation. However, a good estimate of the total runoff is required.

The flow measured at each upper gaging station is composed of both 

floodflow and base flow. Floodflow, which is water that enters the stream 

by direct runoff during and immediately after a rainstorm, reaches the 

gage rapidly and is characterized by a sharp increase in discharge. Floodflows quickly recede and cease until the next rainstorm.

Base flow is the part of the streamflow that is sustained mainly by 

springs, and to a lesser degree of seepage from bank storage. The springs, 

which drain the water-table aquifer in the catchment area of the Edwards 

Plateau (figs. 1 and 2), occur where the water table is intersected by the 

deep valleys. Most of the springs occur at an altitude above flood stages 

of the streams; therefore, rises in stream level do not affect the discharge 

of the springs.

Hydrographs of streamflow data from the upper gaging stations can be 

separated to indicate the relative amounts of base flow and floodflow from 

the Edwards Plateau. For each storm, floodflow within the areas between 

the upper and lower gages is assumed to occur at the same rate per unit

-6-
area as above the upper gage. Stream losses due to evapotranspiration 

are assumed to be proportionately the same for both the infiltration area 

and the catchment area on the Edwards Plateau.

The general equation used for estimating the total runoff derived 

from direct precipitation in the areas between the upper and lower gages 

is expressed as follows:

CT AA n SI = ^ Qtu

where SI is the volume of water (runoff plus infiltration) contributed by 

precipitation in the intervening drainage area, in cfs-days;

AA is the intervening drainage area between the upper and lower gage, 

in square miles;

Au is the drainage area above the upper gage, in square miles; and

Qtu is the volume of water contributed by the storm above the upper 

gage, in cfs-days.

Figure 3 is a hypothetical hydrograph that illustrates graphical 

separation of the streamflow components and their relation to rainstorms. 

The floodflow component (C) is empirically separated from the base flow 

and should be considered as an approximation. The floodflow component 

(C) is only a part of Qtu5 the other part is the amount of water that infiltrates the surface of the Edwards Plateau and becomes base flow during and 

after a storm. The increase in base flow from a storm (cross-hatched area 

on fig. 3) can be computed from the differences between the base-flow recession curves before and after the storm. However, succeeding storms often 

overlap streamflow recessions, and base-flow components for each storm 

are difficult to separate.

The magnitude of the base flow of streams draining the Edwards Plateau 

is directly dependent on the quantity of water stored in the water-table 

aquifer underlying the plateau. Long-term streamflow records at upper gaging stations have been used to develop base-flow recession curves on the 

basis of data collected during the winter, when storms and evapotranspiration losses are at a minimum. These recession curves have been used to 

construct the graphs (figs. 4-10) showing the relation between groundwater storage in the Edwards Plateau aquifer contributing to base flow and 

the base flow of the streams.

An increase in the base flow as the result of a storm can be applied 

to the graphs (figs. 4-10) to determine the corresponding increase in storage in the Edwards Plateau aquifer, which is equivalent to the base-flow 

contribution of the storm during the streamflow recession (component B on 

fig. 3). Graphs relating base flow to storage in the Edwards Plateau aquifer have been developed for the following gaging stations: Nueces River 

at Laguna (fig. 4); Frio River at Concan (fig. 5); Dry Frio River near 

Reagan Wells (fig. 6); Sabinal River near Sabinal (fig. 7); Seco Creek at

-7-
CO

o 

o

LU 

CO

a:

UJ

a.

IUJ 

UJ

u~ 

o

OQ 

O 

2

UJ

a:

o

CO

Base flow 

before storm, Base flow 

after storm

Note: See page)8for explanation 

of symbols

MAR|APR|MAY |JUNE |JULY JAUG |SEPTt OCT|NOVj DEC j JAN j FEB|

FIGURE 3.-Graphical separation of a hypothetical hydrograph for estimating the components of 

direct precipitation
280

>o

08 16 24 32 40 48 56 64 72 

GROUND WATER IN STORAGE CONTRIBUTING TO BASE FLOW, IN THOUSAND

CUBIC FEET PER SECOND-DAYS

FJGURE4.-Relationship between the base flow of the Nueces River at Laguna and ground-water storage 

contributing to base flow in the Nueces River basin
140

8 120

UJ in 

cr

UJ

CL

UJ 

Ld

o

m

=5 

O

LU 

O

cr

o

CO

I

UJ 

CO

CD

100

80

60

40

20

I I

024 

GROUND WATER IN

6 8 10 12 14 

STORAGE CONTRIBUTING TO BASE FLOW, 

CUBIC FEET PER SECOND-DAYS

IN

16 18 

THOUSAND

FIGURE 5.-Relationship between the base flow of the Frio River at Concan and ground-water storage 

contributing to base flow in the Frio River basin
70

o ^/-\ o 60 LJ

LJ 

Q_

LU 

UJ

o 

co

co 

<

CD

50

o

co 40

o

LU 

<S> 

CC.

20

10

0

0 4 8 12 16 20 24 28

GROUND WATER IN STORAGE CONTRIBUTING TO BASE FLOW, IN

CUBIC FEET PER SECOND-DAYS

32 36 

HUNDRED

FIGURE 6.-Relationship between the base flow of the Dry Frio River near Reagan Wells and ground-water 

storage contributing to base flow in the Dry Frio River basin
140

o 

o

LJ

co

cr LJ 

a.

LU 

LJ 

Lu

O

CD

:D 

O

120

100

80

10 LJ 

O

cc 

<

X

o

CO

60

40

LJ 

CO 

<

m

20

0123456789 

GROUND WATER IN STORAGE CONTRIBUTING TO BASE FLOW, IN THOUSAND

CUBIC FEET PER SECOND-DAYS

FIGURE /.-Relationship between the base flow of the Sabinal River near Sabinal and ground-water 

storage contributing to base flow in the Sabinal River 

basin
a

z 

o

UJ 

CO

cr

UJ

o_

hUJ 

UJ

u_ 

o 

m

o

LJ 

O

cr.

o 

co

UJ 

CO

<

GO

42

36

30

24

18

12

0

024 6 8 10 12 14 

GROUND WATER IN STORAGE CONTRIBUTING TO BASE FLOW, IN

CUBIC FEET PER SECOND-DAYS

16 18 

HUNDRED

FIGURE 8.-Relationship between the base flow of Seco Creek at Miller Ranch near Utopia and ground-water 

storage contributing to base flow in the Seco Creek basin
140

8 120

LU 

CO

CC 

UJ 

0.

LU 

LU 

U_

O

CD

13 

O

a: 

<

x

O 

CO

i

LU 

U) 

< 

QQ

100

80

60

40

20

0 5 10 15 20 25 30 35 

GROUND WATER IN STORAGE CONTRIBUTING TO BASE FLOW, IN

CUBIC FEET PER SECOND-DAYS

40 45 

HUNDRED

FIGURE 9.-Relationship between the base flow of Hondo Creek near Tarpley and ground-water storage 

contributing to base flow in the Hondo Creek basin
ft TO

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2. Q

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3 

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ro -P> CD oo o ro 

o o o o o o

1 i I

O 

(Q O
Miller Ranch near Utopia (fig. 8); Hondo Creek near Tarpley (fig. 9); and 

Blanco River at Wimberley (fig. 10). It should be noted that these graphs 

show storage in the Edwards Plateau aquifer between the upper and lower 

limits of base-flow discharge; total storage in the Edwards Plateau aquifer 

is not known.

Component (A), which is the initial increase in base flow after a 

storm, can be computed easily from the hydrograph (fig. 3) to determine 

all of the base flow contributed by a storm. Component (A) is equivalent 

to the volume encompassed within the empirical base-flow curve from the 

beginning of storm runoff (zero base-flow increase) to the point of maximum base-flow increase (point D, fig. 3). The lower boundary is an extrapolation of the previous base-flow recession curve to the same point in time 

(point E, fig. 3) as the maximum base-flow increase (point D, fig. 3).

The increment (DE) is the particular increase in base flow that is 

used to determine component (B) from graphs such as those on figures 4-10. 

The value of Qtu for each individual storm in the area above the upper 

gaging station is the sum of components (A), (B), and (C).

Adjustments for Distribution of Precipitation

Runoff estimates related to determination of recharge in ungaged areas 

are based on the assumption that the runoff characteristics in these areas 

are similar to those in nearby or adjacent gaged areas. The available 

rainfall records are used to trace the distribution of rainfall for each 

major storm, and an adjustment to the estimated runoff is made when a wide 

variation occurs in the rainfall distribution.

The adjustment to the estimated unit runoff is based on a rainfall, 

ratio that is determined from the average rainfall in each of the areas. 

The average rainfall is the arithmetic mean derived from records of the 

precipitation stations within each area. For example, gaged area M receives 

an average rainfall of 5 inches during a storm; ungaged area N receives 

only 2 inches. The estimated unit runoff in area N, based on the gaged 

unit runoff in area M, is adjusted by a rainfall ratio of 2/5. When this 

ratio is plus or minus 20 percent of unity, which is an arbitrary designation, no adjustments are made for the distribution of rainfall.

Adjustments made on a "storm-by-storm" basis within an individual 

month have been compared with the adjustments made by using the total monthly 

rainfall data. The differences in the adjustments are small, probably 

because most of the precipitation occurs in storms of short duration. Large 

discrepancies may occur, however, when a rare storm of long duration overlaps into the following month. The computer program for estimating annual 

recharge was designed so that the adjustments for the distribution of 

precipitation are made monthly.

-16-
RECHARGE IN INDIVIDUAL BASINS 

Nueces-West Nueces River Basin

The Nueces-West Nueces River basin is the westernmost basin that contributes recharge to the Edwards aquifer (fig. 1). The total inflow available for recharge includes the following: (1) All flow that passes the 

gaging station Nueces River at Laguna; (2) all flow that passes the gaging 

station West Nueces River near Brackettville; and (3) all estimated runoff 

from direct precipitation between these two upper gages and the gaging 

station on the Nueces River below Uvalde. Only floodflow passes the gage 

on the West Nueces River, and it is assumed that only this floodflow affects 

recharge to the aquifer extending south and east of the gaging station. 

The estimated runoff in the area between the upper gages and the lower 

gage is based on the contributions (Qtu) from individual storms in the 

gaged area above the gaging station Nueces River at Laguna. The total 

outflow is measured at the gaging station on the Nueces River below Uvalde.

Estimates of recharge to the Edwards aquifer in the Nueces-West Nueces 

River basin are made by use of a computer program containing the following 

equation:

Rnwn = [Qn + Qwn + (SIn ) (RFn) - Qnbu] (1 .9835) (1)

where Rnwn is the monthly recharge to the aquifer in the basin, in acrefeet;

Qn is the total monthly flow at the gaging station Nueces River 

at Laguna, in cfs-days;

Qwn is the total monthly flow at the gaging station West Nueces 

River near Brackettville, in cfs-days;

Qnbu is the total monthly flow at the gaging station Nueces River 

below Uvalde, in cfs-days;

RFn is the rainfall ratio obtained by the equation: 

—— LP

where LP is the average monthly precipitation in the area 

between the upper and lower gages, and

UP_ is the average monthly precipitation in the drainage

area above the upper gage.

The computer program sets RFn equal to unity if the input 

value is between 0.8 and 1.2.

SI n is the estimated monthly runoff, in cfs-days, in the area between 

—— the two upper gages and the lower gage (483 square miles) . 

Runoff is assumed to be proportionate to the runoff in the 

drainage area (764 square miles) above the gage at Laguna. 

The calculation for SIn is as follows:

SIn =

483

764

-17-
where Cn is the floodflow or component (C) , in cfs-days, that is obtained 

from analyses of the streamflow records at the Laguna gage.

An is the initial increase in base flow during a flood, in cfs-days.

It is the same as the base-flow component (A) , which is obtained 

from the analyses of the streamflow records at the Laguna gage.

Bn is the base-flow component (B) , in cfs-days, which is obtained 

from the graph relating ground-water storage in the Edwards 

Plateau aquifer and the base flow at the Laguna gage (fig. 4).

Estimates of SIn are made only for the months in which significant 

storms occur. If a storm overlaps into the following month, the runoff 

is distributed proportionately according to the floodflow occuring during 

each month.

Frio-Dry Frio River Basin and Adjacent Areas

The procedure for estimating recharge in the Frio-Dry Frio River 

basin is similar to the procedure used for the Nueces-West Nueces River 

basin. Total inflow to the basin consists of the flow past the gaging 

stations Frio River at Concan and Dry Frio River near Reagan Wells, plus 

the estimated runoff resulting from precipitation in the area between 

these two gages and the gaging station Frio River below Dry Frio River near 

Uvalde (fig. 1). Total outflow from the infiltration area is measured at 

the gage on the Frio River below Dry Frio River near Uvalde.

The combined drainage area between the upper and lower gages, which 

is 139 square miles, is assumed to have the same runoff characteristics as 

the 522 square miles of combined drainage area above the gaging stations 

at Concan and Reagan Wells. The Leona River basin and the Blanco Creek 

basin are adjacent to the Frio-Dry Frio River basin on the west and east, 

respectively. The combined drainage area of these two small basins is 68 

square miles and is included in the annual recharge estimates for the FrioDry Frio River basin. It is assumed that the recharge in this ungaged area 

is proportionate to the recharge in the Frio-Dry Frio River basin.

Estimates of annual recharge to the Edwards aquifer in the Frio-Dry 

Frio River basin, including the Leona River and Blanco Creek basins, are 

made by use of a computer program containing the following equation:

Rfdf = [Qf + Qdf + (SIfdfMRFf) - Qfbdf 1(1- 9835) (2)

where Rf^f is the monthly recharge to the aquifer in the basin, in acre- ._ —— feet;

Qf is the total monthly flow at the gaging station Frio River at 

Concan, in cfs-days;

Qdf is the total monthly flow at the gaging station Dry Frio River 

" near Reagan Wells, in cfs-days;

Qfbdf is tne total monthly flow at the gaging station Frio River below 

Dry Frio River near Uvalde, in cfs-days;

-18-
RFf is the rainfall ratio, obtained by use of the same equation

—— given in the preceding discussion of the Nueces-West Nueces

River basin; and

SIfdf is the estimated monthly runoff, in cfs-days, in the area between 

the upper gages and the lower gage, which is calculated as 

follows:

SIfdf = (Cf + Af + Bf + Cdf + Adf + Bdf)

The parameters, Cf, Af, and Bf are the components (C) , (A), and (B) , 

respectively, as determined from hydrograph data from the gaging station 

Frio River at Concan. The parameters Cdf, Adf , and Bd f are components 

(C), (A), and (B) , respectively, and are determined from hydrograph data 

from the gaging station Dry Frio River near Reagan Wells. Figures 5 and 

6 show the relations between base flow and aquifer storage that were used 

in deriving the values of Bf and Bdf .

Recharge in the ungaged adjacent areas (Leona River and Blanco Creek 

basins) is estimated only when floodflows occur. Streamflow records from 

the outflow (lower) gage near Uvalde indicate that only large storms result 

in runoff exceeding recharge in the infiltration area of the Frio-Dry Frio 

River basin; it is assumed that a proportionate amount of runoff occurs 

in the infiltration area of the adjacent basins. Recharge in these ungaged 

areas is assumed to be proportional to the recharge as computed for the 

Frio-Dry Frio River basin. The recharge in the adjacent areas is estimated 

to be 68/637 of the recharge in the Frio-Dry Frio River basin because this 

is the ratio of the drainage areas above the lower edge of the infiltration 

area. When flow at the lower gage (Qfbdf) is greater than zero, the equation used to compute the adjacent-area recharge (Rfaa) i- s as follows:

/• o

Rfaa = 637 (Rfdf)

Many storms that produce floodflows in this basin are not large enough 

to produce flow at the lower gage. When flow does not occur at the lower 

gage, recharge in the adjacent areas is included as part of the estimated 

runoff resulting from precipitation in the infiltration area of the FrioDry Frio River basin. Under these conditions, equation (2) is modified 

by increasing the area between the upper gages and the lower gage to account 

for the infiltration area of the ungaged areas. The equation used is as 

follows:

Rfdf = [Qf + Qdf + (SIfdf) ' (RFf) - Qfbdf 1(1.9835) (3)

207 where (SI £d£ ) ' = (Cf + Af + B f + Cdf + Ad£ + Bdf ) .

-19-
Sabinal River Basin and Adjacent Areas

The procedure for estimating recharge in the Sabinal River basin and 

the adjacent ungaged areas is similar to the procedure used in the previously described basins. Inflow is measured by the upper gage, Sabinal 

River near Sabinal; outflow is measured by the lower gage, Sabinal River 

at Sabinal (fig. 1).

The drainage area between the upper and lower gages is 41 square miles, 

of which the infiltration area comprises 15 square miles. It has been 

assumed that the area between the gages has the same runoff characteristics as the area (206 square miles) above the upper gaging station. The 

equation for computing recharge in the Sabinal basin is as follows:

Rsb = [Qsns + (SIsns) (RF S ) - Qsas ] (1 .9835) (4) 

where Rsb is the monthly recharge to the aquifer in the basin, in acre-feet;

Qsns is the total monthly flow at the gaging station Sabinal River 

near Sabinal, in cfs-days;

Qsas is the total monthly flow at the gaging station Sabinal River 

at Sabinal, in cfs-days;

RF S is the rainfall ratio, obtained by use of the same equation given 

in the preceding discussion of the Nueces-West Nueces River 

basin; and

SI sns is the estimated monthly runoff, in cfs-days, in the area between 

the upper and lower gages, which is calculated as follows:

41 SI sns = 206" ( Csns + Asns +

The parameters (Csns , Asns , and Bsns ) are the hydrograph components 

(C) , (A), and (B) , respectively, as determined from streamflow data plus 

the storage-discharge relation shown on figure 7.

Twenty-four square miles in the infiltration area adjacent to the 

Sabinal River basin are drained by parts of Little Blanco, Nolton, and 

Ranchero Creeks. Recharge in this small area is assumed to be proportional 

to recharge in the Sabinal River basin and is included as part of the total 

recharge in the Sabinal basin. The following equation was used to estimate recharge to this adjacent area:

24

Rsba ~ 41

Rsb

sns +

where Rsba ^ s the monthly recharge to the adjacent area, in acre-feet, and 

all other terms are as described previously.

-20-
The item expressed in brackets in the equation is a ratio of the 

recharge in the Sabinal River basin to the total available inflow (measured flow plus estimated runoff) above the infiltration area. Occasionally, 

during minor floods, this recharge ratio exceeds slightly the value of 

1.00, probably because the errors in the estimated runoff (SI sns ) are large 

in relation to the quantity of flow. The computer program is designed so 

that this ratio is set to 1.00 when this condition occurs.

Area Between the Sabinal River Basin and the Medina River Basin

This area includes the drainage basins of the Seco Creek, Hondo Creek, 

and an ungaged area of 152 square miles between the Hondo Creek and Medina 

River basins that are drained by Verde and Quihi Creeks and their tributaries. Recharge in the Seco Creek and the Hondo Creek basins is estimated by using the same basic approach as previously explained. In the 

Seco Creek basin, inflow is measured at the upper gage, Seco Creek at 

Miller Ranch near Utopia (fig. 1), and outflow is measured at the lower 

gage, Seco Creek at Rowe Ranch near D'Hanis (fig. 1). The runoff for the 

125-square-mile area between the upper and lower gages in the Seco Creek 

basin is assumed to be proportional to the runoff in the 43-square-mile 

area above the upper gaging station. The recharge equation used for the 

Seco Creek basin is as follows:

RSC = [Qsu + (SI SU )(RF) - Qsd ] (1.9835) (5) 

where Rsc is the monthly recharge, in acre-feet;

Qsu is the total monthly flow at the gaging station Seco Creek at 

Miller Ranch near Utopia, in cfs-days;

Qsd i s the total monthly flow at the gaging station Seco Creek at 

Rowe Ranch near D'Hanis, in cfs-days;

RF is the rainfall ratio; and

SI SU is the estimated monthly runoff, in cfs-days, in the area between 

the upper and lower gages in the Seco Creek basin, which -is 

computed as follows:

125 SI su = "43" ( c su + Asu + Bsu)

The parameters Csu , Asu , and Bsu are the components (C) , (A), and 

(B) , respectively, that were derived from the separation analysis of the 

daily streamflow hydrograph (Seco Creek at Miller Ranch near Utopia) plus 

the storage-discharge relation shown on figure 8.

Inflow in the Hondo Creek basin is measured by the upper gage, Hondo 

Creek near Tarpley (fig. 1); and outflow is measured by the lower gage, 

Hondo Creek at King Waterhole near Hondo (fig. 1). The runoff in the 56- 

square-mile area between the upper and lower gages is assumed to be propor-

-21-
tional to the runoff in the area of 86 square miles above the upper gaging 

station. The recharge equation for the Hondo Creek basin is as follows:

Rh = [Qht + (SIht ) (RFh ) - Qhh] (1.9835) (6) 

where Rh is the monthly recharge, in acre-feet;

Qht is the total monthly flow at the gaging station Hondo Creek near 

Tarpley, in cfs-days;

Qhh is the total monthly flow at the gaging station Hondo Creek at 

—— King Waterhole near Hondo, in cfs-days;

is the rainfall ratio; and

is the estimated monthly runoff, in cfs-days, in the area between 

the upper and lower gages, which is calculated as follows:

SIht = 35" ( cht + Aht + Bht)

The parameters Cht, Aht, and Bht are the components (C), (A), and 

(B), respectively, that were derived by the same procedures as described 

previously and by the use of the storage-discharge relation shown on figure 9.

An area of 12 square miles within the infiltration area adjacent to 

the Hondo Creek basin is drained by Parkers Creek and Live Oak Creek (fig. 

1). Recharge in this small area is assumed to be proportional to recharge 

in the Hondo Creek basin. The following equation was used to estimate 

recharge in this adjacent area:

Rha = M- (Slht)

Qht + |f (SIht)

where Rha is the monthly recharge, in acre-feet, to the adjacent area; 

all other terms have been described previously.

The item expressed in brackets in the equation is a ratio of the 

recharge in the Hondo Creek basin to the total available inflow (measured 

flow plus estimated runoff) above the infiltration area. The 46 square miles 

represent the infiltration area within the 56 square miles of drainage area 

between the upper and lower gages in the Hondo Creek basin. During minor 

floods, this recharge ratio exceeds slightly the value of 1.00, probably 

because the errors in the estimated runoff (SIht) are l ar£e in relation to 

the quantity of flow. The computer program is designed to set this recharge 

ratio equal to 1.00 if this condition occurs.

The estimates of recharge for the large ungaged area (152 square 

miles) drained by Verde and Quihi Creeks (fig. 1) are based on runoff 

data obtained for the gaged areas of the Hondo Creek and Medina River 

basins. The units of surface runoff for the areas above the gage Hondo 

Creek near Tarpley and above the gage Medina River near Pipe Creek (fig. 

1) are averaged and applied to the ungaged area. The recharge is esti-

-22-
mated as a percentage of the computed runoff, and the basis used is the 

Hondo Creek recharge ratio. The equation for estimating the recharge in 

the ungaged area between the Hondo Creek and the Medina River is as follows;

/Qht . Qmpc'

= 152

Qht + £ (SIht)

(7)

where Rhm is the monthly recharge in the area between the Hondo Creek and 

—— the Medina River, in acre-feet; and

Qmpc is the monthly flow at the gaging station Medina River near Pipe

——— Creek, in cfs-days.

All other terms in the equation have been described previously, including 

the Hondo Creek recharge ratio, the last item in brackets. The drainage 

area above the Tarpley gage is 86 square miles; the drainage area above 

the Pipe Creek gage is 474 square miles.

Medina River Basin

The procedure used for estimating recharge in the Medina River basin 

is different from the procedure used in other basins because streamflow in 

the basin is regulated by a large reservoir (fig. 1). Medina Lake, impounded 

by Medina Dam, is located over the infiltration area and seepage losses 

from the lake recharge the Edwards aquifer.

The basis for estimating recharge from Medina Lake is a correlation 

of reservoir stage and seepage into the aquifer. Lowry (1953) constructed 

two correlation curves, one representing seepage losses with a rising stage 

and the other representing losses with a falling stage. The curves are 

substantially different and apparently reflect the influence of bank storage.

The correlations were developed from data on streamflow, evaporation, 

and reservoir stage, and on inflow-outflow analyses that reflected water 

balances of the reservoir over a long period of time. A graph relating 

the monthly average contents of Medina Lake with the seepage losses to the 

Edwards aquifer is shown on figure 11. This graph also reflects the losses 

from Diversion Reservoir (4 miles downstream from Medina Lake), which is 

used to divert water through an irrigation canal into southern Medina and 

Bexar Counties. Lowry (1953) estimated that when Diversion Reservoir is 

full or nearly full, a constant seepage of 1,500 acre-feet per month recharges 

the Edwards aquifer. Continuous leakage around and below Medina Dam has 

maintained Diversion Reservoir at near capacity except during periods of 

extended droughts.

Therefore, figure 11 was used to estimate monthly recharge to the 

Edwards aquifer in the Medina River basin. The average monthly contents 

in Medina Lake are derived from reservoir-stage records of the U.S. Geological Survey. These records are also used to determine whether reservoir 

stage is rising or falling.

-23-
1

° • 

5-

1-1:

O -0

2- °"

*• 0

o 7

isS* =

s «

3 3

ri

Q. 3"

3- •<•

° Q 

Q £

r Q

§•

n

S ?:

1 S

" o

5"

o

Q5

o

co

0

ro 

o o

MONTHLY AVERAGE CONTENTS, IN THOUSANDS OF ACRE-

— _ — _ _ 

-F>a>oooro-F>a>oo

ooooooooo

~1IIII1III
Area Between the Medina River Basin and the Cibolo Creek Basin

The drainage area between the Medina River and Cibolo Creek, and 

upstream from the lower edge of the infiltration area of the Edwards aquifer, is approximately 282 square miles, of which 131 square miles are in 

the infiltration zone. This area is partly drained by San Geronimo, Leon, 

and Salado Creeks, plus other smaller creeks and tributaries.

Much of the area is ungaged and recharge must be estimated by using 

data from adjacent basins. The gaging station Salado Creek at San Antonio 

(fig. 1) is used to measure the outflow from a drainage area of 137 square 

miles; the area above the lower edge of the infiltration area is 105 square 

miles and that above the upper edge of the infiltration area is 35 square 

miles. The infiltration area is 70 square miles, and the area between the 

gaging station and the lower edge of the infiltration area is 32 square 

miles. Runoff above the infiltration area in the Salado Creek basin is not 

measured directly because suitable gaging sites are not available. Therefore, estimates of total runoff to the area of upper Salado Creek are based 

on the assumption that the computed unit of runoff in the Cibolo Creek basin 

is applicable to this area. The recharge equation for the Salado Creek 

basin is as follows:

Rs = 137 (URC )(RF SC ) - (Qs )(1.9835) (8) 

where Rs is the monthly recharge to the aquifer, in acre-feet;

URC is the unit of runoff in the Cibolo Creek basin, in acre-feet 

per square mile;

Qs is the total monthly flow at the gaging station Salado Creek at 

San Antonio, in cfs-days; and

RF SC is the ratio of average rainfall in the Salado Creek basin to 

average rainfall in the Cibolo Creek basin.

The estimate of total inflow to the remaining 177 square miles of 

ungaged area is based on an average unit of runoff as derived from the 

unit of runoff at the gaging station Medina River near Pipe Creek and the 

unit of runoff computed for the Cibolo Creek basin. It is also assumed 

that the ratio of recharge in the Salado Creek basin to the total inflow 

above and within the infiltration area is applicable to the ungaged area. 

The recharge equation for this ungaged area is as follows:

R•ms = 177

UR

_105 URC RF SC _

(9)

where Rms is the monthly recharge in the area between Medina River and

Salado Creek, in acre-feet.

All other terms and drainage areas in the equation have been identified 

previously. The first bracketed item represents the average unit of runoff 

derived from streamflow records from the gaging station Medina River near

-25-
Pipe Creek and the unit of runoff for the Cibolo Creek basin. The second 

bracketed item is the ratio of recharge in the Salado Creek basin to the 

total available inflow above the lower edge of the infiltration area.

The total recharge for the area between the Medina River basin and 

the Cibolo Creek basin is the sum of Rs and Rms .

Cibolo Creek and Dry Comal Creek Basins 

Cibolo Creek Basin

Streamflow in the Cibolo Creek basin is measured at the gaging stations Cibolo Creek near Boerne and Cibolo Creek at Selma (fig. 1). Runoff 

from the uppermost 68 square miles of the Cibolo Creek basin is measured 

at the gage near Boerne. The gage near Selma, which is downstream from 

the lower edge of the infiltration area, has a drainage area of 274 square 

miles, of which 19 square miles are within the infiltration area.

At one time, a gaging station just upstream from the infiltration area 

was installed to measure all inflow to the infiltration area. However, 

considerable streamflow losses occurred in the Glen Rose Formation upstream 

from this gage, which precluded assessing the unit of runoff for inflow 

analyses. This gage was removed and reestablished near Boerne. The Glen 

Rose Formation along and near the Cibolo Creek is highly permeable in the 

shallow subsurface, and a substantial part of the runoff of Cibolo Creek 

infiltrates this section and is assumed to move laterally into the Edwards 

aquifer.

Runoff in the adjacent Guadalupe River basin is gaged by streamflow 

stations at Comfort and at a site near Spring Branch (fig. 1). Runoff in 

the large drainage area between these gages provides a basis for computing 

the unit of runoff that may be applicable to the Cibolo Creek basin on and 

above the infiltration area.

The distribution of average precipitation in the Cibolo Creek basin 

and in the Guadalupe River basin between the two gages can be derived from 

the records of several precipitation stations in these areas. The average 

monthly precipitation has been used (1) to select the unit of runoff applicable to the Cibolo Creek basin (URC ) from either the unit of runoff for 

the Guadalupe River basin (URg) or the unit of runoff as derived from the 

area above the gage Cibolo Creek near Boerne (URcb), and (2) to adjust the 

selected unit of runoff by use of an appropriate rainfall ratio.

The computer program has been designed so that the selection of the 

unit of runoff of the representative basin is as follows: (1) The difference (J) between the average rainfall in the Cibolo Creek basin above the 

gaging station at Selma and the average rainfall in the upper Cibolo Creek 

basin (above the gage near Boerne) is determined; (2) the difference (K) 

between the average rainfall in the Cibolo Creek basin above the gaging 

station at Selma and the average rainfall in the Guadalupe River basin

-26-
above the gaging station at Sattler is determined; (3) if (J) is equal to 

or greater than (K) , the value of URg is selected; and (4) if (J) is smaller 

than (K), the value of URC^ is selected.

Recharge in the Cibolo Creek basin is computed by use of the following equation:

Rc = 274 (URC )(RF C ) - (Qcs ) (1 .9835) (10) 

where Rc is the monthly recharge in the Cibolo Creek basin, in acre-feet;

URC is the unit of runoff in the Cibolo Creek basin, in acre-feet 

per square mile;

RFC is the rainfall ratio that relates average rainfall in the Cibolo 

Creek basin on and above the infiltration area to the average 

rainfall on the area of known runoff; and

Qcs is the total monthly flow at the gaging station Cibolo Creek at 

Selma, in cfs-days.

URC = URg or URcb

URcb =

where URg is the unit of runoff in the Guadalupe River basin, in acre-feet 

~ per square mile;

URcb is the unit of runoff in the Cibolo Creek basin near Boerne, in 

acre-feet per square mile;

Qgsp is the total monthly flow at the gaging station Guadalupe River 

near Spring Branch, in cfs-days;

QgC is the total monthly flow at the gaging station Guadalupe River 

at Comfort, in cfs-days; and

Qcb is the total monthly flow at the gaging station Cibolo Creek 

near Boerne, in cfs-days.

The Guadalupe River drains 477 square miles between the gage at Comfort and the gage near Spring Branch. The drainage area of the Cibolo 

Creek basin above the gage near Boerne is 68 square miles.

Dry Comal Creek Basin

The outflow of Dry Comal Creek (fig. 1) is measured at the gaging 

station Comal River at New Braunfels. The drainage area above the gage 

is 130 square miles; the infiltration area consists of the upper 89 square 

miles, which is not gaged. The discharge of Comal Springs at New Braunfels flows into Dry Comal Creek above the gaging station, and the measured 

flow is adjusted to determine the runoff from Dry Comal Creek. The Comal 

River is the short channel distance between Comal Springs and Dry Comal 

Creek.

-27-
Runoff in the Dry Comal Creek basin above the gaging station Comal 

River at New Braunfels is computed by applying the unit of runoff derived 

from measured streamflow on the Guadalupe River between the gage at Sattler 

and the gage at New Braunfels (fig. 1). Contributions to the Guadalupe 

River from Hueco Springs, situated upstream from the gage Guadalupe River 

above Comal River at New Braunfels (fig. 1), are accounted in the computation of the unit of runoff in the area between the gages. The equation 

for estimating recharge in the Dry Comal Creek basin is as follows:

= 130 Qg"b " S " QhS (RFd ) - (Qcnb - Qcos)l (1-9835) (11)

where R(j c is the monthly recharge in the Dry Comal Creek basin, in acre- 

—— feet.

Qgnb i s the total monthly flow measured at the gaging station Guadalupe River above Comal River at New Braunfels, in cfs-days.

Qgs is the total monthly flow measured at the gaging station Guadalupe River at Sattler, in cfs-days. The Guadalupe River drains 

82 square miles between the gage.at Sattler and the gage at 

New Braunfels.

Qhs is the total monthly flow of Hueco Springs, in cfs-days. The

springflow is estimated from monthly spring-flow measurements.

Qcnb i s the total monthly flow measured at the gaging station Comal 

River at New Braunfels, in cfs-days.

Qcos i s tne total monthly flow of Comal Springs, separated from the 

measurements at the gaging station Comal River at New Braunfels, in cfs-days.

RF(j is the rainfall ratio of the average rainfall in the Dry Comal 

Creek basin to the average rainfall in the Guadalupe River 

basin between the gages at Sattler and at New Braunfels.

Guadalupe River Basin

The Guadalupe River crosses the infiltration area of the Edwards aquifer (fig. 1), but does not contribute recharge in significant quantities. 

Although 48 square miles of area in the Guadalupe River basin is within 

the infiltration area, seepage studies indicate that the net streamflow 

losses and gains in the area are small and insignificant. The potentiometric surface of the aquifer in the New Braunfels area is generally at 

the level of the streambed of the Guadalupe River and is relatively stable 

because of the large and almost perennial flow of Comal Springs.

Blanco River Basin and Adjacent Area

Recharge in the Blanco River basin, the easternmost basin in the infiltration area, is estimated by using the same procedure as used in the Nueces 

River basin.

-28-
Inflow is composed of the flow past the upper gaging station Blanco 

River at Wimberley, and outflow is measured by the gaging station Blanco 

River near Kyle. Total runoff in the 57-square-mile area between the upper 

gage and the lower gage is determined by assuming that the area has the 

same runoff characteristics as the 355-square-mile area above the upper 

gage. The equation for estimating recharge in the Blanco River basin is 

as follows:

Rb = [Qbw + (SIbwHRFb) - QbkKl-9835) (12) 

where Rj, is the monthly recharge in the Blanco River basin, in acre-feet;

Qbw is the total monthly flow measured at the gaging station Blanco 

River at Wimberley, in cfs-days;

Qbk is the total monthly flow measured at the gaging station Blanco 

River near Kyle, in cfs-days;

is the rainfall ratio of the average rainfall in the area between 

the two gages to the average rainfall in the area above the 

Wimberley gage; and

is the estimated monthly runoff, in cfs-days, in the area between 

the upper and lower gage, which is computed as follows:

57 SIbw = 355" (Cbw + Abw

The parameters Cbw> Abw , and Bbw are equivalent to the (C) , (A), and 

(B) components, respectively, determined from the separation analysis of 

the daily streamflow hydrograph and the storage-discharge graph (fig. 10) 

for the Blanco River at Wimberley.

At some places in the infiltration area, the Blanco River has eroded 

deeply into the outcrop of the Edwards Limestone. During times when the 

aquifer head is high, the aquifer accepts little recharge and at other 

times may discharge to the river. The computer program is designed so that 

when the measured outflow exceeds the estimated inflow, the estimated 

recharge is zero. Under these conditions, the water-balance equation will 

indicate a discharge from the aquifer to the river.

The area between the Guadalupe River basin and the Blanco River basin 

is an ungaged area of 94 square miles within the infiltration area, which 

is drained by Sink, Purgatory, York, and Alligator Creeks. This area is 

surrounded by the following gaged drainage areas: The Blanco River basin 

above the Wimberley gage; the Guadalupe River basin between the Sattler 

and New Braunfels gages; and the Plum Creek basin above the gage at Lockhart (fig. 1).

Inflow to this area is estimated from the average of the units of 

runoff derived from (1) the runoff at the gage Blanco River at Wimberley, 

(2) the runoff in the Guadalupe River basin between the Sattler and New 

Braunfels gages (adjusted for the flow of Hueco Springs) , and (3) runoff

-29-
at the gage Plum Creek at Lockhart. Outflow from this area is assumed to 

be proportional to the outflow from the infiltration area in the Dry Comal 

Creek basin. Recharge in this ungaged area, termed as adjacent to the 

Blanco River basin, is the difference between the estimated inflow on and 

above the infiltration area and the estimated outflow below the infiltration 

area.

The equation for estimating recharge in the area adjacent to the Blanco 

River basin is expressed as follows:

Rba = 94

/QbwA ^Qgnb - Qgs - Qhs^ /QplN

V355 I \_____82_____) U12/ /Qcnb - QCOS\

3 ~ V V 130 /

where Rba is the monthly recharge, in acre-feet; and

(1.9835) (13)

Qpl is the total monthly flow measured at the gaging station Plum 

—— Creek at Lockhart, in cfs-days. The drainage area above the 

Lockhart gage is 112 square miles. The other variables in 

equation (13) have been defined in the sections describing 

recharge in the Blanco River basin and the Dry Comal Creek 

basin.

CONCLUSIONS

The method described in this report for estimating annual recharge to 

the Edwards aquifer is basically the same method used by various investi- • 

gators who have made recharge estimates dating back to 1934. Table 1 shows 

the estimated annual recharge to the Edwards aquifer by basin from 1934 to 

1975. Recharge estimates made since 1953 are considered to be more accurate because gaging stations were installed after 1953 in several areas that 

were previously ungaged.

Errors in the estimates of recharge are the results of inaccuracies 

in streamflow measurements in gaged areas and of discrepancies in runoff 

estimates in ungaged areas. The errors in streamflow measurements are 

minor, because most of the records in the San Antonio area are regarded 

by the U.S. Geological Survey as "excellent," which means that about 95 

percent of the daily discharges are accurate within 5 percent. Some estimates of runoff in ungaged areas, which represent about 30 percent of the 

infiltration area, may have large errors for individual storms.

Occasionally, the water-balance equations used for computed recharge 

are complicated by the occurrence of heavy rainstorms in the infiltration 

area and in areas between the lower edge of the infiltration area and the 

lower gaging stations. In some of the computations, the outflow at the 

lower gage may be greater than the inflow at the upper gage. Under these 

conditions, the recharge is assumed to be zero only during certain times 

in the Blanco River basin, where high aquifer-head conditions produce discharge from the aquifer. In all other basins, estimates of runoff are

-30-
Table 1.—Estimated annual recharge to the Edwards aquifer by basins, 1934-75 

(in thousands of acre-feet)

Calendar 

year

1934

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1949

1950

1951

1952

1953

1954

1955

1956

1957

1958

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

TOTAL

AVERAGE

Nueces-West 

Nueces 

River basin

8.6

411.3

176.5

28.8

63.5

227.0

50.4

89.9

103.5

36.5

64.1

47.3

80.9

72.4

41.1

166.0

41.5

18.3

27.9

21.4

61.3

128.0

15.6

108.6

266.7

109.6

88.7

85.2

47.4

39.7

126.1

97.9

169.2

82.2

130.8

119.7

112.6

263.4

108.4

190.6

91.1

71.8

4,291.5

102.2

Frio-Dry 

Frio 

River 

basin I/

27.9

192.3

157.4

75.7

69.3

49.5

60.3

151.8

95.1

42.3

76.0

71.1

54.2

77.7

25.6

86.1

35.5

28.4

15.7

15.1

31.6

22.1

4.2

133.6

300.0

158.9

128.1

151.3

46.6

27.0

57.1

83.0

134.0

137.9

176.0

113.8

141.9

212.4

144.6

256.9

135.7

143.6

4,147.3

98.7

Sabinal 

River 

basin

7.5

56.6

43.5

21.5

20.9

17.0

23.8

50.6

34.0

11.1

24.8

30.8

16.5

16.7

26.0

31.5

13.3

7.3

3.2

3.2

7.1

.6

1.6

65.4

223.8

61.6

64.9

57.4

4.3

5.0

16.3

23.2

37.7

30.4

66.4

30.7

35.4

39.2

49.0

123.9

36.1

47.9

1,487.7

35.4

Area between 

Sabinal River 

and Medina 

River basins

19.9

166.2

142.9

61.3

54.1

33.1

56.6

139.0

84.4

33.8

74.3

78.6

52.0

45.2

20.2

70.3

27.0

26.4

30.2

4.4

11.9

7.7

3.6

129.5

294.9

96.7

127.0

105.4

23.5

10.3

61.3

104.0

78.2

64.8

198.7

84.2

81.6

155.6

154.6

286.4

115.3

195.9

3,611.0

86.0

Medina 

Lake

46.5

71.1

91.6

80.5

65.5

42.4

38.8

54.1

51.7

41.5

50.5

54.8

51.4

44.0

14.8

33.0

23.6

21.1

25.4

36.2

25.3

16.5

6.3

55.6

95.5

94.7

104.0

88.3

57.3

41.9

43.3

54.6

50.5

44.7

59.9

55.4

68.0

68.7

87.9

97.6

96.2

93.4

2,344.1

55.8

Area between 

Cibolo Creek 

and Medina 

River basins

21.0

138.2

108.9

47.8

46.2

9.3

29.3

116.3

66.9

29.5

72.5

79.6

105.1

55.5

17.5

41.8

17.3

15.3

50.1

20.1

4.2

4.3

2.0

175.6

190.9

57.4

89.7

69.3

16.7

9.3

35.8

78.8

44.5

30.2

83.1

60.2

68.8

81.4

74.3

237.2

68.1

127.4

2,697.4

64.2

CiboloDry Comal 

Creek 

basin

28.4

182.7

146.1

63.9

76.8

9.6

30.8

191.2

93.6

58.3

152.5

129.9

155.3

79.5

19.9

55.9

24.6

12.5

102.3

42.3

10.0

3.3

2.2

397.9

268.7

77.9

160.0

110.8

24.7

21.3

51.1

115.3

66.5

57.3

120.5

99.9

113.8

82.4

104.2

211.7

76.9

195.7

4,028.2

95.9

Bla

Ri

ba

I

1

3

4

2

3

1

1

5

2

2

4

3

4

3

1

2

1

1

2

2

1

7

7

3

6

4

1

1

2

6

3

1

4

4

3

2

3

8

3

8

1,45

3

I/ Includes recharge from gaged and ungaged areas within the basin.
adjusted to reflect the heavy rainfall in the area between the upper and 

lower gages. The long-tenn average estimate of annual recharge is probably 

representative of the true average, because the averaging procedure of the 

many estimates tends to cancel out the major errors. The inflow-outflow 

analyses made by Garza (1962, 1966) indicate that the methods of estimating recharge are adequate in assessing long-term recharge and the annual 

average recharge over a long period. Monthly estimates of recharge during 

periods of high runoff probably contain the major errors. 

The FORTRAN IV computer program developed for computing recharge to 

the Edwards aquifer in the San Antonio area is not included in this report, 

but is available from the U.S. Geological Survey upon request. 

-32-
SELECTED REFERENCES

Garza, Sergio, 1962, Recharge, discharge, and changes in ground-water

storage in the Edwards and associated limestones, San Antonio area, 

Texas, a progress report on studies, 1955-59: Texas Board Water 

Engineers Bull. 6201, 51 p.

____1963, Records of precipitation, aquifer head, and ground-water 

recharge to the Edwards and associated limestones, 1960-62, San 

Antonio area, Texas: Edwards Underground Water Dist. Bull. 3, 7 p. 

_1964, Records of precipitation, aquifer head, and ground-water

recharge to the Edwards and associated limestones, San Antonio area, 

Texas, 1963: Edwards Underground Water Dist. Bull. 6, 7 p. 

_1966, Ground-water resources of the San Antonio area, Texas, a progress report on studies, 1960-64: Texas Water Devel. Board Rept. 34, 

36 p.

Guyton, W. P., and Associates, 1965, Report on recharge to the Edwards

ground-water reservoir, 1962-63: Consulting ground-water hydrologist 

rept. to the San Antonio City Water Board, 22 p.

Lowry, R. L., 1953, Hydrologic report, Medina River above Applewhite Dam 

site: Consulting engineer rept. to the San Antonio City Water Board.

____1955, Recharge to the Edwards ground-water reservoir: Consulting 

engineer rept. to San Antonio City Water Board.

Maclay, R. W., and Small, T. A., 1976, Progress report on the geology of 

the Edwards aquifer, San Antonio area, Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate 

rocks: U.S. Geol. Survey Open-File Rept. 76-627, 65 p.

Maclay, R. W., and others, 1973, Hydrology of the Edwards Limestone aquifer: Geol. Soc. America Hydrogeology Field Trip, San Antonio, Texas, 

16 p.

Petitt, B. M., Jr., and George, W. 0., 1956, Ground-water resources of 

the San Antonio area, Texas, a progress report on current studies: 

Texas Board Water Engineers Bull. 5608, v. 1, 85 p.

Puente, Celso, 1971, Records of precipitation, water levels, and groundwater recharge to the Edwards and associated limestones, San Antonio 

area, Texas, 1970: Edwards Underground Water Dist. Bull. 27, 11 p.

____1972, Records of precipitation, water levels, and ground-water recharge 

to the Edwards and associated limestones, San Antonio area, Texas, 

1971: Edwards Underground Water Dist. Bull. 30, 11 p.

____1973, Ground-water discharge from the Edwards and associated limestones, San Antonio area, Texas, 1972: Edwards Underground Water 

Dist. Bull. 31, 8 p. 

_1974, Records of precipitation, water levels, and ground-water recharge

to the Edwards and associated limestones, San Antonio area, Texas,

1972-73: Edwards Underground Water Dist. Bull. 33, 13 p.

_1975, Relation of precipitation to annual ground-water recharge to

the Edwards aquifer, San Antonio area, Texas: U.S. Geol. Survey 

Open-File Rept. 75-298, 31 p.

Rettman, P. L., 1966, Records of precipitation, aquifer head, and groundwater recharge to the Edwards and associated limestones, San Antonio 

area, Texas, 1965: Edwards Underground Water Dist. Bull. 12, 8 p.

-33-
SELECTED REFERENCES--Continued

__1967, Records of precipitation, aquifer head, and ground-water recharge

to the Edwards and associated limestones, San Antonio area, Texas,

1966: Edwards Underground Water Dist. Bull. 15, 9 p. 

__1968, Records of precipitation, aquifer head, and ground-water recharge

to the Edwards and associated limestones, San Antonio area, Texas,

1967: Edwards Underground Water Dist. Bull. 18, 9 p. 

_1969, Records of precipitation, aquifer head, and ground-water recharge

to the Edwards and associated limestones, San Antonio area, Texas,

1968: Edwards Underground Water Dist. Bull. 21, 9 p.

_1970, Records of precipitation, aquifer head, and ground-water recharge

to the Edwards and associated limestones, San Antonio area, Texas,

1969: Edwards Underground-Water Dist. Bull. 24, 11 p. 

Rose, P. R., 1972, Edwards Group, surface and subsurface, Central Texas:

Univ. Texas, Bur. Econ. Geol. Rept. Inv. 74, 198 p. 

Texas Board Water Engineers and U.S. Geological Survey, 1960, Channel gain

and loss investigations, 1918-58: Texas Board Water Engineers Bull.

5807-D, 270 p. 

U.S. Geological Survey, 1974, Water resources data for Texas, 1973; part 1,

surface-water records: U.S. Geol. Survey basic-data rept., 639 p.

-34-