Prepared in cooperation with the Texas Water Development Board

Analysis of Trends in Selected Streamflow Statistics for the 

Concho River Basin, Texas, 1916–2009

Scientific Investigations Report 2012–5193

IRION COUNTY

REAGAN COUNTY

TOM GREEN

COUNTY

STERLING COUNTY

GLASSCOCK COUNTY

San

Angelo

Tankersley

Christoval

Carlsbad

Paint Rock

Twin Buttes

Reservoir 

O C Fisher Lake

Lake

Nasworthy 

Concho River

North Concho River

Middle Concho River

Spring Creek

Dove Creek

South Concho River

£¤87

£¤87 £¤67

08136500

08136000

08134000

08128500

08128400

08128000 08127500

#

#

#

#

#

#

UPTON COUNTY

U.S. Department of the Interior

U.S. Geological Survey
Front cover: South Concho River looking upstream at U.S. Geological Survey streamflow-gaging station 

08128000 South Concho River at Christoval, Texas (photograph taken January 5, 2010 by Travis Dowell, 

U.S. Geological Survey).

Background: Concho River looking downstream from U.S. Geological Survey streamflow-gaging station 

08136500 Concho River at Paint Rock, Texas (photograph taken on March 9, 2012, by Henry Jacques, 

U.S. Geological Survey).

Back cover:

Top, The dry streambed of Middle Concho River looking downstream from U.S. Geological Survey

streamflow-gaging station 08128400 Middle Concho River above Tankersley, Texas (photograph taken 

November 6, 2009, by Greg Webb, U.S. Geological Survey).

Bottom, North Concho River looking upstream from U.S. Geological Survey streamflow-gaging station

08134000 North Concho River near Carlsbad, Texas (photograph taken November 6, 2009, by Patrick 

Boyd, U.S. Geological Survey). 
Analysis of Trends in Selected Streamflow 

Statistics for the Concho River Basin, 

Texas, 1916–2009

By Dana L. Barbie, Loren L. Wehmeyer, and Jayne E. May

Prepared in cooperation with the Texas Water Development Board

Scientific Investigations Report 2012–5193

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Barbie, D.L., Wehmeyer, L.L., and May, J.E., 2012, Analysis of trends in selected streamflow statistics for the Concho 

River Basin, Texas, 1916–2009: U.S. Geological Survey Scientific Investigations Report 2012–5193, 15 p. 
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................3

Concho River Basin Description .......................................................................................................3

Methods ..........................................................................................................................................................4

Analysis of Trends in Selected Streamflow Statistics ............................................................................4

South Concho River Basin ..................................................................................................................4

Middle Concho River Basin ................................................................................................................6

North Concho River Basin ..................................................................................................................6

Lower Concho River Basin .................................................................................................................6

Concho River at San Angelo ..............................................................................................................8

Concho River at Paint Rock, Texas ...................................................................................................8

Summary .......................................................................................................................................................14

References Cited .........................................................................................................................................15

Figures

1. Map showing selected U.S. Geological Survey surface-water monitoring 

stations in the Concho River Basin, Texas ..............................................................................2

2. Graph showing annual mean daily discharge for U.S. Geological Survey 

surface-water monitoring stations 08128000 South Concho River near Christoval, 

Texas and 08127500 South Concho Irrigation Canal at Christoval, Tex., along with 

the estimated combined annual mean daily discharge ........................................................5

3. Graph showing annual mean daily discharge for U.S. Geological Survey 

streamflow-gaging stations 08128400 Middle Concho River above Tankersley, 

Texas, and 08128500 Middle Concho River near Tankersley, Tex. .......................................7

4. Graph showing annual mean daily discharge for U.S. Geological Survey 

streamflow-gaging station 08134000 North Concho River near Carlsbad, Texas .............9

5. Graph showing annual mean daily discharge for U.S. Geological Survey 

streamflow-gaging station 08136000 Concho River at San Angelo, Texas ......................11

6. Graph showing annual mean daily discharge for U.S. Geological Survey 

streamflow-gaging station 08136500 Concho River at Paint Rock, Texas ........................13

Tables

1. Selected U.S. Geological Survey surface-water monitoring stations in the 

Concho River Basin, Texas, 1915–2009 ....................................................................................3

2. Results of Kendall’s Tau statistical test for detection of monotonic trends in 

select streamflow statistics for U.S. Geological Survey streamflow-gaging 

station 08128000 South Concho River at Christoval, Texas ...................................................6

3. Results of Kendall’s Tau statistical test for detection of monotonic trends in 

select streamflow statistics for A, U.S. Geological Survey streamflow-gaging 

station 08128400 Middle Concho River above Tankersley, Texas, and B, U.S. 

Geological Survey streamflow-gaging station 08128500 Middle Concho River 

near Tankersley, Tex. ...................................................................................................................8
iv

4. Results of Kendall’s Tau statistical test for detection of monotonic trends in 

select streamflow statistics for U.S. Geological Survey streamflow-gaging 

station 08134000 North Concho River near Carlsbad, Texas ..............................................10

5. Results of Kendall’s Tau statistical test for detection of monotonic trends in 

select streamflow statistics for U.S. Geological Survey streamflow-gaging 

station 08136000 Concho River at San Angelo, Texas .........................................................12

6. Results of Kendall’s Tau statistical test for detection of monotonic trends in 

select streamflow statistics for U.S. Geological Survey streamflow-gaging 

station 08136500 Concho River at Paint Rock, Texas ...........................................................14

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 
Analysis of Trends in Selected Streamflow Statistics for 

the Concho River Basin, Texas, 1916–2009

By Dana L. Barbie, Loren L. Wehmeyer, and Jayne E. May

Abstract

The Concho River Basin is part of the upper Colorado 

River Basin in west-central Texas. Monotonic trends in 

streamflow statistics during various time intervals from 

1916–2009 were analyzed to determine whether substantial 

changes in selected streamflow statistics have occurred within 

the Concho River Basin. Two types of U.S. Geological Survey 

streamflow data comprise the foundational data for this report: 

(1) daily mean discharge (daily discharge) and (2) annual 

instantaneous peak discharge. Trend directions are reported 

for the following streamflow statistics: (1) annual mean daily 

discharge, (2) annual 1-day minimum discharge, (3) annual 

7-day minimum discharge, (4) annual maximum daily 

discharge, and (5) annual instantaneous peak discharge.

The South Concho, Middle Concho, and North Concho 

Rivers drain the upper part of the Concho River Basin. The 

North and South Concho Rivers converge in San Angelo, Tex., 

to form the Concho River. The Concho River flows east from 

San Angelo to its confluence with the Colorado River east 

of Paint Rock, Tex. The trend analyses principally focused 

on application of the nonparametric Kendall’s Tau statistical 

test to detect monotonic trends (dependency) in streamflow 

with time; in other words, Kendall’s Tau is a test of temporal 

independence of streamflow with time. A positive Tau 

indicates an upward monotonic streamflow trend; conversely, 

a negative Tau indicates a downward monotonic streamflow 

trend. Hence, the trend analysis reported here is limited to 

direction and not magnitude of streamflow change. 

Six U.S. Geological Survey streamflow-gaging stations 

were selected for analysis. Streamflow-gaging station 

08128000 South Concho River at Christoval has downward 

trends for annual maximum daily discharge and annual 

instantaneous peak discharge for the combined period 

1931–95, 2002–9. Streamflow-gaging station 08128400 

Middle Concho River above Tankersley has downward trends 

for annual maximum daily discharge and annual instantaneous 

peak discharge for the combined period 1962–95, 2002–9. 

Streamflow-gaging station 08128500 Middle Concho River 

near Tankersley has no significant trends in the streamflow 

statistics considered for the period 1931–60. Streamflowgaging station 08134000 North Concho River near Carlsbad 

has downward trends for annual mean daily discharge, annual 

7-day minimum daily discharge, annual maximum daily 

discharge, and annual instantaneous peak discharge for the 

period 1925–2009. Streamflow-gaging stations 08136000 

Concho River at San Angelo and 08136500 Concho River 

at Paint Rock have downward trends for 1916–2009 for all 

streamflow statistics calculated, but streamflow-gaging station 

08136000 Concho River at San Angelo has an upward trend 

for annual maximum daily discharge during 1964–2009. The 

downward trends detected during 1916–2009 for the Concho 

River at San Angelo are not unexpected because of three 

reservoirs impounding and profoundly regulating streamflow.

Introduction

The Concho River Basin is part of the upper Colorado 

River Basin in west-central Texas, principally in Tom 

Green, Irion, Reagan, and Sterling Counties and to a lesser 

degree in Coke, Concho, Crockett, Glasscock, Howard, 

Midland, Runnels, Schleicher, and Upton Counties (fig. 1). 

The population of the Concho River Basin is approximately 

120,000, and most of the residents (approximately 93,200 in 

2010) live in San Angelo (fig. 1) (U.S. Census Bureau, 2012). 

The major water demands (withdrawals) in the basin are for 

public (municipal) supply, ranching, irrigated agriculture, and 

oil and gas production. Public water supply in San Angelo 

historically has been provided by streamflow stored in 

O.H. Ivie and Twin Buttes Reservoirs, Lakes Nasworthy 

and E.V. Spence, and O.C. Fisher Lake (fig. 1). Although 

streamflow supplies some water for irrigation, most 

agriculture land is irrigated with groundwater. Tom Green 

County has the largest area under irrigation (about 40,000 

acres in 2007) (Region F Water Planning Group, 2009). 

Groundwater is the predominant water source for the 

rural population, ranching, and oil and gas production in 

the study area. Because the Concho River Basin provides 

water stored for public water supply for most residents in 

the study area, trends in streamflow in the Concho River 

Basin are of interest to water managers in this part of west 

Texas. Accordingly, the U.S. Geological Survey (USGS), 

in cooperation with the Texas Water Development Board, 
Figure 1. Selected U.S. Geological Survey surface-water monitoring stations in the Concho River Basin, Texas. 

IRION COUNTY

UPTON COUNTY

COKE COUNTY

CROCKETT COUNTY

REAGAN COUNTY

TOM GREEN

COUNTY

RUNNELS COUNTY

CONCHO COU

STERLING COUNTY

MIDLAND

COUNTY

SCHLEICHER

COUNTY

MENARD COU

ECTOR

COUNTY

CRANE

COUNTY

GLASSCOCK COUNTY

NOLAN COUNTY TAYLOR COUNTY

MARTIN

COUNTY 

MITCHELL COUNTY

HOWARD

COUNTY ANDREWS COUNTY 

PECOS COUNTY

Midland

San

Angelo

Odessa

Tankersley

Christoval

Carlsbad

Paint Rock

Twin ButtesReservoir 

O C Fisher Lake

Lake

Nasworthy 

Lake

E V Spence 

Oak Creek

Reservoir

Colorado River

Concho River

North Concho River

Middle Concho River

Spring Creek

Dove

Creek

So

u

th

Concho

Rive

r

EXPLANATION

Concho River BasinStream Perennial IntermittentU.S. Geological Survey surface-water monitoring station and number—Table 1

#

08127500

Study a

TEXA

102° 101° 100°

32°31°

£¤277

£¤277

£¤87

£¤385

§ ̈¦20

 ̈¦10 §

£¤87 £¤67

£¤67

Base from U.S. Geological Survey, The National Atlas of the United States, 1:2,000,000 Texas Centric Mapping System Albers Equal AreaNorth American Datum of 1983

0 20 40 MILES

0 20 40 KILOMETERS

08136500

08136000

08134000

08128500

08128400

08128000 08127500

#

#

#

#

#

#
Introduction 3

evaluated trends during 1916–2009 in selected streamflow 

statistics for the Concho River Basin. Six USGS streamflowgaging stations were selected for analysis: 08128000 South 

Concho River at Christoval, Tex.; 08128400 Middle Concho 

River above Tankersley, Tex.; 08128500 Middle Concho 

River near Tankersley, Tex.; 08134000 North Concho River 

near Carlsbad, Tex.; 08136000 Concho River at San Angelo, 

Tex.; and 08136500 Concho River at Paint Rock, Tex. 

(fig. 1, table 1). In addition to the six streamflow-gaging 

stations listed in table 1, a USGS surface-water monitoring 

station used to monitor discharge in a canal is listed (station 

08127500 South Concho Irrigation Company Canal at 

Christoval, Tex.). The use of the discharge data for this 

station and how it interacts with the analyses presented in 

this report is described in the section “Analysis of Trends 

in Selected Streamflow Statistics” pertaining to the South 

Concho River Basin.

The USGS has published statistical and historical 

summaries of streamflow data in Texas with great emphasis 

on visualization of streamflow data and statistics (Asquith 

and others, 2007a, b; Asquith and Heitmuller, 2008). These 

previous reports include similar and complementary statistical 

and graphical presentations of streamflow data for the six 

streamflow-gaging stations analyzed in this report. Lastly, 

the periods of annual instantaneous peak discharge for the 

six streamflow-gaging stations contribute to the regression 

equations by Asquith and Roussel (2009) that estimate peakstreamflow frequency in Texas.

Purpose and Scope

Monotonic trends in selected streamflow statistics in the 

Concho River Basin were evaluated for using a nonparametric 

test (Kendall’s Tau) at selected USGS streamflow-gaging 

stations. Two types of USGS streamflow data comprise the 

foundational data for this report: (1) daily mean discharge 

(daily discharge) and (2) annual instantaneous peak discharge. 

Trends are reported for the following streamflow statistics: 

(1) annual mean daily discharge, (2) annual 1-day minimum 

discharge, (3) annual 7-day minimum discharge, (4) annual 

maximum daily discharge, and (5) annual instantaneous 

peak discharge. The scope of this report does not include 

an analysis of how possible changes in precipitation, land 

use, or groundwater withdrawals might have affected 

streamflow trends.

Concho River Basin Description

The South Concho, Middle Concho, and North Concho 

Rivers drain the upper part of the Concho River Basin. The 

Concho River and its tributaries generally flow from west 

to east with the exception of the South Concho River, which 

flows from south to north (fig. 1). Middle and South Concho 

Rivers join at Twin Buttes Reservoir (fig. 1) and continue 

downstream as the South Concho River. The Concho River 

is formed by the confluence of the South Concho and North 

Concho Rivers in San Angelo. From where it rises in San 

Angelo, the Concho River flows east about 60 miles (mi) to 

its confluence with the Colorado River, east of Paint Rock 

(fig. 1). Although the published drainage area of the Concho 

River Basin is about 6,765 square miles (mi2) (U.S. Geological 

Survey, 2012), approximately 1,130 mi2 of the basin are 

characterized by playas or other natural depressions that do not 

contribute runoff to the Concho River (Tovar and Maldonado, 

1981). The contributing drainage area of the Concho River is 

about 5,634 mi2 (Tovar and Maldonado, 1981).

Table 1. Selected U.S. Geological Survey surface-water monitoring stations in the Concho River Basin, Texas, 1915–2009.

[USGS, U.S. Geological Survey; mi2, square miles; NA, not applicable]

USGS 

station 

number

USGS station name

Gaging 

station 

type

Total 

drainage 

area 

(mi2

)

Contributing 

drainage 

area 

(mi2

)

Period of 

continuous daily 

mean discharge 

record

Water 

years 

statistically 

evaluated

08127500 South Concho Irrigation Company Canal at Christoval, Tex. canal NA NA Dec. 1939–Oct. 1983 none

08128000 South Concho River at Christoval, Tex. stream 413 354 Mar. 1930–Sept. 1995; 

May 2001–Sept. 2009

1931–95, 

2002–9

08128400 Middle Concho River above Tankersley, Tex. stream 2,084 1,116 Apr. 1961–Sept. 1995; 

Apr. 2001–Sept. 2009

1962–95, 

2002–9

08128500 Middle Concho River near Tankersley, Tex. stream 2,653 1,685 Mar. 1930–Mar. 1961 1931–60

08134000 North Concho River near Carlsbad, Tex. stream 1,266 1,191 Apr. 1924–Sept. 2009 1925–2009

08136000 Concho River at San Angelo, Tex. stream 5,542 4,411 Oct. 1915–Sept. 2009 1916–2009

08136500 Concho River at Paint Rock, Tex. stream 6,572 5,443 Oct. 1915–Sept. 2009 1916–2009
4 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

The USGS has collected streamflow data in the 

Concho River Basin since October 1915. Since then and at 

various time intervals, up to 24 daily surface water stations 

(19 streamflow-gaging stations, 3 lake or reservoir stations, 

and 2 canal stations) have collected data in the basin through 

the present (2012). Two streamflow-gaging stations (the oldest 

in the basin) were installed in 1915 (Concho River at San 

Angelo and Concho River at Paint Rock), one was installed 

in 1924 (North Concho River near Carlsbad), and two were 

installed in 1930 (South Concho River at Christoval and 

Middle Concho River near Tankersley). The canal gage was 

installed in 1939 and discontinued in 1983. The South Concho 

River at Christoval, Middle Concho River near Tankersley, 

and Middle Concho River above Tankersley streamflowgaging stations are upstream from Twin Buttes Reservoir 

and Lake Nasworthy. The North Concho River near Carlsbad 

streamflow-gaging station is upstream from O.C. Fisher Lake. 

The Concho River at San Angelo and Concho River at Paint 

Rock streamflow-gaging stations are downstream from the 

three identified impoundments.

Methods

Trend analysis used in this study is based on application 

of the nonparametric Kendall’s Tau statistical test to detect 

monotonic trends in streamflow with time (Helsel and 

Hirsch, 2002). A positive Tau indicates an upward monotonic 

streamflow trend; conversely, a negative Tau indicates a 

downward monotonic streamflow trend. The p-value of 

Kendall’s Tau is a measure of the statistical significance of 

the trend. For this study, a p-value of less than 0.05 indicates a 

significant trend, whereas a p-value of 0.05 or greater indicates 

no significant trend. The trend analysis reported here is limited 

to direction and not magnitude of streamflow change. 

Annual mean daily discharge is computed as the average 

of daily mean discharges during a water year (a water 

year is defined as the 12-month period from October 1 to 

September 30). The annual 1-day minimum discharge is the 

discharge on the day of lowest daily mean discharge in a given 

water year. Similarly, the annual 7-day minimum discharge is 

the lowest daily mean discharge during 7 consecutive days in a 

given water year. The annual maximum daily discharge is the 

discharge on the day of highest annual daily mean discharge in 

a given water year. The annual instantaneous peak discharge 

is the discharge associated with the highest instantaneous 

streamflow computed in a given water year; this statistic 

is fundamentally different from the other four streamflow 

statistics because there is no time averaging involved. 

The date of the annual instantaneous peak discharge is not 

constrained to occur on the same day as the annual maximum 

daily discharge. To conduct the trend analysis, the software 

package Statistica (StatSoft, 2009) was used to perform the 

Kendall’s Tau test. For each of the six streamflow-gaging 

stations, Kendall’s Tau statistics were computed for the five 

streamflow statistics; the precise period of records evaluated 

by streamflow-gaging station are listed in table 1.

There is an important limitation of the Kendall’s Tau 

statistical test when applied to streamflow time series data 

that might contain a substantial number of years with no 

flow for annual 1-day (or 7-day) minimum discharge. The 

Tau and the attendant p-value become highly unreliable and 

direct interpretation of their numerical meaning can become 

tenuous because of ties. In many nonparametric statistical 

tests, considerable accommodation of ties in the data set can 

be made but are only approximations. For this study, the trend 

analysis of annual 1-day and 7-day minimum daily discharge 

for streamflow-gaging stations 08128400 Middle Concho 

River above Tankersley and 08128500 Middle Concho River 

near Tankersley yield Tau and p-values that are difficult to 

interpret and hence specially noted when the trends analysis is 

presented in tabular form.

Analysis of Trends in Selected 

Streamflow Statistics 

South Concho River Basin

The headwaters of the South Concho River are in 

north Schleicher County (fig. 1) approximately 40 mi south 

of San Angelo. The river flows north into Twin Buttes 

Reservoir. The South Concho Irrigation Company Canal 

diverts water upstream from the South Concho River at 

Christoval streamflow-gaging station and returns water to 

the river downstream from the streamflow-gaging station. 

The canal gaging station was operated from December 1939 

through October 1983 (table 1) and the South Concho River 

at Christoval streamflow-gaging station was operated from 

March 1930 through September 1995 and reactivated from 

May 2001 to the present (2012). The return discharge from 

the canal into the river is less than the discharge diverted into 

the canal upstream because of withdrawals for irrigation and 

evaporation losses (Sauer, 1972). The sum of the discharge in 

the canal and the discharge at the nearby streamflow-gaging 

station can be used as an estimate for the total South Concho 

River discharge during the period of mutual operation. The 

three time series of annual mean discharges shown in figure 2 

indicate that the estimated combined annual mean streamflow 

does not materially vary or diverge from patterns seen in the 

South Concho River at Christoval annual mean streamflow. 

Hence, for this report, the five statistics of South Concho 

River at Christoval streamflow were evaluated for monotonic 

trends, and a benefit of this decision is that a longer period 

of record (blue line in fig. 2) is available for analysis without 

inclusion of the streamflow in the canal.

For the South Concho River at Christoval streamflowgaging station, statistically significant downward trends 
Figure 2. Annual mean daily discharge for U.S. Geological Survey surface-water monitoring stations 08128000 South Concho River near Christoval, T

Concho Irrigation Canal at Christoval, Tex., along with the estimated combined annual mean daily discharge. 

1930

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

Water year

0

20406080

100120140160180200220

Annual mean daily discharge, in cubic feet per second No data, 

1996–2001

08127500 South Concho Irrigation Canal at Christoval, Texas08128000 South Concho River at Christoval, TexasEstimated combined annual mean daily discharge

EXPLANATION
6 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

in streamflow for the combined period 1931–95 and 

2002–9 were detected for annual maximum daily discharge 

and annual instantaneous peak discharge (table 2). No 

statistically significant trends were detected for the other three 

streamflow statistics.

Middle Concho River Basin

The headwaters of the Middle Concho River are 

approximately 90 mi west of San Angelo. The river flows 

east into Twin Buttes Reservoir (fig. 1). The two Middle 

Concho River streamflow-gaging stations were not operated 

concurrently (table 1), visually indicated by the plotted 

annual mean daily discharges for the streamflow-gaging 

stations shown in figure 3. The upstream streamflow-gaging 

station 08128400 Middle Concho River above Tankersley, 

Tex., (drainage area 1,116 mi2) was a replacement for 

streamflow-gaging station 08128500 Middle Concho River 

near Tankersley, Tex. (drainage area 1,685 mi2). The Middle 

Concho River near Tankersley streamflow-gaging station was 

discontinued on March 31, 1961, in order to accommodate the 

construction of Twin Buttes Reservoir. The Middle Concho 

River above Tankersley streamflow-gaging station began 

operating April 1, 1961. This streamflow-gaging station 

transition occurred at the beginning of a multi-year period of 

unusually low streamflow. For example in 1962, the annual 

mean discharge was no flow (fig. 3).

For the Middle Concho River above Tankersley 

streamflow-gaging station, statistically significant downward 

trends in streamflow for the combined period 1962–95 and 

2002–9 were detected for annual maximum daily discharge 

and annual instantaneous peak discharge at (table 3a). No 

statistically significant trends were detected for the other 

three streamflow statistics. For the Middle Concho River 

near Tankersley streamflow-gaging station, no statistically 

significant trends were detected for all five streamflow 

statistics for the period 1931–60 (table 3b).

North Concho River Basin

The headwaters of the North Concho River are 

approximately 70 mi northwest of San Angelo. The North 

Concho River flows from northwest to southeast into O.C. 

Fisher Lake (fig. 1). The North Concho River near Carlsbad 

streamflow-gaging station is a particularly important sentinel 

for streamflow conditions for much of the greater Concho 

River Basin. Specifically, the streamflow at this streamflowgaging station is considered natural to the extent that the 

upstream watershed has effectively no diversions or regulation 

during the 85 years of record (April 1924–September 2009). 

The annual mean discharge for the North Concho River near 

Carlsbad is shown in figure 4.

For the North Concho River near Carlsbad streamflowgaging station, statistically significant downward trends for 

the period 1925–2009 were detected for annual mean daily 

discharge (fig. 4), annual 7-day minimum discharge, annual 

maximum daily discharge, and annual instantaneous peak 

discharge (table 4).

Lower Concho River Basin

Three impoundments regulate streamflow in the Concho 

River Basin: Lake Nasworthy (since 1930), O.C. Fisher Lake 

(since 1952), and Twin Buttes Reservoir (since 1963) (Dowell, 

1964; U.S. Geological Survey, 2012). These impoundments 

are upstream from the Concho River mainstem that originates 

just downstream from the reservoirs at San Angelo. These 

impoundments then substantially regulate low to high 

streamflows observed at the Concho River at San Angelo and 

Concho River at Paint Rock streamflow-gaging stations. Both 

O.C. Fisher Lake and Twin Buttes Reservoir did not initially 

yield the public water supply anticipated from the reservoir 

designs that were based on the existing streamflow records in 

Concho River Basin (Sauer, 1972). Since impoundment 

began in 1952, the only time the conservation pool level in 

Table 2. Results of Kendall’s Tau statistical test for detection of monotonic trends in select streamflow statistics for U.S. Geological 

Survey streamflow-gaging station 08128000 South Concho River at Christoval, Texas.

[p-value, probability value; <, less than]

Streamflow 

statistic

Interval 

analyzed

Kendall’s 

Tau p-value Trend

Annual mean daily discharge 1931–95, 2002–9 -0.07 0.40 not significant

Annual 1-day minimum discharge 1931–95, 2002–9 0.14 0.08 not significant

Annual 7-day minimum discharge 1931–95, 2002–9 0.12 0.13 not significant

Annual maximum daily discharge 1931–95, 2002–9 -0.19 <0.05 downward

Annual instantaneous peak discharge 1931–95, 2002–9 -0.22 <0.05 downward
Figure 3. Annual mean daily discharge for U.S. Geological Survey streamflow-gaging stations 08128400 Middle Concho River above Tankersley, Texa

Concho River near Tankersley, Tex.

1930

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

Water year

0

20406080

100120140160

Annual mean daily discharge, in cubic feet per second

Downstream station, 1931–1960

Upstream station, 1962–1995

Upstream sta

No data, 

1996–2001

EXPLANATION

08128400 Middle Concho River above Tankersley, Texas08128500 Middle Concho River near Tankersley, Texas
8 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

O.C. Fisher Lake was attained occurred in 1957 (U.S. Army 

Corps of Engineers, 2005). Water levels in Twin Buttes 

Reservoir did not reach the conservation pool elevation 

until 1974 (approximately 11 years after completion) 

(Zuniga, 1999). Controlled releases, or lack thereof, from the 

impoundments have significantly reduced flood peaks at the 

Concho River streamflow-gaging stations. The two Concho 

River streamflow-gaging stations were analyzed for their 

period of record (1916–2009) and for the period in which of 

all three impoundments provided regulation (1964–2009).

Concho River at San Angelo

The Concho River at San Angelo streamflow-gaging 

station is 0.4 mi downstream from the confluence of the North 

and South Concho Rivers in the eastern part of the city of San 

Angelo. The streamflow-gaging station is approximately 6 

mi downstream from Lake Nasworthy, approximately 5 mi 

downstream from O.C. Fisher Lake, and approximately 60 mi 

upstream from the mouth (fig. 1). The annual mean discharge 

for the Concho River at San Angelo is shown in figure 5.

For the Concho River at San Angelo streamflow-gaging 

station, statistically significant downward trends for the 

period 1916–2009 were detected for all five streamflow 

statistics (table 5). No statistically significant trends for the 

period 1964–2009 were detected except that annual maximum 

daily discharge has an upward trend (table 5). The downward 

trends detected for the period 1916–2009 are not unexpected 

because of three reservoirs impounding and controlling 

streamflow.

Concho River at Paint Rock, Texas

The Concho River at Paint Rock streamflow-gaging 

station is 20 mi upstream from the mouth of Concho River 

(fig. 1) and is approximately 42 mi downstream from San 

Angelo. The streamflow-gaging station represents the terminal 

streamflow data collection point in the Concho River Basin. 

The annual mean daily discharge for the Concho River at Paint 

Rock is shown in figure 6.

For the Concho River at Paint Rock streamflow-gaging 

station, statistically significant downward trends for the period 

1916–2009 were detected for all five streamflow statistics 

(table 6). No statistically significant trends were detected 

for the period 1964–2009 for all five statistics (table 6). The 

downward trends detected for the period 1916–2009 are 

not unexpected because of three reservoirs impounding and 

controlling streamflow.

Table 3. Results of Kendall’s Tau statistical test for detection of monotonic trends in select streamflow statistics for A, U.S. Geological 

Survey streamflow-gaging station 08128400 Middle Concho River above Tankersley, Texas, and B, U.S. Geological Survey streamflowgaging station 08128500 Middle Concho River near Tankersley, Tex.

[USGS, U.S. Geological Survey; p-value, probability value; <, less than]

A, USGS streamflow-gaging station 08128400 Middle Concho River above Tankersley, Tex.

Streamflow 

statistic

Interval 

analyzed

Kendall’s 

Tau p-value Trend

Annual mean daily discharge 1962–95, 2002–9 -0.17 0.10 not significant

Annual 1-day minimum discharge 1962–95, 2002–9 1

0.75 0.2 not significant

Annual 7-day minimum discharge 1962–95, 2002–9 1

0.67 0.1 not significant

Annual maximum daily discharge 1962–95, 2002–9 -0.25 <0.05 downward

Annual instantaneous peak discharge 1962–95, 2002–9 -0.24 <0.05 downward

B, USGS streamflow-gaging station 08128500 Middle Concho River near Tankersley, Tex.

Annual mean daily discharge 1931–60 0.10 0.44 not significant

Annual 1-day minimum discharge 1931–60 0.04 0.5 not significant

Annual 7-day minimum discharge 1931–60 0.04 0.5 not significant

Annual maximum daily discharge 1931–60 0.10 0.45 not significant

Annual instantaneous peak discharge 1931–60 0.07 0.57 not significant

1

The numerical values for Kendall’s Tau and attendant p-value are considered approximate and difficult to interpret because of substantial number of zero flow 

values for the annual 1-day and 7-day minimum discharges.
Figure 4. Annual mean daily discharge for U.S. Geological Survey streamflow-gaging station 08134000 North Concho River near Carlsbad, Texas. 

Water year

1925

1927

1929

1931

1933

1935

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

Annual mean daily discharge, in cubic feet per second

0

50

100150200250300350400
10 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

Table 4. Results of Kendall’s Tau statistical test for detection of monotonic trends in select streamflow statistics for U.S. Geological 

Survey streamflow-gaging station 08134000 North Concho River near Carlsbad, Texas.

[p-value, probability value; <, less than]

Streamflow 

statistic

Interval 

analyzed

Kendall’s 

Tau p-value Trend

Annual mean daily discharge 1925–2009 -0.39 <0.05 downward

Annual 1-day minimum discharge 1925–2009 0.14 0.06 not significant

Annual 7-day minimum discharge 1925–2009 -0.17 <0.05 downward

Annual maximum daily discharge 1925–2009 -0.35 <0.05 downward

Annual instantaneous peak discharge 1925–2009 -0.36 <0.05 downward
Figure 5. Annual mean daily discharge for U.S. Geological Survey streamflow-gaging station 08136000 Concho River at San Angelo, Texas. Water year

1916

1918

1920

1922

1924

1926

1928

1930

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

Annual mean daily discharge, in cubic feet per second

0

200400600800

1,0001,200

Lake Nasworthy

impoundment 

O C Fisher Lake

impoundment 

Twin Buttes Reservoir

impoundment 

Unregulated flow
12 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

Table 5. Results of Kendall’s Tau statistical test for detection of monotonic trends in select streamflow statistics for U.S. Geological 

Survey streamflow-gaging station 08136000 Concho River at San Angelo, Texas.

[p-value, probability value; <, less than]

Streamflow 

statistic

Interval 

analyzed

Kendall’s 

Tau p-value Trend

Annual mean daily discharge 1916–2009 -0.41 <0.05 downward

Annual 1-day minimum discharge 1916–2009 -0.35 <0.05 downward

Annual 7-day minimum discharge 1916–2009 -0.25 <0.05 downward

Annual maximum daily discharge 1916–2009 -0.37 <0.05 downward

Annual instantaneous peak discharge 1916–2009 -0.42 <0.05 downward

Annual mean daily discharge 1964–2009 0.20 0.05 not significant

Annual 1-day minimum discharge 1964–2009 -0.13 0.20 not significant

Annual 7-day minimum discharge 1964–2009 -0.10 0.34 not significant

Annual maximum daily discharge 1964–2009 0.24 <0.05 upward

Annual instantaneous peak discharge 1964–2009 0.06 0.54 not significant
Figure 6. Annual mean daily discharge for U.S. Geological Survey streamflow-gaging station 08136500 Concho River at Paint Rock, Texas. Water year

1916

1918

1920

1922

1924

1926

1928

1930

1932

1934

1936

1938

1940

1942

1944

1946

1948

1950

1952

1954

1956

1958

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

Annual mean daily discharge, in cubic feet per second

0

200400600800

1,0001,2001,4001,600

Lake Nasworthy

impoundment 

Twin Buttes Reservoir

impoundment 

Unregulated flow

O C Fisher Lake

impoundment 
14 Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009

Summary

The Concho River Basin is part of the upper Colorado 

River Basin in west-central Texas. Streamflow (surface 

water) supplies some irrigation water and the majority of 

the public water supply. Public water supply historically has 

been provided by streamflow stored in O.H. Ivie and Twin 

Buttes Reservoirs, Lakes Nasworthy and E.V. Spence, and 

O.C. Fisher Lake. Because the Concho River Basin provides 

water stored for public water supply for most residents in 

the study area, trends in streamflow in the Concho River 

Basin are of interest to water managers in this part of west 

Texas. Accordingly, the U.S. Geological Survey (USGS), 

in cooperation with the Texas Water Development Board, 

evaluated trends during 1916–2009 in selected streamflow 

statistics for the Concho River Basin. Monotonic trends 

in streamflow were evaluated using a nonparametric test 

(Kendall’s Tau) to evaluate whether statistically significant 

changes in the selected streamflow statistics have occurred 

over time within the Concho River Basin at six USGS 

streamflow-gaging stations: 08128000 South Concho 

River at Christoval, Tex.; 08128400 Middle Concho River 

above Tankersley, Tex.; 08128500 Middle Concho River 

near Tankersley, Tex.; 08134000 North Concho River near 

Carlsbad, Tex.; 08136000 Concho River at San Angelo, Tex.; 

and 08136500 Concho River at Paint Rock, Tex. Two types 

of USGS streamflow data comprise the foundational data for 

this report: (1) daily mean discharge (daily discharge) and 

(2) annual instantaneous peak discharge. Trends are reported 

for the following streamflow statistics: (1) annual mean daily 

discharge, (2) annual 1-day minimum discharge, (3) annual 

7-day minimum discharge, (4) annual maximum daily 

discharge, and (5) annual instantaneous peak discharge.

The USGS has collected streamflow data in the Concho 

River Basin since October 1915. Since then and at various 

time intervals, as many as 24 daily surface-water monitoring 

stations (19 streamflow-gaging stations, 3 lake or reservoir 

stations, and 2 canal stations) have been used to collect data 

in the basin through the present (2012). Trend analyses used 

in this study are based on application of the nonparametric 

Kendall’s Tau statistical test to detect monotonic trends in 

streamflow with time. A positive Tau indicates an upward 

monotonic streamflow trend; conversely, a negative Tau 

indicates a downward monotonic streamflow trend. The 

p-value of Kendall’s Tau is a measure of the statistical 

significance of the trend. For this study, a p-value of less than 

0.05 indicates a significant trend, whereas a p-value of 0.05 or 

greater indicates no significant trend.

For USGS streamflow-gaging station 08128000 South 

Concho River at Christoval, Tex., statistically significant 

downward trends in streamflow for the combined period 

1931–95 and 2002–9 were detected for annual maximum 

daily discharge and annual instantaneous peak discharge. 

For USGS streamflow-gaging station 08128400 Middle 

Concho River above Tankersley, Tex., statistically significant 

downward trends in streamflow for the combined period 

1962–95 and 2002–9 were detected for annual maximum 

daily discharge and annual instantaneous peak discharge. For 

USGS streamflow-gaging station 08128500 Middle Concho 

River near Tankersley, Tex., no statistically significant trends 

were detected for all five streamflow statistics for the period 

Table 6. Results of Kendall’s Tau statistical test for detection of monotonic trends in select streamflow statistics for U.S. Geological 

Survey streamflow-gaging station 08136500 Concho River at Paint Rock, Texas.

[p-value, probability value; <, less than]

Streamflow 

statistic

Interval 

analyzed

Kendall’s 

Tau p-value Trend

Annual mean daily discharge 1916–2009 -0.37 <0.05 downward

Annual 1-day minimum discharge 1916–2009 0.14 <0.05 downward

Annual 7-day minimum discharge 1916–2009 0.14 <0.05 downward

Annual maximum daily discharge 1916–2009 -0.41 <0.05 downward

Annual instantaneous peak discharge 1916–2009 -0.43 <0.05 downward

Annual mean daily discharge 1964–2009 -0.03 0.77 not significant

Annual 1-day minimum discharge 1964–2009 -0.12 0.22 not significant

Annual 7-day minimum discharge 1964–2009 -0.07 0.45 not significant

Annual maximum daily discharge 1964–2009 -0.03 0.75 not significant

Annual instantaneous peak discharge 1964–2009 -0.06 0.58 not significant
References Cited 15

1931–60. For USGS streamflow-gaging station 08134000 

North Concho River near Carlsbad, Tex., statistically 

significant downward trends for the period 1925–2009 were 

detected for annual mean daily discharge, annual 7-day 

minimum discharge, annual maximum daily discharge, and 

annual instantaneous peak discharge. For USGS streamflowgaging station 08136000 Concho River at San Angelo, Tex., 

statistically significant downward trends for the period 

1916–2009 were detected for all five streamflow statistics. No 

statistically significant trends for the period 1964–2009 were 

detected except that annual maximum daily discharge has an 

upward trend. For USGS streamflow-gaging station 08136500 

Concho River at Paint Rock, Tex., statistically significant 

downward trends for the period 1916–2009 were detected for 

all five streamflow statistics. No statistically significant trends 

for the period 1964–2009 were detected. The downward 

trends detected for the period 1916–2009 are not unexpected 

because of three reservoirs impounding and controlling 

streamflow.

References Cited

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Summary of percentages of zero daily mean streamflow 

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in Texas through 2003: U.S. Geological Survey Data 

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Geological Survey streamflow-gaging stations in Texas 

through 2003: U.S. Geological Survey Data Series 248, 

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Publishing support provided by

Lafayette and Menlo Publishing Service Centers

Information regarding water resources in Texas is available at 

http://tx.usgs.gov/
 
Barbie, Wehmeyer, and May—Analysis of Trends in Selected Streamflow Statistics for the Concho River Basin, Texas, 1916–2009—SIR 2012–5193

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