1

TRINITY

RIVER

WEST

FORK SAN

JACINTO RIVER

BR AZOS

R IVER

TRINITY

 BAY

GALVESTON BAY LAKE

 CONROE

LAKE HOUSTON

GULF OF MEXICO

GALVESTON

CONROE

LIBERTY

HOUSTON

LIBERTY

HARRIS

FORT BEND

CHAMBERS

BRAZORIA

GALVESTON

US 290

I10

US 59

US

90

I10

I45

State 6

State 288

US

59

State 36

State 65

105

State

I45

o

o

94 30’

o 94 45’

o 95

95 15’ o

95 30’

o 95 45’

o 30 30’

o 30 15’

o 30

o

o

o 96

o 29 45’

29 30’

29 15’

o 29

15

1

2

3

6

11

9 8

7

13

17

12

16

14

5

4

10

103

102

107

101

110

104

108

105

106

109

0 10 20 30 MILES

MONTGOMERY

, 

station data can be used to estimate discharge characteristics 

at ungaged sites. Regionalization techniques such as regression analyses relate 

discharge-frequency characteristics to 

selected physical and climatic characteristics of drainage basins. A particular 

discharge-frequency characteristic that 

can be regionalized is the minimum 

7-day, 2-year discharge1 (7Q2). In Texas, 

the 7Q2 is used at stream sites to analyze 

permit applications for water allocation, 

water-supply planning, aquatic maintenance (instream flow) requirements, 

and waste-load allocation for point and 

nonpoint source discharges.

In 1997 and 1998 the USGS, in cooperation with the City of Houston and the 

Houston-Galveston Area Council, conducted a study to estimate low-flow 

discharge characteristics for selected 

streams in southeast Texas. The objectives of this study were to estimate the 

7Q2 for 17 ungaged stream sites (fig. 1) 

where such data were unavailable and 

to summarize selected water-quality 

properties and constituents at each of 

the ungaged sites during low-flow 

conditions. Discharge measurements 

were made and water-quality data were 

collected using USGS methods as 

described by Buchanan and Somers 

(1969), Rantz and others (1982), and 

Wells and others (1990).

This fact sheet presents estimated 

7Q2 for the 17 ungaged sites and statistical summaries of four water-quality 

properties and three water-quality 

 1 Annual lowest mean discharge for 7 

consecutive days with a 2-year recurrence 

interval.

g

gaging stations to estimate the 7Q2 for the ungaged s

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S

Figure 1. Locations of ungaged stream sites and index stations in or near the 

metropolitan area, southeast Texas.

TEXA

Study 

LOCATIO

5 Ungaged stream site—Number is 

referenced in table 1

EXPLANATION

Index station—Number is referenced 

in table 2

101
2

Estimation of Minimum 7-Day, 2-Year Discharge

One technique to estimate low-flow discharge characteristics 

(such as 7Q2) in a region uses data from one or more index stations (Riggs, 1972; Stedinger and Thomas, 1985). These typically 

are USGS stations with a continuous record of discharge for a 

length of time sufficient (minimum 8 years) to provide reliable 

low-flow discharge characteristics. For an index station to be used 

in the analysis, the low-flow data should not have any significant 

temporal trends, which would skew the frequency characteristics. 

To test for a trend in the data, the Mann-Kendall test (Helsel and 

Hirsch, 1992) was used to determine the significance of Kendall’s 

tau. In this application, a Kendall’s tau p-value less than or equal 

to 0.10 indicated a statistically significant trend in the data. If a 

trend was detected in the data set, the record was truncated to a 

shorter period without a trend.

Initially, the 7-day minimum flow was determined for each 

year the index station has been operated (or for the shorter period 

if the data set had a trend). The 7Q2 for the index station was 

determined by computing the median discharge for the station 

time series. The median is the 50th percentile and by 

is the discharge with a 2-year recurrence interval, on 

station data and assuming no specified statistical dist

the data set.

To transfer the 7Q2 from the index station to the u

a relation was developed between low-flow discharge 

station and at the ungaged site. Periodic discharge me

were made during various low-flow conditions at the 

sites where the 7Q2 was to be estimated. At least five 

measurements were made at each site. The low-flow 

measurements then were used with concurrent discha

determined at the appropriate index station(s) to deve

tion between low-flow discharge characteristics for th

site and for the index station(s). An example of such 

shown in figure 2.

In figure 2, discharge measured at an ungaged site 

Creek at FM 1485; site 4 in fig. 1, table 1) is plotted 

ordinate (y-axis), and daily mean discharge for an ind

(08070500 Caney Creek near Splendora, Tex.; station 

Figure 2. Relation between discharge at ungaged stream site and index station.

0.01 0.1 1 10 100 1,000 5,000

DAILY MEAN DISCHARGE IN CUBIC FEET PER SECOND

08070500 CANEY CREEK NEAR SPLENDORA, TEXAS (USGS INDEX STATION)

0.01

0.1

1

INSTANTANEOUS DISCHARG

PEACH CREEK AT FM 

EXPLANATION

Line of ordinary least squares 

Line of organic correlation

Concurrent discharge

at ungaged site and

index station

Computed minimum 7-day, 

2-year discharge (7Q2) at 

index station
3

This technique was used to estimate the 7Q2 for e

17 ungaged sites. If more than one index station was i

the analysis for a particular ungaged site, multiple 7Q

were determined. Multiple 7Q2 estimates for an unga

averaged to determine a final 7Q2 estimate. The final 

mate for each ungaged site is listed in table 1. The in

used for these analyses are listed in table 2.

fig. 1) corresponding to the date of the discharge measured at the 

ungaged site is plotted on the abscissa (x-axis). Two relations are 

developed using these data pairs; a line of ordinary least squares 

and a line of organic correlation. The line of ordinary least squares 

minimizes errors in the y direction only and defines the best-fit 

straight line as the line that minimizes the sum of the squares of 

the distances of the data points from the best-fit straight line 

(Helsel and Hirsch, 1992, p. 275). The line of organic correlation 

minimizes errors in both the x and y directions and defines the 

best-fit straight line as the line that minimizes the sum of the 

areas of right triangles formed by horizontal and vertical lines 

extending from observations to the fitted line (Helsel and Hirsch, 

1992, p. 276). In most instances, the line of organic correlation is 

the preferable statistical tool for 7Q2 estimation because the line 

of ordinary least squares tends to produce biased, in some cases 

highly biased, 7Q2 estimates (W.H. Asquith, U.S. Geological 

Survey, written commun., 1999).

The computed 7Q2 for the index station (fig. 2) is shown as a 

dashed line that perpendicularly intersects the abscissa at the 7Q2 

value. To estimate the 7Q2 for the ungaged site, the intersection of 

the line of organic correlation and the index station 7Q2 line is 

located. A horizontal line is projected from this intersection to the 

ordinate, and the estimated 7Q2 for the ungaged site is determined 

from the scale. Although the line of organic correlation is preferable, the ordinary least-squares line can be used if, from observation, the least-squares line clearly and unequivocally fits the data 

better in the region of the graph for which a 7Q2 estimation is 

required.

9 Hunting Bayou Homestead Rd. 29°48'35" 95°18'06" 107 8.50 

10 Garners Bayou US 59 29°58'45" 95°16'41" 108 6.00 

11 Halls Bayou Airline Dr. 29°53'56" 95°25'46" 109 13.0 

12 Sims Bayou1

1 USGS partial-record station at site.

Hiram Clarke St. 29°37'07" 95°26'44" 106 20.2 

13 East Fork Double Bayou State 65 29°47'23" 94°33'15" 101, 104 12.0 

14 Rabbs Bayou Crabb River Rd. 29°32'54" 95°41'49" 105, 110 7.60 

15 Mustang Bayou Trammel Fresno Rd. 29°32'09" 95°27'19" 110 6.10 

16 Oyster Creek FM 1462 29°21'08" 95°31'14" 110 n/a 

17 Dickinson Bayou FM 517 29°26'08" 95°10'11" 105, 106 15.0 

Table 2. Index stations used for estimation of m

7-day, 2-year discharge 

Index

station

(fig. 1)

USGS

streamflowgaging

station no.

Name Latitu

101 08067500 Cedar Bayou near Crosby, Tex. 29°58

102 08070200 East Fork San Jacinto River near 

New Caney, Tex.

30°08

103 08070500 Caney Creek near Splendora, Tex. 30°15

104 08071280 Luce Bayou above Lake Houston, 

near Huffman, Tex.

30°06

105 08072300 Buffalo Bayou near Katy, Tex. 29°44

106 08075400 Sims Bayou at Hiram Clarke St., 

Houston, Tex.

29°37

107 08075770 Hunting Bayou at I–610, Houston, 

Tex.

29°47

108 08076180 Garners Bayou near Humble, Tex. 29°56

109 08076500 Halls Bayou at Houston, Tex. 29°51

110 08077000 Clear Creek near Pearland, Tex. 29°35
4

Statistical Summary of Water-Quality Data

Selected water-quality data also were determined at each of 

the 17 ungaged sites. During site visits for low-flow discharge 

measurements, USGS personnel measured water temperature, 

specific conductance, pH, and dissolved oxygen with field meters 

and collected water-quality samples from the centroid of flow. 

These samples were analyzed by the City of Houston Wastewater 

Quality Control Laboratory for concentrations of total suspended 

solids, hardness, and dissolved chloride. Statistical summaries of 

these water-quality data are listed in table 3.

These water-quality data (specifically total suspended solids, 

hardness, and dissolved chloride) are used in trace metal limit 

determinations as part of the waste-load allocation process. For 

instance, the limiting criteria for several trace metals, such as 

cadmium, copper, and lead, are defined as a function of hardness. 

These data typically are collected during various flow conditions, 

which can affect concentrations. 

Selected References

Buchanan, T.J., and Somers, W.P., 1969, Discharge measurements 

at gaging stations: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 3, chap. A8, 65 p.

Helsel, D.R., and Hirsch, R.M., 1992, Studies in environmental 

science 49—Statistical methods in water resources: 

Amsterdam, Elsevier, 522 p.

Rantz, S.E., and others, 1982, Measurement and com

streamflow—Volume 1. Measurement of stage an

U.S. Geological Survey Water-Supply Paper 217

Riggs, H.C., 1972, Low-flow investigations: U.S. Ge

Survey Techniques of Water-Resources Investiga

4, chap. B1, 18 p.

Stedinger, J.R., and Thomas, W.O., Jr., 1985, Low-flo

estimation using base-flow measurements: U.S. 

Survey Water-Resources Investigations Report 8

Wells, F.C., Gibbons, W.J., and Dorsey, M.E., 1990, G

for collection and field analysis of water-quality 

from streams in Texas: U.S. Geological Survey O

Report 90–127, 79 p.

—Jeffery W. East

Any use of trade, product, or firm names is for descri

purposes only and does not imply endorsement by the 

Government.

Information on technical reports and hydrologic d

to this study can be obtained from:

Subdistrict Chief

U.S. Geological Survey

2320 La Branch Street

Houston, TX 77004

Phone: (713) 718–3655

FAX: (713) 718–3661

World Wide Web: http://tx.usgs.gov

1 Analyses by City of Houston Wastewater Quality Control Laboratory.

2 Summary statistics from one sample.

8 15.4 24.8 30.2 402 824 1,170 7.4 8.0 8.6 4.3 6.3 8.4 16.0 29.2 42.8 122 215 310 52.6 

9 20.9 31.5 37.4 388 691 1,060 7.9 8.5 9.1 2.1 10.8 20.0 18.0 27.0 42.4 89.2 117 158 23.7 

10 17.3 25.8 32.7 565 729 803 8.2 8.3 8.6 4.5 7.4 9.7 23.5 41.7 60.0 125 168 211 74.1 

11 21.4 28.6 32.0 749 780 829 7.5 7.7 8.0 5.0 7.3 9.5 10.0 14.7 20.8 168 172 179 90.7 

12 31.1 31.8 32.5 1,210 1,360 1,510 7.4 7.7 8.0 6.4 8.4 10.4 14.0 18.6 23.2 213 244 276 192 

13 15.9 22.0 29.4 388 422 481 7.0 7.5 7.9 7.4 8.2 9.6 33.4 86.2 139 138 145 152 28.3 

14 20.0 23.8 27.6 877 900 924 8.5 9.0 9.5 5.2 6.8 8.5 86.0 126 165 156 176 196 133 

215 25.9 25.9 25.9 617 617 617 7.4 7.4 7.4 7.7 7.7 7.7 83.2 83.2 83.2 174 174 174 83.0 

16 21.0 21.1 21.3 656 783 911 7.5 7.6 7.7 5.7 6.2 6.7 16.8 29.9 43.0 230 237 244 61.4 

17 19.8 26.1 28.8 610 773 960 7.4 7.7 8.2 5.0 5.6 5.9 37.0 54.3 88.0 188 217 234 78.6