USGS 

science for a changing world 

Streamflow in the Quinnipiac River Basin, 

Connecticut-Statistics and Trends, 1931-2000 

Water-Resources Investigations Report 02-4217 

Prepared in cooperation with the 

Connecticut Department of Environmental Protection 

U.S. Department of the Interior 

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

U.S. Geological Survey 

Streamflow in the Quinnipiac River Basin, 

Connecticut-Statistics and Trends, 1931-2000 

By Elizabeth A. Ahearn 

Water-Resources Investigations Report 02-4217 

Prepared in cooperation with the 

Connecticut Department of Environmental Protection 

East Hartford, Connecticut 

2002 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey. 

For additional information write to: 

District Chief 

U.S. Geological Survey 

101 Pitkin Street 

East Hartford, CT 06108 

http://ct.water.usgs.gov 

Copies of this report can be purchased 

from: 

U.S. Geological Survey 

Branch of Information Services 

Box 25286, Federal Center 

Denver, CO 80225 
CONTENTS 

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

Introduction ............................................................................................................................................................... 2 

Purpose and scope ............................................................................................................................................ 2 

Previous studies ................................................................................................................................................ 3 

Description of basin.......................................................................................................................................... 3 

Acknowledgments ............................................................................................................................................ 3 

Streamflow statistics ................................................................................................................................................. 3 

Flow-duration statistics .................................................................................................................................... 5 

LO\V-flow frequency statistics . . . . . . . . . . .. . .. . .. . .. .......................... ... ... ...... ....................... ... .. .. ... . . . .. . . . .. . . . . .. ... ... . .. ... . . 5 

Monthly mean and monthly median streamflow .................................................................. ............................ 12 

August median streamflow ............................................................................................................................... 16 

Streamflow trend analysis at the Wallingford gaging station ................................................................................... 17 

Trend detection methods .................................................................................................................................. 17 

Trends in streamflow ........................................................................................................................................ 19 

Factors that affect trends in streamflow ........................................................................................................... 23 

Precipitation ............................................................................................................................................. 23 

Wastewater discharges . . .. . . . .. ... ... ....... ..... . . . .... . . . . . .. . .. ... .......... ... .. . . .. . . . . ... . .. .......... ... .. . . . . . . .. . .. ... .................... 25 

Summary ................................................................................................................................................................... 29 

References cited ........................................................................................................................................................ 30 

Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum and annual 

maximum, Quinnipiac River at Wallingford, Conn., 1931-2000 ....................................................................... 33 

Appendix 2. Trend results of annual and monthly precipitation data for selected stations in Connecticut, 

1930-2000 ........................................................................................................................................................... 39 

Appendix 3. Trend results of wastewater-discharge data from municipal wastewater-treatment facilities in 

Meriden, Cheshire, and Southington, Conn. 1985-2001 .................................................................................... 41 

FIGURES 

1. Map showing location of the Quinnipiac River Basin, south-central Connecticut, and 

data-collection points ................................................ ................................................................................. 4 

Figures 2. to 19. Graphs showing: 

2. Minimum, maximum, and period of record flow-duration curves (A), and monthly flow-duration 

curves (B) at Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 ............................... 8 

3. Minimum, maximum, and period of record flow-duration curves (A), and monthly flow-duration 

curves (B) at Quinnipiac River at Southington, Conn. (01195490), 1988-2000 ............................... 9 

4. Low-flow frequency curves, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 (A) 

and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 (B) ........................................ 11 

5. Monthly median and monthly mean streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 

1931-2000 (A) and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 (B) .............. 13 

6. Boxplots showing monthly streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 

1931-2000 (A) and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 (B) .............. 14 

Contents ill 
FIGURES-Continued 

7. Trends in annual August median streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 

1931-2000 ........................................................................................................................................... 16 

8. Annual maximum, mean, median, and minimum streamflow, Quinnipiac River at Wallingford, Conn. 

(01196500), 1931-2000 ...................................................................................................................... 18 

9. Trends in annual minimum streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 

1931-2000 ........................................................................................................................................... 21 

10. Trends in annual maximum streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 

1931-2000 ........................................................................................................................................... 22 

11. Trends in annual median streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 ... 22 

12. Annual total precipitation for five rain gages in and near the Quinnipiac River Basin, Conn., and 

LOWESS smooth line, 1930-2000 ...................................................................................................... 24 

13. Annual minimum streamflow at two U.S. Geological Survey streamflow-gaging stations 

(Quinnipiac River at Wallingford, Conn.-01196500 and Quinnipiac River at Southington, 

Conn.-01195400), 1988-2000 .......................................................................................................... 26 

14. Monthly total permitted wastewater discharges into the Quinnipiac River from the wastewater-treatment 

facility in Meriden, Conn., 1985-2001 ............................................................................................... 26 

15. Trends in wastewater discharges into the Quinnipiac River from the wastewater-treatment facility in 

Meriden, Conn., 1985-2001 ............................................................................................................... 28 

TABLES 

1. Annual and monthly flow duration, Quinnipiac River at Wallingford, Conn. (01196500), 1931-00 

and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 ..................................................... 6 

2. Annual lowest mean flows for indicated periods of consecutive days and indicated recurrence 

intervals, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 and Quinnipiac River at 

Southington, Conn. (01195490), 1988-2000 ............................................................................................ 10 

3. Monthly streamflow in percentiles, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 

and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 ..................................................... 15 

4. Monthly median streamflows for August and September, Quinnipiac River at Wallingford, Conn. 

(01196500), 1931-2000 and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 ............. 16 

5. Trends for selected percentiles of streamflow between the annual minimum and annual maximum 

streamflows, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 ...................................... 19 

6. Trends in wastewater discharge at selected wastewater-treatment facilities, 

Quinnipiac River Basin, Conn. ................... ............................................................................................... 27 

CONVERSION FACTORS 

Multiply 

inch (in.) 

mile (mi) 

square mile (mP) 

cubic foot per second (ft3/s) 

million gallons per day (Mgal/d) 

By 

25.4 

1.609 

2.590 

0.02832 

0.04381 

To obtain 

millimeter 

kilometer 

square kilometer 

cubic meter per second 

cubic meter per second 

iv Streamflow in the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Streamflow in the Quinnipiac River Basin, 

Connecticut-Statistics and Trends, 1931-2000 

by Elizabeth A. Ahearn 

ABSTRACT 

Streamflow statistics were updated for two 

U.S. Geological Survey continuous record streamflow-gaging stations on the Quinnipiac River 

(01196500-Wallingford and 01195490-Southington). The streamflow record from the Wallingford station was analyzed to determine trends in 

streamflow quantity from October 1930 to 

September 2000. Trends were analyzed using the 

non-parametric Mann-Kendall test and magnitudes estimated for selected percentiles of streamflow, ranging from the oth (annual minimum) to 

lOOth (annual maximum) percentile. Trend tests 

were performed on various time periods (30-, 40-, 

50-, 55-, 60-, 65- to 70-year periods), all ending in 

2000, except for two periods (1931-1960 and 

1941-1971) that were studied as 30-year reference 

periods. 

The two most prevalent trends were 

increases in the annual minimum and annual 

maximum streamflow during the period 1931-00. 

The annual minimum streamflow increased by an 

average of 0.44 cubic feet per second per year 

(ft3

/s/yr), and the annual maximum streamflow 

increased by an average of 17 ft3

/s/yr. Increasing 

streamflows were detected predominantly in the 

lower half of the flow distribution (Oth to 

5oth ~ercentile) and in the annual maximum 

(lOOt percentile) streamflow for periods 1931-

00, 1941-00, and 1951-00. During 1961-00, the 

pattern changed-increasing streamflows were 

detected in the middle to upper half of the flow 

distribution (30th to lOath percentile). No trends 

were detected during 1971-00. 

Trend analyses of precipitation (data from 

1930 to 2000) and wastewater discharge (data 

from 1985 to 2001) provide some evidence about 

the effects these factors have on streamflow 

trends. Changes in rainfall intensity and, to a lesser 

extent, increases in residential and commercial 

development and expansion of storm drainage 

systems are probable causes for increases in the 

annual maximum streamflow. No evidence was 

found to indicate that a precipitation trend is 

causing the increase in the annual minimum to 

annual median streamflow. Upward trends in 

wastewater discharge from municipal (Meriden, 

Cheshire and Southington) wastewater-treatment 

facilities were detected during 1985-01. Statistical 

evidence suggests that the wastewater-discharge 

trend increased at a rate consistent with the annual 

minimum streamflow trend during 1985-01 and 

that the increases in the annual minimum streamflow appear to be caused by increases in wastewater discharges. 

Flow regulation appears to have an effect on 

streamflow trends, particularly on base flows 

(annual minimum daily mean streamflow) from 

1932 to 1939 when exceptionally low flows were 

recorded at a time of average to above-average 

precipitation. Droughts during the early 1930's, 

early 1940's, and 1960's, and above-average 

precipitation during the 1970's indicate that the 

magnitude of the streamflow trends (annual 

minimum to annual median) particularly for 

1931-00 may, in part, be a result of the relation 

between precipitation variability and the time 

periods analyzed. The highest number of streamflow trends were detected during 1931-00, 1941-

00 and 1961-00, coincident with the beginning of 

multi-year droughts. Flow regulation during the 

early period of record (1930's) appears to be the 

predominant reason for the trend magnitude in the 

annual minimum streamflow from 1931 to 2000. 

Abstract 
INTRODUCTION 

The Quinnipiac River is an important resource 

for economic development, industry, agriculture, water 

supply, and biological diversity in south-central 

Connecticut. Despite the river's abundant flow, water 

demands periodically are greater than minimum 

streamflow. Water scarcity caused by an over-allocation of the resource and (or) drought poses a challenge 

to all water users. Three tributaries (Sodom Brook, 

segments of Misery Brook, and Harbor Brook) and the 

upper Quinnipiac main stem headwaters are listed in 

the Draft 2002 Connecticut Waterbodies Not Meeting 

Water Quality Standards (Impaired Water List), 

pursuant to Sections 303(d) and 305(b) of the Federal 

Clean Water Act, prepared by the Connecticut Department of Environmental Protection (DEP) for the U.S. 

Environmental Protection Agency (USEPA) (Connecticut Department of Environmental Protection, written 

commun., 2002). Flow alteration was a criterion used 

in designating these waterbodies as impaired. Low 

streamflows call attention to the seriousness of limited 

supplies, and degradation of the resource has widespread implications. Federal, State, and local agencies 

are concerned that declining streamflows are degrading 

the ecological health of the river. As demands for and 

uses of water increase, consideration is directed to the 

effects of human influences on ecological health of the 

river, and the balance between streamflow needed for 

fish and wildlife habitat integrity and water use. The 

stream habitat and stream ecosystem are affected by 

changes to the natural flow regime. 

The natural flow of the Quinnipiac River has 

been altered by numerous flow diversions resulting in 

a complex, modified river system. This complexity is 

evident during low-flow periods when diversions may 

cause some stream reaches to dry up, whereas other 

reaches may gain flow from treated wastewater 

discharge. Flow-regulation patterns in the basin affect 

the tributaries and main stem in different ways (Quinnipiac River Watershed Partnership, 2000). Streamflow 

is decreasing (low flow) in the tributaries and in the 

headwaters where water primarily is withdrawn. A 

large percentage of water that is diverted from the tributaries eventually is returned as wastewater to the main 

stem of the Quinnipiac River, thereby increasing flows 

on the main stem. 

In response to the water allocation and streamflow concerns in the basin, the U.S. Geological Survey 

(USGS) began a cooperative study with the Connecticut DEP in 2000 to (1) compute current streamflow 

statistics from two continuous-record streamflowgaging stations (Wallingford 01196500 and Southington 0 1195490) that can be used for water-resource 

planning, management, and regulatory activities; and 

(2) analyze historical streamflow data for trends to 

document any long-term (30 years or greater) flow 

changes. Streamflow records from the two gaging 

stations in the Quinnipiac River Basin and results of 

streamflow-trend analysis for the Wallingford gaging 

station provide data to improve the understanding of 

past and current streamflow conditions in the basin. 

Purpose and Scope 

This report presents streamflow statistics of 

duration, frequency, and quantity of annual, monthly, 

maximum, and minimum streamflow for two gaging 

stations in the Quinnipiac River Basin. Streamflow 

statistics are presented in tabular and graphical form to 

characterize the basin's water resources and estimate 

flows needed for water supply and allocation. The 

report also presents trends in streamflow for 1931-00 

at the Wallingford gaging station. Statistical techniques 

used to perform the trend analysis and compute the 

basin characteristics that affect streamflow processes 

are presented to provide perspective on the detected 

trends. In addition, the report briefly describes how and 

to what extent climatic and human influences affect 

streamflow characteristics. 

Streamflow records were retrieved from the 

USGS National Water Information System (NWIS) 

accessed on the World Wide Web at URL http://waterdata.usgs.gov/nwis. All analyses were based on historical average daily discharge records from two 

continuous-record streamflow-gaging stations operated by the USGS on the main stem of the Quinnipiac 

River. Records from the downstream station (Wallingford-O 1196500) began in October 1930, and records 

from the upstream station (Southington-01195400) 

began in November 1987. In this report, all time 

periods are expressed in water years 

1

. 

1 A water year is the 12-month period October 1 through September 30 and is designated by the calendar year in which it ends; 

thus, the year from October 1, 1930 through September 30, 1931 is 

called the 1931 water year. 

2 Streamflow in the Quinniplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Previous Studies 

Streamflow duration and frequency statistics 

were compiled in a water-resources inventory of the 

Quinnipiac River Basin by Mazzaferro and others 

( 1979) for the Wallingford gaging station for the period 

1931-60. In the early 1980's, Cervione (1982) and 

Weiss (1983) summarized low- and high-flow 

frequency statistics for rivers in the State with appreciable record length. Low- and high-flow frequency 

statistics for the Quinnipiac River at Wallingford 

gaging station are included in these two studies. The 

later studies benefited from longer records and supersede the statistics in the earlier report by Mazzaferro 

and others. Monthly, annual, and period of record 

streamflow statistics and a limited number of duration 

statistics are published annually by the USGS in the 

Water-Resources Data series. 

Description of Basin 

The Quinnipiac River Basin is in south-central 

Connecticut and drains an area of about 166 mi2 

(fig. 1). The Quinnipiac River originates in the town of 

New Britain and flows southward for approximately 

38 mi through the towns of Plainville, Southington, 

Cheshire, Meriden, Wallingford, North Haven, and 

New Haven to Long Island Sound. The lower 9 mi of 

the Quinnipiac River are affected by tides from Long 

Island Sound. Major tributaries are the Eightmile 

River, Tenmile River, Misery Brook, Broad Brook, 

Harbor Brook, Sodom Brook, Wharton Brook, and 

Muddy River. Subregional basins of the major tributaries range from 5 to 22 mi2. Land use is predominantly forest, agriculture, and residential in the upper 

basin and predominantly densely developed urban and 

industrial in the lower basin. 

The USGS operates two continuous-record 

streamflow-gaging stations on the Quinnipiac 

River-at Wallingford (station 01196500) and at 

Southington (station 01195490). The Wallingford 

station was established in October 1930 and has a 

drainage area of 115 mi2, which is approximately 

70 percent of the basin. The Southington station was 

established in November 1987 and has a drainage area 

of 17.4 mi2, which is 10 percent of the basin. 

An assessment of water diversions in 2000 identified 103 consumptive water withdrawals, with a 

combined maximum withdrawal capacity of 

120.3 Mgal/d, and 19 wastewater discharges, with a 

combined maximum daily discharge of 44.6 Mgalfd 

(Quinnipiac River Watershed Partnership, 2000). 

Acknowledgments 

Charles Fredette of the Connecticut DEP, and 

Glenn A. Hodgkins and Phillip J. Zarriello of the 

USGS provided valuable technical insight and review 

of the report. Barbara Korzendorfer, USGS, provided 

assistance in editorial review and report production. 

STREAMFLOW STATISTICS 

Statistical analysis of hydrologic data is important in understanding streamflow variability, estimating 

future water supplies, and establishing new policies 

and regulations for managing water resources. Some of 

the most commonly used streamflow statistics include 

flow duration, flow frequency (recurrence of a given 

flow), averages, and percentiles of daily flows. Flow 

duration characterizes past streamflow by ranking the 

distribution of flows over the analyzed period of 

record. Flow-frequency estimates the probability of a 

given streamflow occurring in the future. Averages and 

percentiles describe the central tendency and distribution of streamflow data. 

The Wallingford gaging station (0 1196500~ 

fig. 1) had a mean annual flow of217 ft3/s 

(140 Mgalld), a historical minimum daily mean flow of 

9.0 ft3/s (5.8 Mgalld) recorded on November 2, 1930, 

and a historical maximum daily mean flow of 

7,210 ft31s (4,660 Mgalld) recorded on June 6, 1982. 

The monthly median streamflow ranged from a low of 

79.2 ft31s (51.2 Mgalld) in August to a high of294 ft3/s 

(190 Mgalld) in March. 

The Southington gaging station (01195490; 

fig. 1) had a mean annual flow of 34.2 ft3 Is 

(22.1 Mgalld), a historical minimum daily mean flow 

of 3.8 ft31s (2.5 Mgalld) recorded on August 20, 1999, 

and a historical maximum daily mean flow of 810 ft3

1s 

(523 Mgalld) recorded on October 21, 1999. The 

monthly median streamflow ranged from a low of 

11.7 ft31s (7.56 Mgalld) in September to a high of 

38.4 ft3 Is (24.8 Mgalld) in April. 

Streamflow Statistics 3 
41"36' 

41 "24' 

73"0' 

Base from U.S. Geological Survey 

digital line graph (1988) 

72"48' 

$ ' I 

\ 

\ 

\ \ 

\ 

( 

Inset shows location of 

Ouinnipiac River Basin 

EXPLANATION 

'\.. · · .r Basin ·\ boundary 

01196500 Streamflow0 

0 

.& gaging station 

ana number 

650n.o.. National Oceanic .,.... and Atmospheric 

Administration rain 

gage and number 

I 

2 

2 

I 

I 

4 MILES 

I 

4 KILOMETERS 

Figure 1. Location of the Quinnipiac River Basin, south-central Connecticut, and data-collection points. 

4 Streamflow in the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Flow-Duration Statistics 

Annual and monthly flow-duration statistics 

were calculated for the Wallingford and Southington 

gaging stations (table 1; figs. 2 and 3). Flow-duration 

statistics represent the percentage of time specified 

streamflows were equaled or exceeded over the analysis period. For example, the daily flows at the Wallingford gaging station were greater than or equal to 

85.9 ft3/s 50 percent of the time (table 1) in October 

during the period 1931-00. Because flow-durations 

statistics are performed on the rank position of a flow 

value, the chronology of the flows is concealed. No 

indication is given of the date on which the flows 

occurred. Flow-duration statistics are estimated by 

arranging (by month) all the daily mean streamflows 

for the given time period in ascending order without 

regard to the sequence of the occurrence. The data then 

are grouped into class intervals by ascending order and 

the probabilities of specified streamflow being equaled 

or exceeded are computed. Flow-duration statistics 

were estimated by using the DVSTAT Computation 

computer program in the USGS National Water Information System following USGS standard methods for 

estimating flow duration (Searcy, 1959) at gaging 

stations. The duration analysis provides values for 

exceedance percentages based on the class limits and 

the percentage of all days in which a class limit was 

equaled or exceeded. 

For the Wallingford gaging station, the wettest 

and driest water years on record (fig. 2A) show a wide 

range in the percentage of time specified streamflows 

were equaled or exceeded in a single year. During the 

wettest year on record (water year 1984), streamflow 

was equal to or greater than 259 ft3 /s (167 Mgalld) 

50 percent of the time; during the driest year (water 

year 1966), streamflow was equal to or greater than 

56.3 ft3

/s (36.4 Mgal/d) 50 percent of the time. From 

the long-term average (1931-00) at the Wallingford 

gagin~ station, streamflow was equal to or greater than 

152ft /s (98.2 Mgal/d) 50 percent of the time. 

The driest and wettest years are defined as water 

years with the overall greatest number of minimum and 

maximum flow durations in a water year. During water 

year 1966 (driest water year), the 10- to 90-percent 

flow duration ranged from 26 to 168 ft3 Is and were the 

lowest flows for that duration range in 70 years. During 

water year 1984 (wettest water year), the 20- to SOpercent flow duration ranged from 138 to 473 ft3

/s and 

were the highest flows for that duration range in 

70 years. Although water year 1931 had the lowest 

daily mean streamflow (9.0 ft3

/s) recorded at the Wallingford gaging station, water year 1966 was the driest 

water year because of the extent and severity of the low 

flows. The minimum daily mean streamflow in water 

year 1966 at the Wallingford gaging station was 

14.0 ft3/s. 

Because of the relatively short record of the 

Southington gaging station (12 years), compared to the 

Wallingford station, the minimum and maximum flow 

durations for each duration grouping in the period of 

record (1988-00) are used to emphasize the variation in 

durations (fig. 3A). At the minimum limit, streamflow 

was equal to or greater than 15 ft3

/s (9.7 Mgalld) 

50 percent of the time in a single year (water year 

1999). At the maximum limit, streamflow was equal to 

or greater than 33 ft3

/s (21 Mgalld) 50 percent of the 

time in a single year (water year 1990). From the 12-

year average (1988-00) at the Southington gaging 

station, streamflow was equal to or greater than 

25.1 ft3

/s (16.2 Mgalld) 50 percent of the time. 

Generally, flow-duration statistics apply to the 

period of record for which the data were analyzed. 

Where the period of record used to compute the statistics is sufficiently long, flow-duration statistics can be 

used as an indicator of probable future flows (Searcy, 

1959); however, if diversions, return flows, or climatic 

factors affecting flow are expected to change substantially, flow-duration statistics cannot be used to indicate probable future flows with statistical certainty. 

Low-Flow Frequency Statistics 

Frequency analysis can be used to obtain the 

probability that specific streamflow will be equaled or 

exceeded. The major application of low-flow 

frequency analysis is for setting flow standards associated with diversion permits and for prediction purposes 

in water-resources planning. Two commonly used 

indices of low flow are the 7-day, 10-year low flow, 

(7Q10), the lowest streamflow for a period of7 consecutive days that is expected to occur once in a 10-year 

period, and the 30-day, 2-year low flow, (30Q2), the 

lowest streamflow for a period of 30 consecutive days 

that is expected to occur once every other year. The 

most widely used low-flow frequency statistic in 

Connecticut is the 7Q10 flow, which is used by many 

State and local agencies and by the USEPA to regulate 

wastewater discharges to streams. The Connecticut 

DEP adopted as State policy the 7Q 10 statistic as a 

minimum flow event for which water-quality standards 

are expected to be met (Connecticut Department of 

Environmental Protection, 1997). 

Streamflow Statistics 5 
m Table 1. Annual and monthly flow duration, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 and Quinnipiac River at Southington, Conn. 

(01195490), 1988-2000 

Duration 

(in 

percent) 

99 

98 

97 

96 

95 

90 

85 

80 

75 

70 

65 

60 

55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

2 

October 

22.7 

35.3 

39.7 

41.0 

42.4 

48.3 

52.4 

56.6 

60.7 

64.8 

69.5 

74.3 

79.6 

85.9 

92.3 

100 

109 

121 

137 

164 

200 

249 

360 

576 

843 

November 

30.8 

39.7 

43.8 

47.1 

50.3 

63.3 

71.9 

78.0 

84.0 

92.2 

101 

111 

122 

134 

148 

163 

180 

199 

222 

250 

297 

368 

533 

760 

896 

December 

42.9 

49.8 

55.7 

60.6 

64.7 

76.5 

87.6 

99.2 

110 

122 

133 

147 

160 

173 

187 

202 

222 

245 

273 

307 

355 

439 

601 

888 

1,220 

Streamflow, In cubic feet per second 

January February March April May June 

U.S. Geological Survey station 01196500, Quinnipiac River at Wallingford 

50.0 

61.6 

67.1 

71.9 

76.2 

91.3 

104 

116 

126 

136 

146 

156 

170 

184 

199 

215 

232 

257 

288 

331 

384 

487 

724 

1,140 

1,460 

62.3 

70.8 

82.0 

89.5 

92.5 

107 

119 

130 

144 

157 

170 

184 

199 

215 

234 

253 

272 

295 

322 

358 

408 

482 

643 

957 

1,270 

105 

115 

123 

130 

137 

164 

186 

203 

218 

233 

246 

260 

276 

295 

317 

342 

370 

400 

439 

488 

551 

661 

969 

1,320 

1,730 

97.4 

107 

115 

123 

129 

152 

169 

184 

200 

215 

230 

245 

261 

280 

299 

321 

346 

373 

411 

455 

520 

628 

842 

1,240 

1,530 

71.8 

82.2 

87.3 

92.3 

95.6 

Ill 

124 

135 

148 

161 

172 

184 

196 

207 

220 

233 

250 

268 

292 

322 

358 

413 

521 

702 

962 

53.0 

57.0 

61.0 

63.7 

66.3 

75.9 

83.5 

90.2 

96.3 

102 

108 

114 

121 

129 

138 

147 

159 

173 

192 

215 

251 

311 

431 

694 

1,070 

July 

30.1 

36.1 

40.6 

43.2 

45.6 

53.9 

59.4 

64.2 

68.5 

72.9 

77.4 

81.9 

87.5 

93.1 

98.6 

104 

111 

120 

132 

147 

166 

195 

266 

410 

535 

August 

19.6 

26.8 

33.2 

35.3 

37.4 

44.1 

50.3 

55.3 

59.4 

63.4 

67.2 

71.1 

75.2 

79.5 

83.9 

89.7 

96.1 

104 

113 

130 

152 

190 

260 

408 

637 

September 

25.6 

33.7 

35.5 

37.2 

38.9 

44.3 

48.5 

52.1 

55.8 

59.4 

63.0 

67.3 

71.8 

76.4 

81.6 

86.9 

93.3 

101 

110 

124 

145 

184 

277 

510 

822 

Annual 

34.7 

40.7 

44.1 

47.5 

50.0 

60.9 

70.3 

79.4 

88.7 

98.4 

110 

I 

122 

136 

152 

169 

188 

208 

232 

258 

297 

347 

424 

587 

904 

1,210 

----·····------------
Table 1. Annual and monthly flow duration, Ouinnipiac River at Wallingford, Conn. (01196500), 1931-2000 and Ouinnipiac River at Southington, Conn. 

(01195490), 1988-2000-Continued 

Duration 

(in 

percent) 

99 

98 

97 

96 

95 

90 

85 

80 

75 

70 

65 

60 

55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

2 

October 

5.30 

5.50 

5.70 

6.00 

6.36 

7.22 

7.76 

8.50 

9.50 

11.1 

12.5 

13.7 

14.8 

15.8 

17.6 

19.4 

21.3 

24.1 

27.3 

32.5 

39.0 

48.3 

81.5 

179 

276 

November 

8.70 

.9.00 

9.20 

9.40 

9.62 

11.4 

12.5 

13.4 

14.7 

17.2 

19.8 

21.3 

23.2 

24.9 

26.8 

29.1 

31.7 

35.6 

40.5 

44.8 

51.6 

66.2 

97.2 

136 

172 

December 

8.00 

8.30 

8.70 

9.10 

9.60 

13.5 

15.5 

17.4 

19.0 

20.4 

21.5 

22.7 

24.0 

25.3 

26.7 

28.5 

30.8 

33.6 

37.3 

42.7 

50.3 

63.4 

90.2 

121 

153 

Streamflow, in cubic feet per second 

January February March April May June 

U.S. Geological Survey station 01195490, Quinnipiac River at Southington 

11.2 12.9 15.6 16.8 13.6 7.30 

11.6 13.8 16.2 18.2 14.4 7.90 

12.0 15.3 16.9 19.3 15.1 8.40 

12.4 16.2 17.7 20.2 15.5 8.70 

12.8 17.1 19.4 20.7 15.9 9.06 

14.7 19.2 25.2 22.7 18.1 10.8 

16.5 20.8 27.5 24.2 19.3 12.3 

18.2 22.7 29.0 25.9 20.7 13.1 

19.8 24.1 30.5 27.8 22.4 13.9 

21.5 25.5 31.9 29.7 24.2 14.6 

23.9 

25.9 

27.7 

29.2 

31.0 

33.1 

35.4 

37.9 

41.8 

47.3 

56.6 

69.1 

106 

193 

223 

26.8 

28.4 

30.2 

32.1 

34.2 

36.2 

38.2 

40.6 

44.5 

48.3 

55.3 

63.5 

85.7 

104 

125 

33.4 

34.8 

36.1 

37.5 

39.0 

40.8 

42.9 

46.1 

51.5 

58.6 

71.5 

92.2 

151 

184 

199 

31.6 

33.5 

35.9 

38.4 

40.8 

43.2 

46.1 

49.2 

52.7 

58.5 

63.4 

73.3 

98.0 

133 

250 

25.8 

27.6 

29.5 

31.4 

33.2 

35.3 

37.6 

41.0 

45.8 

52.1 

61.4 

74.6 

102 

187 

258 

15.6 

16.7 

17.9 

19.2 

20.6 

22.3 

24.4 

26.6 

29.3 

33.5 

40.5 

50.5 

74.7 

95.2 

186 

July 

5.00 

5.20 

5.60 

5.90 

6.16 

7.34 

8.15 

8.85 

9.75 

11.1 

12.1 

13.0 

13.9 

14.9 

16.0 

17.1 

18.3 

20.1 

22.5 

24.9 

29.1 

36.4 

66.4 

98.7 

144 

August 

4.00 

4.10 

4.30 

4.50 

4.63 

5.34 

6.52 

7.81 

8.77 

9.75 

10.9 

11.8 

12.6 

13.4 

14.2 

15.1 

16.0 

17.7 

19.7 

23.3 

28.4 

37.6 

64.7 

140 

234 

September 

4.10 

4.30 

4.50 

4.90 

5.15 

6.25 

7.12 

7.80 

8.51 

9.12 

9.61 

10.2 

11.0 

11.7 

12.5 

13.5 

15.0 

16.4 

18.2 

21.2 

24.7 

31.5 

49.5 

106 

166 

Annual 

4.80 

5.50 

6.20 

6.80 

7.35 

9.42 

11.6 

13.3 

15.0 

16.8 

18.8 

20.8 

22.9 

25.1 

27.4 

29.8 

32.7 

35.7 

39.5 

44.6 

52.2 

64.6 

91.8 

154 

200 
A. 

B. 

1o.ooo r--------,---,----r---.--r---.------,,..--~~ 

0 

z 

0 

(,) 

liJ 

Ul 

a: 

liJ 

c.. 

1-

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u. 

(,) 

iii 

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(,) 

~ 

~ 0 

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a: 

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1,000 

100 

MINIMUM (DRIEST YEAR 0 

PERIOD OF RECORD··1966) 

WALLINGFORD 

10 ~~------~-...J_ ___ ...J__~_...J_ _ _L ___ ~--~~ 

0.9 1.0 10.0 20.0 50.0 70.0 80.0 90.0 98.0 99.0 99.2 

PERCENTAGE OF TIME STREAMFLOW EQUALED OR EXCEEDED 

2,000 

WALLINGFORD 

1,000 

100 

0L-L-----~-~-~~_L~_L__~_~ _ _L _ _L __ L_ _ _L~ 

0.9 1.0 5.0 10.0 20.0 30 040.050.0 70.0 80.0 90.0 95.0 98.0 99.0 99.2 

PERCENTAGE OF TIME STREAMFLOW EQUALED OR EXCEEDED 

Figure 2. Minimum, maximum, and period of record flow-duration curves (A), and monthly flow-duration curves (B) at 

Ouinnipiac River at Wallingford, Conn. (01196500), 1931 - 2000. 

8 Streamflow In the Quinniplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
A. 1.000 

SOUTHINGTON 

0 

z 

0 

() 

w 

(/) 

cr 

w 100 c.. 

1-

w 

w 

u.. 

() 

iii 

::> 

() 

~ 

0 10 ..J 

u.. 

::!: 

<X 

w 

cr 

1-

(/) 

1 

0.9 1.0 5.0 10.0 20.0 50.0 70.0 80.0 90.0 99.0 99.2 

B. PERCENTAGE OF TIME STREAMFLOW EQUALED OR EXCEEDED 

300 

SOUTHINGTON OCT 

NOV 

DEC 

JAN 

FEB 

MAR 

0 APR 

z 100 - MAY 

0 JUNE () - JULV w 

(/) AUG 

cr SEPl 

w 

c.. 

1-

w 

w 

u.. 

() 

iii 

::> 

() 

~ 

~ 0 

..J 10 u.. 

::!: 

<X 

w 

cr 

1-

(/) 

3 

0.9 1.0 2.0 5.0 10.0 20.0 30.040.050.060.070.0 80.0 90.0 98.0 99.0 99.2 

PERCENTAGE OF TIME STREAMFLOW EQUALED OR EXCEEDED 

Figure 3. Minimum, maximum, and period of record flow-duration curves (A), and monthly flow-duration curves (B) at 

Ouinnipiac River at Southington, Conn. (01195490), 1988-2000. 

Streamflow Statistics 9 
The 7QIO computed for the period of record 

(1931-00 Wallingford; 1988-00 Southington) is 

33.4 fr31s (21.6 Mgalld) for the Wallingford gaging 

station and 4.32 ff Is (2.79 MgaUd) for the Southington 

gaging station (table 2; fig. 4). The 30Q2 computed for 

the period of record is 61.8 ft3

/s (39.9 Mgalld) for the 

Wallingford gaging station and 9.01 ft31s (5.82 MgaUd) 

for the Southington gaging station (table 2; fig. 4). 

Stated in terms of probability (1 divided by the recurrence interval), the 7-day annual minimum streamflow 

at the Wallingford gaging station has a 10-percent 

chance in any year of being less than 33.4 ft3

1s (21.6 

Mgal/d); the 30-day annual minimum streamflow at the 

Wallingford gaging station has a 2-percent chance in 

any year of being less than 61.8 ft3 Is (39 .9 Mgal/d). 

Frequency statistics relate magnitude of a given 

streamflow to recurrence interval. The recurrence 

interval is the average length of time, usually stated in 

years, between the exceedance of particular streamflow 

magnitudes. Low-flow frequency statistics are 

computed from a series of annual minimum flows and 

can be computed for any combination of days of 

minimum flows. 

Low-flow frequency statistics for Wallingford 

and Southington gaging stations (table 2; fig. 4) were 

calculated with the USGS computer program SWSTAT 

(Lumb and others, U.S. Geological Survey, written 

colllinun., 1994) for minimum 1-day flows and the 

minimum average daily flow for n-days, where n 

equals 2, 3, 7, 10, 30, 60, 90, 120, 183, and 365 consecutive days for each year of record. Then-day low flows 

were fitted to a log-Pearson Type III distribution curve 

to calculate recurrence frequencies. 

Table 2. Annual lowest mean flows for indicated periods of consecutive days and indicated recurrence intervals, 

Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 and Quinnipiac River at Southington, Conn. (01195490), 

1988-2000 

[--, low-flow frequency estimates beyond a 20-year recurrence interval are considered unreliable and were not calculated] 

Period of Annual lowest mean flows (in cubic feet per second) for indicated recurrence Intervals (in years) 

consecutlve days 1.25 2 5 10 20 50 100 

U.S. Geological Survey station 01196500, Quinnipiac River at Wallingford, April1, 1932 to March 31,2000 

1 56.6 40.6 26.2 20.0 15.6 11.5 9.2 

2 58.5 42.9 28.7 22.4 17.8 13.5 11.0 

3 59.4 45.8 32.5 26.2 21.6 16.9 14.2 

7 64.9 51.5 39.2 33.4 29.0 24.5 21.8 

10 67.4 52.5 40.2 34.8 30.7 26.7 24.2 

30 81.2 61.8 48.1 42.6 38.6 34.8 32.6 

60 96.9 72.7 55.7 48.9 44.1 39.4 36.7 

90 108 80.5 61.7 54.2 49.1 44.1 41.2 

183 155 112 83.8 72.6 64.9 57.5 53.2 

365 264 212 167 147 131 114 104 

U.S. Geological Survey station 01195490, Quinnipiac River at Southington, April!, 1989 to March 31, 2000 

1 8.64 6.36 4.72 4.06 3.59 

2 8.77 6.41 4.76 4.09 3.63 

3 8.90 6.47 4.81 4.15 3.69 

7 9.43 6.83 5.03 4.32 3.82 

10 9.98 7.14 5.18 4.41 3.87 

30 13.28 9.01 6.26 5.23 4.52 

60 16.1 11.2 7.89 6.61 5.73 

90 20.0 13.9 9.76 8.19 7.10 

183 27.6 19.9 15.3 13.7 12.6 

365 39.9 32.8 26.7 24.0 21.9 

10 Streamflow In the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
A. 

c 

z 

0 

(.) 

w 

en 

a: 

w 

0.. 

1-

w 

w 

u. 

(.) 

iii 

:::> 

(.) 

~ 

~ 0 

...J 

u. 

:E 

c:x: 

w 

a: 

1-

en 

z 

c:x: 

w 

:E 

1-

en 

w 

3: 

0 

...J 

c 

z 

0 

(.) 

w 

en 

a: 

w 

0.. 

1-

w 

w 

u.. 

(.) 

iii 

:::> 

(.) 

~ 

~ 0 

...J 

u. 

:E 

c:x: 

w 

a: 

1-

m 

z 

c:x: 

w 

:E 

1-

m 

w 

3: 

0 

...J 

B. 

1.000 

(/) 

>- WALLINGFORD <( 

500 0 

l!J 

> 

>= ::J 

u 

l!J 

(/) 

z 

0 

u 

u. 100 0 

0 

0 

a: 

50 l!J 

0.. 

10 -

EXAMPLE: The 7-day annual low flow will be 

5 less than or equal to 33.4 cubic feet per second 

(2 1.6 million gallons per day) at intervals averaging 10 years . 

1.25 2 5 10 20 50 

100 

(/) 

>-

<( 

0 

l!J 

> 

>= ::J 

u 

l!J 

(/) 

z 

0 

u 

u. 

0 

0 

10 0 

a: 

l!J 

0.. 

1.25 

CURRENCEINTERVA ~I NYEARS 

SOUTH INGTON 

EXAMPLE: The 7-day annual low flow will be 

less than or equal to 4.32 cubtc feet per second 

(2.79 million gallons per day) at intervals averaging 10 years . 

2 5 10 

RECURRENCE INTERVAL, IN YEARS 

365 

183 

90 

tO 

100 

365 

183 

90 

60 

30 

20 

Figure 4. Low·flow frequency curves, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 (A) and 

Quinnipiac River at Southington, Conn. (01195490), 1988-2000 (8 ). 

Streamflow Statistics 11 
The reliability of low-flow frequency statistics is 

related closely to the length of record used to compute 

the frequency statistics. At least 10 years of record are 

needed to determine the statistics (7Q10) with reasonable confidence (Ries, 2000). The results from the lowfrequency analysis can be used as an indication of 

probability that certain flow events will occur in the 

future, assuming the human influences on streamflow 

are expected to remain unchanged. The low-flow 

frequency statistics for both the Wallingford and Southington gaging stations should not be used to predict 

future flows because of the changing flow regulation 

during the period of record affecting natural streamflow. The frequency statistics presented in table 2 

reflect the overall period of record that includes past 

flow regulation at the Wallingford and Southington 

gaging stations. Because the Southington gaging 

station has a relatively short record (12 years), 

compared to the Wallingford gaging station, the 

maximum low-flow frequency estimated at the Southington gaging station is 20 years. Low-flow frequency 

estimates beyond a 20-year recurrence interval at the 

Southington gaging station are considered unreliable 

and, therefore, were not calculated. 

When streamflows are altered by diversions, 

reservoirs, and wastewater discharges, the natural 

7Q I 0 flow cannot be estimated from a frequency analysis of streamflow data. For rivers that are altered 

largely by regulation, the 7Ql0 low flow can be 

computed from a regional regression equation. Low 

streamflows (such as the 7Ql0) are highly dependent 

on the geologic characteristics of the basin (Thomas, 

1966). The regression equation developed by Cervione 

(1982) for estimating the 7Ql0 in Connecticut is based 

on the percentage of the basin underlain by coarsegrained deposits. The 7Ql0 computed from the regression equation is 30.2 ft3

/s (19.5 Mgal/d) for the Wallingford gaging station and 6.21 ft3

/s (4.01 Mgal!d) for 

the Southington gaging station. The estimated 7Q10 

value for Wallingford from the regression equation is 

10 percent less than the 7Q10 calculated from the 

station data. The estimated 7Q10 value for Southington 

from the regression equation is 30 percent greater than 

the 7Q 10 calculated from the station data. 

Monthly Mean and Monthly Median 

Streamflow 

Monthly mean and monthly median streamflow 

(figs. SA and 5B) were calculated from the daily mean 

streamflow for the Wallingford gaging station for the 

years 1931-00 and for the Southington gaging station 

for the years 1988-00. Mean streamflow shows a 

maximum in March at the Wallingford gaging station 

of383 ft3/s (247 Mgalld) and at the Southington gaging 

station of 50.1 ft3/s (32.4 Mgalld) in response to precipitation, snowmelt, and soil-moisture content. 

Increasing evaporation and transpiration during the 

growing season contribute to a minimum mean flow at 

the Wallingford gaging station of 110 ft3

/s 

(71.1 Mgal/d) in August and at the Southington gaging 

station of 1g.1 ft3

/s (12.3 Mgal!d) in September. Mean 

streamflow generally is much larger than the median 

streamflow because very large streamflow values often 

associated with storm events affect the mean appreciably more than the median. 

Boxplots (figs. 6A and 6B) illustrate the central 

tendency and variability in monthly streamflow, and 

provide a graphical representation of distribution and 

range of the data. The top and bottom of the box represent the 75th and 25th percentiles, respectively, and 

bracket the central 50 percent of the flow data for a 

given month. The centerline splitting the box represents the median (50th percentile) and is defined as the 

median of all the daily discharges during the period of 

record for that month. The lines drawn from the ends of 

the box extend to the 1oth and 9oth percentiles of the 

data. Streamflows that are less than the 10th and greater 

than the goth percentiles d.efine and emphasize the 

extreme values. The largest 10 percent (for example, 

major flood events) and smallest 10 percent (for 

example, droughts) of the streamflow data values are 

not shown. 

From the boxplots (figs. 6A and 6B), it can be 

seen that August and September have similar flow 

characteristics at each station, as indicated by almost 

identical percentiles used to define the boxplots. From 

25 to 7 5 percent of the observed streamflows in August 

and September range from about 60 to 110 ft3

/s (3g to 

71 Mgalld) at the Wallingford gaging station, and from 

about g.o to 18 ft3

/s (5.8 to 12 Mgalld) at the Southington gaging station. 

Monthly streamflow percentiles used to create 

the boxplots (10-, 25-, 50-, 75-, 90-percentiles) and 

monthly minimum, maximum, and mean streamflows 

are listed in table 3. The 10th percentile indicates that 

90 percent of the streamflow equals or exceeds this 

value for a given month. Conversely, the goth percentile 

indicates that 10 percent of the streamflow equals or 

exceeds this value for a given month. These descriptive 

statistics summarize information about the distribution 

of streamflow in a given month for the period of record. 

The percentiles are reciprocals of flow durations. 

12 Streamflow in the Qulnnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
A. 

450 

c 

z 

0 400 

0 

w 

en 350 a: 

w 

a. 

t- 300 w 

w 

u. 

0 250 

m 

:::;) 

0 200 

~ 

~ 150 0 

_J 

u. 

~ 100 

< w 

a: 

t- 50 en 

WALLINGFORD 

- - c rronthty rood ian 

o rronthly rrean 

- ,...... 

- -

....- ....-

rr- r-- -

> 

< c 

250 a: 

w 

a. 

en 

z 

0 

200 _J 

_J 

< (!J 

z 

150 0 

:J 

_J 

~ 

~ 100 ~ 0 

_J 

u. 

~ 

50 < w 

a: 

ten 

0 ,•( _ __.__ 0 

JAN F83 fv\1\R AFR MA.Y JUN JUL AUG SEP OCT NOV DEC 

'---------------------------------------------

B. SOUTHINGTON 

60 > 

c < z 

0 

0 

w 50 

en 

a: 

w 

a. 

tij 40 

w 

-

....-

....- [!] rronthly rredian 

r- o rronthly rrean r35 c 

a: 

w 

a. 

30 en 

z 

0 

_J 

25 _J 

< 

(!) 

u. 

0 

m 30 

- - - -

z 

20 0 

:J 

;:) _J 

0 

~ 

~ 20 

0 

_J 

u. 

~ 10 < w 

a: 

ten 0 

r-- - .. -

I 

~ 

. 

L..J _LLJ .__LJ._ _.fl.· ~ .. 

15 :E 

~ 

10 ~ 0 

_J 

u. 

5 ~ 

< w 

a: 

t0 en 

JAN F83 MA.R APR WIAY JUN JUL AUG SEP OCT NOV DEC 

'-----------------------------

Figure 5. Monthly median and monthly mean streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 (A), 

and Quinnipiac River at Southington, Conn. (01195490), 1988-2000 (B). 

Streamflow Statistics 13 
A. 

0 

z 

8 600 

UJ 

(/) 

a: 

UJ 

a. 500 

1-

UJ 

UJ 

u. 

~ 400 

al 

:::> 

(.) 

~ 300 

i 

0 

_J 

u. 200 

:E 

~ 

UJ 

a: 

t;; 100 

B. 

0 

z 

0 

(.) 

UJ 

(/) 

a: 

UJ 

a. 

1-

UJ 

UJ 

u. 

~ 

m 

:::> 

(.) 

z 

i 

0 

_J 

u. 

:E 

~ 

UJ 

a: 

1-

(j) 

0 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

WALLINGFORD 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 

SOUTHINGTON 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

1Oth percentile 

Figure 6. Monthly streamflow, Quinnipiac River at Wallingford, Conn. (011 96500). 1931-2000 (A) and Quinnipiac River at 

Southington, Conn. (01195490), 1988-2000 (B) 

14 Streamflow in the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Table 3. Monthly streamflow in percentiles, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 and 

Ouinnipiac River at Southington, Conn. (01195490), 1988-2000 

(Percentiles were determined from daily mean flows using SPLUS software] 

Maximum 

90th- 75th- 50th- 25th- lOth- Minimum Daily Month (daily percentile 

mean) percentile percentile (median) percentile percentile (daily mean) mean 

U.S. Geological Survey station 01196500, Quinnipiac River at Wallingford, water years 1931-2000 

January 4,100 480 285 183 126 91 37 261 

February 3,290 481 322 215 142 105 36 273 

March 2,800 651 438 295 218 164 80 383 

April 3.360 630 410 277 199 152 80 358 

May 3,300 411 294 207 149 110 27 248 

June 7,210 306 193 128 96 76 40 185 

July 1,610 193 131 92 68 53 16 118 

August 3,160 190 112 79 60 43 13 110 

September 3,770 180 109 76 55 44 12 114 

October 3,350 250 136 85 61 48 10 134 

November 2,080 361 220 134 83.8 63 9 188 

December 2,620 437 270 172 110 76 17 230 

U.S. Geological Survey station 01195490, Quinnipiac River at Southington, water years 1988-2000 

January 374 67.8 41 28 19 14 7.7 39.4 

February 251 62.3 44 31 24 19 12 37.9 

March 348 90 51 37 30 24.2 15 50.1 

April 360 72.1 52 38 27 22 15 45.9 

May 313 73.8 45 31 22 18 12 42.1 

June 724 51 29 19 13 10 6.5 28.4 

July 230 36.6 22 14 9.6 7.4 4.7 20.9 

August 283 35.8 19 13 8.9 5.3 3.8 21.8 

September 704 30.1 18 11 8.5 6.3 3.9 19.1 

October 810 47.9 27 15 9.5 7.1 5.1 28.9 

November 289 62 37.3 24 14 11 7.9 33.3 

December 227 61.6 37 25 18.5 13 7.7 33.2 

Streamflow Statistics 15 
August Median Streamflow 

The August median flow, also called the aquatic 

base flow (ABF), is a streamflow statistic used by many 

Federal and State regulators as the minimum streamflow 

required for maintaining aquatic habitat in New England 

streams. The U.S. Fish and \Vildlife Service (USFWS) 

recommends computing the aquatic base flow as the 

median of all the August monthly means (U.S. Fish and 

Wildlife Service, 1981). Alternatively, Charles Ritzi and 

Associates (1987) computed the ABF as the median of 

the August daily mean flow. From a time-series plot 

(fig. 7) of August median flows from 1931 to 2000 at the 

Wallingford gaging station, it can be seen that the 

medians differ considerably from year to year. The 

August median flow ranged from a low of 25 ft3/s 

( 16 Mgal/d) in water year 1966 to a high of 218 ft3/s 

(141 Mgalld) in water year 1938. The extremes in 1966 

and 1938 August median streamflows reflect the end of 

multi-year drought period and a hurricane, respectively. 

The LOWESS smooth curve illustrates the general 

trends in the August median flow that have taken place 

since 1931. The smooth line shows the August median 

now dipped to the lowest point in 70 years during the 

multi-year drought in the 1960's. 

Cl 

z 

0 

() 

w 

(/) 

a: 

w 

a. 

... 

w 

w 

LL 

() 

iii 

::::> 

() 

~ 

~ 0 

..J 

LL 

z 

<t 

0 

w 

~ 

... 

(/) 

::::> 

(!) 

::::> 

<t 

..J 

<t 

::J 

z 

z 

<t 

250 I 

200 t150 r100 -

50 tI 0 

1930 

I I I 

-

n 

I 

1940 1950 1960 

The computed August median flow (table 4) may 

not be the appropriate flow for maintaining aquatic 

habitat as recommended by the USFWS because of the 

extent of regulation affecting streamflows. The USFWS 

states that the ABF should be determined from unregulated streamflow. The September median flow is shown 

to illustrate the similarity of the monthly averages during 

late summer when low-flow conditions are present. 

September flows are less than August flows. 

Table 4. Monthly median streamflows for August and 

September, Quinnipiac River at Wallingford, Conn. 

(01196500), 1931-2000 and Quinnipiac River at Southington, 

Conn. (01195490), 1988-2000 

[All values in cubic feet per second. USFWS, U.S. Fish and Wildlife Service] 

U.S. Geological 

Survey station 

Quinnipiac River at 

Wallingford 

(01196500) 

Quinnipiac River at 

Southington 

(01195490) 

I I 

Computed 

from 

Daily means 

Monthly 

means 

(USFWS) 

Daily means 

Monthly 

means 

(USFWS) 

I 

LOWESS smooth --

f-'" .... I-' 

---

~ 1970 1980 1990 

WATER YEAR 

August 

median 

I 

79.0 

90.1 

13.0 

20.7 

-

-

-

2000 

September 

median 

76.0 

89.1 

11.0 

12.4 

Figure 7. Trends in annual August median streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000. 

16 Streamflow in the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
STREAMFLOW TREND ANALYSIS AT THE 

WALLINGFORD GAGING STATION 

Statistical procedures were used to detect trends 

in the streamflow data and to evaluate how streamflow 

has changed over the entire streamflow range (annual 

minimum to annual maximum). Long-term streamflow 

records at the Wallingford streamflow-gaging station 

were examined for trends during the period 1931-00. 

Streamflow records at the Southington streamflowgaging station were not examined for trends because of 

the uncertainty of trend analysis on short periods (less 

than 30 years) of streamflow records. 

Trend Detection Methods 

The approach to assess trends was based on an 

approach used by Lins and Slack (1999) to assess 

national streamflow trends. Although the statistical 

methods are similar, the national study evaluated trends 

in streamflow in climate-sensitive streams with 

drainage basins with limited anthropogenic (humancaused) influences. Streamflow trends in the Quinnipiac River could be caused by flow diversions, 

wastewater discharges, flow regulation by dams and 

reservoirs, ground-water pumping, basin transfers, 

changes in land use and population, and climatic variability. 

Because of the large differences in streamflow 

characteristics between months and seasons, streamflow data were subdivided into 14 indices to represent 

different streamflow distributions. Streamflow from 

the same indices are compared and evaluated for trends 

over time. The indices are based on a range of percentiles from high to low flows. The range of percentiles 

evaluated for trends includes the annual maximum 

(lOoth_ ), 901h-, goth_, 701h-, 601h-, annual median 

csoth_ ), 40th-, 30th-, 2oth_, loth_, sth_, 2nd_, 1st_, and 

annual minimum (01h-) percentiles. The annual 

maximum and minimum percentiles represent the 

highest daily mean and lowest daily mean streamflow 

in a water year, respectively. 

Different time periods were evaluated to provide 

information on how characteristics of streamflow 

trends are affected by the length of record and to 

provide insight into regulation and natural variation of 

streamflow. Trend analyses were performed on time 

periods that were not heavily affected by droughts, 

which may bias the trend results. Trend tests were 

performed for 30-, 40-, 50-, 55-, 60-, 65- to 70-year 

periods, all ending in water year 2000, except for two 

30-yearperiods (1931-1960 and 1941-1970) that were 

studied as reference periods. 

A time-series plot of the annual minimum and 

maximum streamflow bracketing the annual mean and 

median flows was used to make preliminary inferences 

concerning streamflow over time (fig. 8). Statewide, 

multi-year droughts occurred from July 1929 to 

December 1932, from September 1940 to April1945, 

and from August 1961 to November 1971 (Weiss, 

1991). Scatterplots of streamflow percentiles were 

created and visually inspected for trends. To determine 

if the observed trends resulted from a chance arrangement of the data rather than from an actual change in 

streamflow, a statistical test (Mann-Kendall test) was 

performed. The underlying test is for correlation 

between two variables (streamflow and time). The 

correlation coefficient Kendalfs tau was used to 

measure the strength of association between streamflow and time. The Mann-Kendall test is a nonparametric trend test (Helsel and lllisch, 1992) and uses a 

rank-based procedure that tests for one-directional 

(monotonic) changes over time. The test examines 

whether streamflow is increasing or decreasing monotonically with time. 

Three statistical parameters-Kendall's tau, pvalue, and Sen slope--are used to summarize the 

results of the trend analysis. Kendall's tau ranges from 

-1 to + 1 and measures the strength of the trend. The 

strength of the trend increases as the value approaches 

its upper or lower limits; values less than 0 indicate an 

upward trend, and values greater than 0 indicate a 

down ward trend. If there is no correlation between 

streamflow and time, Kendall's tau equals 0. 

The second statistical parameter used to summarize the trend results is the p-value. The p-value 

measures the attained significance level of the statistical test (correlation) and is defined as the probability 

that a detected trend could have arisen by chance rather 

than from an actual change in streamflow. The p-value 

does not indicate the size or importance of a trend (for 

example, magnitude of the streamflow trend); rather, it 

indicates whether the Kendall's tau value has any 

significance. For this study, a p-value of 0.05 was 

selected as the critical attained significance level of the 

test. Where the p-value is less than or equal to 0.05, 

there is a less than or equal to 5-percent chance that the 

indicated significant trend actually is due to chance 

rather than from an actual change in streamflow, and 

the trend is than considered 95-percent reliable or 

"believable." The p-value can be statistically significant and the trend inconsequential at the same time. 

Streamflow Trend Analysis at the Wallingford Gaging Station 17 
0 

z 

0 

(.) 

w 

(J) 

a: 

w 

0.. 

tu 1,000 

w 

u. 

(.) 

ii5 

::I 

(.) 

z 

3" 

0 

..J 

u. 

:E 

<( 

w 

100 

I 

Drought 

/ 

J" 

' I I I \ 

I I 

II 

I I 

J 

\/ I I 

v 

Drought 

I I 

II 

I ~ 

1\ I \ 1\ r 

I I I I I" 

I ', I; I 

\I 

I 

~ ~ 

a: 

.... II ' 1\ I I /''I I • / I II ,... ~ I \I I I ' ~ 1 I 1

1 I '-, ; 4 I ,... ' I 1 ; \ 1 I I ' 1 \I 1 11 /"• I I 1 \ " I 1 I 1"' ',J 1 I \ 1 V 1

1 I 11 (J) J ~ 1 ( II \ ;'' I I I I \ I I I 11 1 I \ 1 1 

I I I ~"' ~~ V ~ -1 I I I Y I I v"" I I 

/

1 1

1

,,! I I I I { 

• I/ II 

I I I If 

I"'\ I II If 

I 1/ f I _, 

10 / 

1930 1940 1950 1960 1970 1980 1990 2000 

WATER YEAR 

EXPLANATION 

Annual maximum flow 

---- Annual mean flow 

Annual median flow 

Annual minimum flow 

Figure 8. Annual maximum, mean, median, and minimum streamflow, Ouinnipiac Rive r at Wallingford, Conn. (01196500), 

1931-2000. Shaded areas represent statewide, multi-year droughts. 

To ill ustrate central patterns in streamflow data 

as a function of time, a smooth line is added to the scatl rplot of streamflow fo r the range of percentiles 

analyzed fo r trends. The smoothing procedure used is 

LOWESS , locally weighted scatterpl ot smoothing 

( leveland , 1979). The smooth line is derived by the 

pattern of the data and indicates trend directions over 

the rime. A smoothing factor is used to control the fit of 

th urve to the data and ranges from 0 to 1. Smaller 

mooth ing factors result in less smoothing of the curve 

to the data. A smoothing factor of 0.5 was used for the 

fi nal fit. 

The Seasonal Kendall Slope Estimator (Sen 

lope) is used to estimate the magnitude of the trend. 

The Sen lope for streamflow is ex pressed as a change 

in treamflow per year, in cubic fe t per s cond. 

Alth ough the relation between streamflow and time 

usuall y is non linea r, a linear fu nction is used to represent the trend magnitude. The actual trend magnitude 

may differ significantly from the trend magnitude estimated from the Sen slope; particularly where anomal ies 

are near the beginning or the end of the period of record 

(Lins and Slack, 1999). The procedure for computing 

the Sen slope is described by Smith and others (1982). 

Trend analyses we re performed on records of 

prec ipitation and wastewater discharge to eva luate 

whether these factors affected trends in streamflow. 

The Mann-Kendall test was applied to monthly precipitati on and wastewater discharge records to determine 

the presence or absence of a trend in indi vidual months 

and seasons. A variation of the Mann -Kendall test, 

known as the seasonal Kendall test, was used to test fo r 

overall trends. The seasonal Kendall test statistic is 

determined by calculati ng the Mann-Kendall test 

statisti c for each month or season and combining the 

individual test statisti cs into a single test statistic. The 

18 Streamflow in the Qulnnlpiac River Basin, Connecticut- Statistics and Trends, 1931-2000 
single test statistic represents the overall trend. Where 

seasons consistently show the same trend pattern, trend 

results can be more significant by combining the individual test statistics into one overall test statistic. The 

Mann-Kendall test statistic can incorporate an upward 

trend in one season and a downward trend in another 

season that cancel each other, resulting in a seasonal 

Kendall test statistic that indicates no overall trend. 

Trends in Streamflow 

Trend test results, by percentile and by period of 

record, are summarized in table 5. Complete trendanalysis results for streamflow are shown in appendix 

l . Many similarities in trends are found across time 

periods, across percentiles, and with respect to direction. Increasing streamflows were detected predominantly in the lower half of the flow distribution (annual 

minimum to annual median) and the extreme upper half 

of the flow distribution (annual maximum flow), 

except during 1961-00, when upward trends were 

detected in only the upper half of the flow distribution. 

No decreasing flow trends were detected in any time 

periods or percentiles. Another important characteristic 

is that most trends represent only modest increases in 

streamflow. 

The most statistically significant trend detected 

(highest Kendall's tau value and lowest p-value) is an 

upward trend in the annual minimum (daily mean) 

streamflow at the Wallingford gaging station. The 

annual minimum streamflow coth percentile) increased 

by an average of 0.44 ft3 Is per year from 1931 to 2000 

(fig. 9). A LOWESS smooth line also shows an upward 

trend in the annual minimum flow since 1931 (fig. 9). 

The smooth line begins at its lowest point during the 

70-year period, which coincides with the lowest 

streamflow on record for the Quinnipiac River, then 

rapidly rises from 1931 until the early 1950's. From the 

early 1950's to 2000, the smooth line, which illustrates 

the relation between streamflow and time, continues to 

rise at a more gradual rate. 

Table 5. Trends for selected percentiles of streamflow between the annual minimum and annual maximum 

streamflows, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000 

[Kendall's tau values range from ·I to I; values less than 0 indicate an upward ttend, and values greater than 0 indicate a downward !rend; 

; p-value. attained significance level of the !rend test; p-values in bold and shaded lines arc at the attained significance level of less than or equal 

to 0.05; median is the median value for the percentile; Sen slope is the change in stteamflow per year, in cubic feet per second;<, less than] 

Percentile 

0 percentile 

(annual minimum daily mean) 

I st percentile 

2nd percentile 

5lh percentile 

lOth percentile 

20th percentile 

30th percentile 

40lh percentile 

50th percentile (median) 

lOOlh percentile 

(annual maximum daily mean) 

0 percentile 

(annual minimum daily mean) 

I st percentile 

2nd percentile 

!OOlh percentile 

(annual maximum daily mean) 

Kendall's tau p-value 

1931-2000 (drought 1929-32) 

0.357 <0.001 

.281 <.001 

.253 .002 

.201 .014 

.171 .037 

.151 .066 

.153 .062 

.145 .077 

.151 .066 

.253 .002 

1935-2000 

.280 <.001 

.194 .021 

.176 .037 

.215 .on 

Median 

40.0 

46.3 

50.2 

55.9 

65.0 

82.8 

102 

125 

!56 

1.780 

41.0 

46.8 

50.9 

1,860 

Sen slope 

0.436 

.331 

.313 

.263 

.235 

.250 

.358 

.444 

.533 

16.8 

.333 

.201 

.204 

15.7 

Streamflow Trend Analysis at the Wallingford Gaging Station 19 
Table 5. Trends for selected percentiles of streamflow between the annual minimum and annual maximum 

streamflows, Ouinnipiac River at Wallingford, Conn. (01196500), 1931-2000-Continued 

[Kendall's tau values range from · I to I; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend; 

; p-value, attained significance level of the trend test; p-values in bold and shaded lines are at the anained significance level of less than or equal 

to 0.05 ; median is the median value for the percentile; Sen slope is the change in streamflow per year, in cubic feet per second;<, less than) 

Percentile 

0 percentile 

(annual minimum daily mean) 

5th percentile 

lOth percentile 

20th percentile 

30th percentile 

40th percentile 

50th percentile (median) 

90th percentile 

IOOth percentile 

(annual maximum daily mean) 

30th percenti 1e 

lOOth percentile 

(annual maximum daily mean) 

I st percentile 

2°d percentile 

5th percentile 

30th percentile 

I ooth percentile 

(annual maximum daily mean) 

30th percentile 

40th percentile 

50th percentile (median) 

60th percentile 

70th percentile 

goth percentile 

90th percentile 

No trends detected 

Kendall's tau p-value 

1941-2000(drought1940-45) 

0.167 0.060 

.159 

.193 

.200 

.214 

.189 

.179 

.156 

.241 

.158 

0.247 

1945-2000 

1951-2000 

.179 

.181 

.162 

.166 

.168 

.073 

.030 

.024 

.016 

.034 

.044 

.078 

.007 

.086 

0.007 

.068 

.064 

.099 

.091 

.086 

1961-2000(drought 1961-69) 

.222 .045 

.203 .067 

.219 .048 

.227 .040 

.212 .056 

.221 .046 

.245 .027 

1971-2000 

Median 

43.0 

57.2 

65.0 

81.9 

100 

123 

158 

418 

1,860 

105 

1,980 

48.3 

52.2 

59.1 

105 

2,100 

109 

132 

160 

194 

232 

285 

414 

20 Streamflow In the Qulnnlplac River Basin, Connecticut-Statistics and Trends, 1931-2000 

Sen slope 

0.202 

.250 

.314 

.391 

.530 

.650 

.772 

1.84 

18.5 

.441 

20.8 

.233 

.272 

.289 

.512 

16.8 

.912 

1.18 

1.53 

1.71 

1.85 

2.53 

4.42 
Although the annual minimum flow trend was 

the most statistically significant trend, it was not as 

significant in the shorter time periods. The annual 

Injnimum streamflow increased by an average of 0.44 

ft Is per year from 1931 to 2000 (p-value less than 

0.001 and Kendall's tau of0.357) as compared to 0.20 

ft 3/s per year from 1941-00 (p-value of 0.060 and 

Kendall's tau of0.167). Exceptionally low streamflows 

were recorded through the 1930's that largely affect the 

magnitude of the annual minimum streamflow trend. 

The unusually low flows of the 1930's are an anomaly 

for the period of record. After the drought of July 1929 

to December 1932 (recurrence interval of more than 

25 years), the annual minimum streamflow remained 

exceptionally low through 1938. The low streamflows 

during that time period probably are caused by the 

extensive flow regulation. No trends were detected in 

the annual minimum streamflow for the 30- or 40-year 

periods ( 1961-00 and 1971-00) at the Wallingford 

gaging station. 

The second most statistically significant trend 

detected is an upward trend in the annual maximum 

streamflow (fig. 1 0). Upward trends also were detected 

in the annual median streamflow (fig. 11). The annual 

maximum (1001

h percentile) streamflow increased by 

an average of 16.8 ft3

/s per year, and annual median 

(50th percentile) streamflow increased by an average of 

0.53 ft3 /s per year during 1931-00. Statistically significant upward trends in the annual maximum daily mean 

streamflow were detected during the 60- and 70-year 

periods. The increasing streamflow trend was detected 

at the 90th percentile (high flow) but not the 1001

h 

percentile during the 40-year period; the trend was 

nearly significant (p-value = 0.086) during the 50-year 

period. No trends were detected in the annual 

maximum streamflow during 1971-00. Land-cover 

changes from pervious to impervious surface, expansion of the storm-drainage systems, and increases in 

rainfall intensity probably result in increases in annual 

maximum streamflow. 

Statistically significant uRward trends in the 

annual median streamflow (501 percentile) were 

detected during 1941-00 and 1961-00. The upward 

trend in the annual median streamflow nearly was 

significant (p-value = 0.066) during 1931-00. No 

trends were detected in the annual median streamflow 

in the non-drought beginning time periods ( 1935-00, 

1945--00, 1951-00 and 1971-00). Severe droughts at 

the beginning of the time period analyzed probably can 

result in the detection of an increase in streamflow. The 

LOWESS smooth line of the annual median streamflow pattern begins at its lowest point, which coincides 

with the 1930's drought, then gradually rises until the 

early 1950's and dips during the 1960's drought (fig. 

11 ). The smooth line rises from 1960 to 1980, then 

gradually falls from approximately 1982 until 2000. 

100rT--~--.---~-.--~--~--~~--~--~--~~--~----

z 

0 

0 

w 

en 

a: 

~ 80 

t 

w 

LL. 

0 

iD 60 ::> 

0 

~ 

~ 0 

...J 40 LL. 

::!: 

c:s: 

w 

a: 

.... 

en 

~ 20 ::> 

~ 

z 

~ 

0 

Annual mimmum flow= 0.44 • {water year- 1965.5) + 40 

[approximate increase of 0.4 cubic feet per second per year) 

p-value = 0.00001 

Kendall's tau = 0.35 

LOWESS smooth --

Sen slope 

... -- ... ·--· ... -- ..... -- .. -- ---- -< 

.. ----- ---- ...... --~· 

---

1930 1940 1950 1960 1970 1980 1990 2000 

WATER YEAR 

Figure 9. Trends in annual minimum streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 1931-2000. 

Streamflow Trend Analysis at the Wallingford Gaging Station 21 
0 

z 

0 u 8,000,-,,------,-------,----.----------,------,-----,---, w 

Ill 

a: 

~ 7,000 

Iii 

w 

u.. 

u 6,000 -

iii 

::::l 

u 

~ 

:i 

0 

5,000 

ii 4,000 -

::E 

~ 

w 

a: 3,000 -

!;; 

::E 

~ 2,000 -

x 

~ 

Annual maximum flow= 16.7 ·rwaler year -1965.5) + 1775 

[approximate increase of 17 cubic feet per second per year) 

p-value = 0.002 

Kendall's tau= 0.25 

::E .. ..J 1,000 -

~ 

::::l 

z 

LOWESS smooth - -

• Sen slope 

~ 0 -------~-------L------~--------L-------~-------L------~~ 1930 1940 1950 1960 1970 1980 1990 2000 

WATER YEAR 

Figure 10. Trends in annual maximum streamfl ow, Ouinnipiac River at Wallingford, Conn. (01196500), 1931-2000. 

300 

0 Annual median flow= 0.53 · [water year . 1965.5) + 156.25 LOWESS smooth-- z [approximate increase of 0.5 cubic teet per second per year) 0 Sen slope 

u p-value = 0.06 w 

Ill 250 Kendall's tau = 0.15 

a: 

w 

a. 

Iii 

w 200 u.. 

u 

iii _....._ __ ::::l --- u .. ~ 150 

0 . . ..J 

u.. .. ::E 100 ~ 

w 

a: 

1-

Ill 

::E 50 ::::l 

::E 

z 

~ 

0 

1930 1940 1950 1960 1970 1980 1990 2000 

WATER YEAR 

Figure 11. Trends in annual median streamflow, Quinnipiac River at Wallingford, Conn. (01196500), 1931 - 2000. 

22 Streamflow in the Quinnipiac River Basin, ConnE!ctlcut-Statistics and Trends, 1931-2000 
The highest number of statistically significant 

trends (p-value <= 0.05) over all the percentiles is seen 

during the 60- and 70-year periods (1941-DO and 

1931-00). A high number of trends also is evident 

during the 40-year period (1961-00) and 50-year 

period (1951-DO). No statistically significant trends 

were detected during the most recent 30-year period 

(1971-00). For the 60- and 70-year periods (1941-DO 

and 1931-DO), trends are detected in the low to middle 

range coth to soth percentile) and the uppermost percentile (lOOth percentile). The trend pattern changes from 

the lower half to the upper half of the streamflowpercentile distribution during the most recent 40-year 

period (1961-00). Statistically significant trends 

(p-value <= 0.05 and Kendalrs tau value> 0.2) are 

detected in the middle to upper percentile range (30th to 

9oth percentile) during 1961-DO. No statistically significant trends were detected in the 6oth to 90th percentiles 

for the periods analyzed, except during 1961-DO. 

Trend-analysis results are sensitive to the 

extreme climatic anomalies near the beginning and the 

end of the record. Statewide droughts, from July 1929 

to December 1932, from September 1940 to April 

1945, and from August 1961 to November 1971 

(Weiss, 1991), can affect the trend-analysis results for 

the 70-, 60-, and 40-year periods (1931-DO, 1941-DO, 

and 1961-DO). Because the results are sensitive to 

extreme climatic anomalies early or late in the analysis 

period, a second series of trend analyses were 

performed on non-drought beginning periods: 1935-

00, 1945-00, 1951-DO, and 1971-DO. 

Fewer increasing streamflow trends were 

detected across the non-drought beginning time periods 

than in the drought beginning time periods. Also, the 

magnitude of the trends was much lower and the 

attained significance of the statistical test was higher 

for the non-drought beginning periods. For example, 

the magnitude of the trend at the 30th percentile is 

higher and the attained significance of the trend test is 

more significant for the time periods beginning in a 

drought (1941-DO-Kendall's tau of0.214 and p-value 

equal to 0.016; 1961-00-Kendall's tau of 0.222 and 

p-value equal to 0.045) than the time period not beginning in a drought (1951-DO-Kendall's tau of 0.166 

and p-value = 0.091). A number of trends were 

detected with an attained significance level slightly 

greater than 0.05 (0.06 to 0.1) in the lower half and 

extreme upper range of the flow distribution, and 

across time periods (droughts and non-drought beginnings). Trends may be possible at significance levels 

greater than 0.05. 

The trend-analysis results only reflect streamflow patterns detected at the Wallingford streamflowgaging station. The station is approximately 14 mi from 

Long Island Sound and receives runoff and ground 

water from 70 percent of the basin. The accuracy of 

transferring flow characteristics or trend results 

upstream or downstream from the station was not evaluated for this study. Diversions, return flows, other 

regulation and natural changes in streamflow need to 

be interpreted before flow characteristics can be transferred to another location on the river. 

Factors That Affect Trends in Streamflow 

Factors that may affect streamflow trends 

include (1) climatic variability, such as precipitation, 

and (2) human influences, such as wastewater 

discharges, streamflow diversions, interbasins transfers, consumptive uses, and land-cover changes, such 

as urbanization. Generally, streamflow diversions, outof-basin transfers, and consumptive uses decrease low 

streamflow. Urbanization may cause an increase in 

runoff and flooding and a decrease in low streamflow. 

Relations between streamflow trends and trends in 

precipitation and municipal wastewater discharges 

were examined to determine the effects of precipitation 

and wastewater discharge on streamflow. Analyses of 

the effects of human influences other than wastewater 

discharges are not included in this study. 

Precipitation 

Precipitation data were analyzed to evaluate 

whether streamflow trends on the Quinnipiac River 

might be related to precipitation. Regional precipitation data were retrieved from the National Oceanic and 

Atmospheric Administration (NOAA), National 

Climatic Data Center database for the period 1930-00 

(data accessed on November 26, 2001 on the World 

Wide Web at URL http://lwf.ncdc.noaa.gov/oal 

climate/climatedata.html). A precipitation dataset, both 

spatial and temporal, was compiled from five rain 

gages in south-central Connecticut (fig. 1; appendix 2). 

Four different precipitation indices were evaluated for 

trends: monthly total, annual total, annual median, and 

annual mean. Monthly precipitation data from five rain 

gages were averaged within each year to produce 

annual mean and annual median values, and summed 

Streamflow Trend Analysis at the Wallingford Gaging Station 23 
each year to produce annual totals. When monthly data 

for one or more of the five rain gages were missing, the 

monthly average was based on the number of rain 

gages with data. A time-series plot of total annual 

precipitation as the arithmetic average of the five rain 

gages is shown in figure 12. Monthly precipitation 

totals were compiled by averaging data from two rain 

80 

CJ) 70 

w 

J: 

(.) 

~ 

~ 60 

z 

0 

i= 

~ 50 - __. t- :.- 0:: 

(3 - w 

a: 

a. 40 ...J 

<t 

t0 

t- 30 ...J 

~ 

:::> 

z 

z 

~ 20 

10 

1930 1940 1950 1960 

gages (NOAA station numbers 65077 and 64767) with 

nearly complete record. These two rain gages used to 

analyze monthly precipitation trends are missing less 

than 1 percent of the rainfall data for the time period 

analyzed. A Kendall tau test was performed to detect 

trends in precipitation. 

LOWESS smooth 

-

1970 1980 1990 2000 

CALENDAR YEAR 

Figure 12. Annual total precipitation for five rain gages in and near the Ouinnipiac River Basin, Conn., and LOWESS 

smooth line, 1930-2000. 

24 Streamflow In the Qulnnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
The trend-analysis results on precipitation indicated that no significant trends are present at the 95-

percent significance level (p-value <= 0.05). The 

monthly precipitation total, annual precipitation total, 

and annual average (median and mean) precipitation 

totals show no statistical evidence of upward or downward trends during 1930-00. From this analysis, no 

evidence is available to indicate that precipitation may 

be causing the increase in the annual minimum flow at 

the Wallingford gaging station. 

A national research study on precipitation trends 

indicates that extreme precipitation events, such as the 

1-day rainfall intensity for coastal areas, are increasing 

(Karl and Knight, 1998). The magnitude of the 

maximum streamflow is a function of rainfall intensity. 

An increase in rainfall intensity as indicated in the 

national study of extreme precipitation events lends 

support to an upward trend detected in the annual 

maximum streamflow from 1931 to 2000. 

Wastewater Discharges 

Wastewater discharges can augment streamflow. 

Of the 19 permitted wastewater discharges in the basin, 

12 are· upstream from the Wallingford gaging station 

with a combined maximum flow of 25 Mgal!d 

(38.7 fr3/s). Three of the 12 permitted discharges are 

municipal wastewater-treatment facilities in Meriden, 

Cheshire, and Southington. Collectively, these three 

municipal wastewater-treatment facilities account for 

92 percent of the permitted discharges (upstream from 

the Wallingford gaging station) into the Quinnipiac 

River with a combined maximum daily flow of 

22.9 Mgal/d (35.5 ft3/s) and an average daily flow of 

15.8 Mgal!d (24.5 ft3/s). The combined wastewater 

discharges above the Wallingford streamflow-gaging 

station represent about 33 percent of the monthly 

median streamflow in August at the station. Data from 

the three municipal wastewater-treatment facilities 

were analyzed for trends because of the large 

percentage of streamflow that these permitted 

discharges represent. The other nine permitted wastewater discharges (upstream from the Wallingford 

gaging station) that make up the remaining 8 percent of 

permitted discharges have a combined maximum daily 

flow of2.1 Mgal/d (3.2 ft3

/s) and were not included in 

the trend analysis. 

To further understand the trend in annual 

minimum streamflow, the annual minimum streamflow 

was plotted for the Southington and Wallingford 

stations for coincident time periods (fig. 13). (The 

Southington station is upstream from the three municipal wastewater-treatment facilities.) Streamflow at the 

Southington station is regulated by 15 industrial and 

public-water diversions with a combined maximum 

daily withdrawal of 13.2 Mgalld, and 4 permitted 

surface-water discharges with a combined maximum 

daily flow of 0.732 Mgalld (Quinnipiac River Watershed Partnership, 2000). The Southington station 

would be expected to yield higher natural flows during 

low-flow conditions than the Wallingford station 

because the percentage of coarse-grained deposits (as 

percent of total drainage area) is higher at Southington 

than at Wallingford. A comparison of annual minimum 

streamflow at the two stations since 1988 shows that 

for the annual minimum streamflow, the yield per 

square mile of basin generally is lower at Southington 

than at Wallingford. Wastewater discharge may be 

increasing the annual minimum streamflow at the Wallingford station, and diversions may be reducing the 

annual minimum flow at Southington. 

Wastewater records were obtained from each 

town's Water Pollution Control Department. Records 

were available in electronic form from 1985 to 2001 

( 17 years) for Meriden and Cheshire and from 1983 to 

2001 (19 years) for Southington. Monthly total 

discharge data were available from the Meriden and 

Southington facilities; daily average discharge was 

available for the Cheshire facility. A boxplot of wastewater discharge from the Meriden wastewater-treatment facility shows monthly and seasonal variability in 

the wastewater discharge data (fig. 14). A Kendall tau 

statistical analysis was used to detect statistically 

significant trends in monthly wastewater discharges. A 

p-value less than or equal to 0.05 was selected to indicate the presence of a statistically significant trend. 

Streamflow Trend Analysis at the Wallingford Gaging Station 25 
c 

z 

0 

(.) 

w 

II) 

a: 0.6 

w 

a.w 

1-..J 

~~ u.w 

~~ Cll:J 

;::) 0 0.4 

Ucn 

~a: ~w 

;:a. 

0 

..J 

u. 

:E 

<t: 0.2 

w 

a: 

Ill) 

Wallingford 

Southington 

' . 

. · .... ;... .... 

Southington •• : •,. . . . I I 

· ...... 

I 

. 

. 

0.0 '---..L.--~-...L--......1....-.......1...-.....L..-........L---1.----IL.---"----...1.--...L--.....J....-.....1 

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 

WATER YEAR 

Figure 13. Annual minimum streamflow at two U.S. Geological Survey streamflow-gaging stations (Quinnipiac River at 

Wallingford, Conn.-01196500 and Quinnipiac River at Southington, Conn.-01195400), 1988-2000. 

[Wallingford station has 39 percent coarse-grained deposits, and Southington has 51 percent coarse-grained deposits.] 

J: 

...,_ 

-z 

wo 

~:E <(CX: 

J:W 

()C. 

en en 

cz 

a:O w...J ..,_...J 

<(<( 

3:~ 

wz ._a 

en- <t:J 

3::;E 

~ 

700 

600 

500 

400 

300 

200 

100 

0 

~8 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

1Oth percentile 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 

Figure 14. Monthly total permitted wastewater discharges into the Quinnipiac River from the wastewater-treatment facility 

in Meriden, Conn., 1985-2001. 

26 Streamflow in the Quinniplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Selected trends in wastewater discharges are 

summarized in table 6. Complete trend-analysis results 

for wastewater discharge are shown in appendix 3. 

Upward trends were detected at all three municipal 

wastewater-treatment facilities (fig. 15). The overall 

upward trends were 2.23 Mgal/month/year (about 

2 ft3

/s) at Meriden; 1.18 Mgal/monthlyear (about 

1.2 ft3

/s) at Southington; and an average 0.04 

Mgal/d/year (about 1 ft3/s) at Cheshire. Trend-analysis 

results by month for Meriden showed statistically 

significant trends in February and March. No trends 

were detected in summer or early fall during low-flow 

conditions. Trend-analysis results by month for 

Cheshire showed statistically significant trends in all 

months except December and August. Trend-analysis 

results for Southington showed statistically significant 

trends in January, February, and March. 

The population of the Meriden, Cheshire, and 

Southington has increased by about 9.3 percent 

between 1980 (115,785) and 2000 (126,515), and by 

148 percent between 1930 (50,981) and 2000 

(126,515) (data accessed on April19, 2002, on the 

World Wide Web at URLs 

http:/ /www.sots. state.ct.us/RegisterManual/ 

Section VII/Population 1900 .htm and 

http:/ /www.sots. state.ct. us/RegisterManual/ 

Section VIIJPopulation 1970.htm). Population increases 

in urban and suburban areas generally cause increases 

in water use, water consumption, and wastewater. In 

recent years, many water-conservation practices (such 

as modem plumbing fixtures and educational programs 

related to conservation) were implemented to reduce 

the volume of wastewater generated~ however, the 

wastewater data (fig. 15) and trend-analysis results 

indicate that wastewater discharge increased from 1985 

to 2000. A trend analysis of water use and consumption 

was beyond the scope of this study. 

Tabte 6. Trends in permitted wastewater discharges at selected wastewater-treatment facilities, Quinnipiac River 

Basin, Conn. 

[Kendall's tau values range from -I to I; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend; 

p-value, attained significance level of the trend test; p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05; median is the median value for the percentile; Sen slope is the change in wastewater discharge per year;<, less than] 

Trend Kendall's tau P-value Median Sen slope 

Meriden wastewater-treatment facility (1985-Ul; units are total million gallons per month) 

Overall trend (seasonal Kendall) 0.178 0.040 264 2.23 

February .456 .012 294 6.86 

March .529 .003 370 11.7 

Cheshire wastewater-treatment facility (1985-01; units are average million gallons per day) 

Overall trend (seasonal Kendall) .477 <.001 1.93 .046 

January .684 <.001 2.18 .065 

February .691 <.001 1.99 .075 

March .603 <.001 2.15 .080 

April .485 .007 2.23 .086 

May .441 .015 1.99 .054 

June .463 .010 1.85 .055 

July .485 .007 1.65 .038 

August .353 .052 1.72 .027 

.September .463 .010 1.77 .031 

October .382 .036 1.84 .035 

November .375 .039 1.91 .030 

Southington wastewater-treatment facility (1983-01; units are total million gallons per month) 

Overall trend (seasonal Kendall) .205 <.001 117 1.18 

January .368 .030 124 2.73 

February .380 .025 113 1.76 

March .555 .001 141 2.99 

Streamflow Trend Analysis at the Wallingford Gaging Station 27 
700 r-----------------------------------------------~ 

600 

u.i 

(!) 500 

Regression line Y= 0.007x + 56.4 

.. .. 

" 

4 

a::tJ) 

~z :I:o 

()..J 

~..J 

c~ 

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..Jz 

400 

'&.. 4 

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i=2 300 " ..... 4... ... . .. \ .... " .. " .. .. + .... " 4 • • 4r 

Z..J 

0::::! 

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.... 

200 

100 

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0> 0> 0> O'l 0> O'l 0> 0 0 0 0> 0> 0> O'l O'l O'l 0> 0 0 0 0 ...- ...- T"" ...- C\J C\1 C\J C\J 

CALENDAR YEAR 

Figure 15. Trends in wastewater discharges to the Quinnipiac River from the wastewater-treatment facility in 

Meriden, Conn., 1985-2001. 

From the wastewater-discharge trend analysis, 

evidence indicates that wastewater discharge may be 

increasing streamflow at the Wallingford gaging 

station. Both the LOWESS line of the annual minimum 

streamflows and Sen slope of annual minimum streamflows are consistent with an increase in wastewater 

discharges. The combined daily average wastewater 

discharge from the three facilities increased by about 4 

ft 3/s (2.5 Mgalld) during 1985-01. Because the Sen 

slope from 1931 to 2000 is affected by the unusually 

low flows in the 1930's, the Sen slope from 1941 to 

2000 is used to interpret the relation between trends in 

wastewater discharge and the annual minimum streamflow. The LOWESS line shows an increase in the 

annual minimum flow of about 2 ft3/s (1.3 Mgal/d) and 

the Sen slope (from 1941 to 2000) shows an increase in 

the annual minimum flow of about 3 ft3/s (1.9 Mgalld) 

at the gaging station during 1985-00. 

An increase in the quantity of wastewater 

discharge may not necessarily cause an increase in 

downstream flow, particularly on a regulated river or 

where the distance between the release point and study 

point is considerable (miles). Other factors affecting 

low flow, such as water withdrawals and land-use 

changes, were not evaluated and cannot confirm or 

refute the possibility that increases in the annual 

minimum streamflow primarily are related to the 

increases in wastewater discharges. The storage 

capacity of the channel reach between the wastewatertreatment facilities and the gaging station, particularly 

the Community Lake reach, has not been analyzed and 

can be an important factor in controlling low-streamflow characteristics. Additional investigation on water 

withdrawals, land-use changes, and the storage 

capacity of the channel reach would be necessary to 

better understand the effects of wastewater discharges 

on streamflow trends. 

28 Streamflow in the Quinnlplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
SUMMARY 

The Quinnipiac River is an important resource in 

south-central Connecticut. In response to water allocation and streamflow concerns in the basin, the USGS, 

in cooperation with the Connecticut DEP, began a 

study in 2000 to compute current streamflow statistics 

and analyze historical streamflow data for trends to 

document long-term changes. Streamflow statistics for 

two continuous-record streamflow-gaging stations on 

the Quinnipiac River (Wallingford-0 1196500 and 

Southington-01195490) were updated using standard 

USGS methods and computer software to estimate 

statistics for flow duration, low-flow frequency, annual 

average streamflow, and monthly average streamflow. 

The flow statistics were estimated from an analysis of 

the historical streamflow record. Because the streamflow is affected by diversions, wastewater discharges, 

reservoirs, ground-water pumping, basin transfers, and 

changes in land use and population, the streamflow 

statistics reflect regulated flow and not the natural flow 

of the river. 

Low-flow frequency estimates from station data 

varied by 10 percent at Wallingford and by 30 percent 

at Southington from the low-flow frequency estimates 

from a regression equation. The 7Q10 for Wallingford 

from the regression equation is 1 0-percent less than the 

7Q10 calculated from the station data, indicating that 

regulation (release of wastewater discharges) may be 

increasing the flow at Wallingford. The 7Q10 for 

Southington from the regression equation is 30-percent 

greater than the 7Ql0 value calculated from the station 

data, indicating that diversions may be reducing flow at 

Southington. 

Streamflow trends were evaluated at the longterm gaging station in Wallingford using the non-parametric Mann-Kendall test. Trends were computed for a 

range of percentiles of streamflow, from the annual 

minimum to annual maximum, and for a range of time 

periods, from 30 to 70 years. Trend results indicate that 

the annual minimum and annual maximum streamflow 

increased by 0.44 ft3

/s and 17 ft3/s per year, respectively, for the Wallingford gaging station during the 

period of record (1931-00). The highest percentage of 

trends were detected in the 40-, 60- and 70-year periods 

(1961-00, 1941-00 and 1931-00, respectively). 

Increases in the annual maximum streamflow were 

conunon during all time periods, except during 1971-

00, when no streamflow trends were detected. No 

downward trends were detected for any time periods or 

percentiles. Significant streamflow trends were 

common in the lower half of the flow distribution 

ranging from the annual minimum to annual median 

(Olh to 50th percentile) for periods longer than 40 years. 

During 1961-00, streamflow trends were detected in 

the middle to upper half of the flow distribution (30th to 

90th percentile). 

Records of precipitation and wastewater 

discharges were analyzed to provide insight into 

streamflow trends. Trend-analysis results of precipitation from rain gages in and near the basin indicate no 

trends in annual or monthly precipitation totals during 

1930-00. The trend-analysis results for precipitation 

could not directly lend support to an upward trend in 

the annual minimum streamflow. A national research 

study on precipitation trends indicate increases in the 

rainfall intensity for Connecticut in the 20th century. 

Increases in rainfall intensity lends support to an 

upward trend in the annual maximum streamflow. 

Increases in residential and commercial development 

and expansion of storm-drainage systems may be 

causes of an upward trend in the annual maximum 

streamflow. 

Wastewater discharges can constitute a substantial percentage of streamflow in August and September 

during low-flow conditions and during periodic 

droughts. A trend analysis of wastewater discharges for 

municipal wastewater-treatment facilities in Meriden, 

Cheshire, and Southington indicates an increase in 

wastewater discharge at each municipal wastewatertreatment facility during 1985-01. Results of the trend 

analyses provide evidence that wastewater discharge is 

increasing at a rate consistent with the annual minimum 

streamflow trend. Wastewater discharge increased by 

about 4 ft3 Is and the annual minimum streamflow at the 

gaging station in Wallingford increased by approximately 2-3 ft3

/s. 

Analyses of precipitation and wastewater 

discharge provide general insight to what extent these 

trends have on the streamflow. Abnormally low flows 

were recorded from 1932 to 1938 when annual precipitation totals were normal to above-normal. Graphical 

evidence suggests that regulation affected streamflow 

in the early period of record (1930's). Conversely, the 

recurrence of droughts in the early period (1930's, 

1940's and 1960's) and above-normal precipitation in 

the 1970's indicates that the upward trends in streamflow (annual minimum to annual median) may, in part, 

SUMMARY 29 
be a result of climatic variability. The magnitude of the 

trend in the annual minimum streamflow from 1931 to 

2000 at the Wallingford gaging station appears to 

reflect both regulation and periodic droughts in the 

early record. The extent that regulation and droughts 

affect the magnitude of the trends could not be determined readily and was outside the scope of the study. 

Wastewater discharges appear to be affecting some 

streamflow trends. Increases in both the wastewater 

discharges and the annual minimum streamflow seem 

to indicate that the annual minimum streamflow at the 

Wallingford gaging station has been augmented by 

upstream wastewater discharges. 

The streamflow statistics and trend-analysis 

results presented here provide a limited understanding 

and assessment of the water quantity in the Quinnipiac 

River; however, many uncertainties are associated with 

factors causing streamflow trends. A watershed-simulation model could, for example, provide valuable 

information about the effects of the withdrawals and 

discharges on streamflow. Both natural effects and 

human influences that affect streamflow trends need 

further investigation to understand the effects of regulation and climate on the Quinnipiac River. 

REFERENCES CITED 

Cervione, M.A., Jr., 1982, A method for estimating the 7-

day, 10-year low flow of streams in Connecticut: Connecticut Water Resources Bulletin 34, p. 17 p. 

Cleveland, W.S., 1979, Robust locally weighted regression 

and smoothing scatterplots: Journal of American Statistical Association, v. 74, no. 368, p. 829-836. 

Connecticut Department of Environmental Protection, 1997, 

Connecticut water quality standards and criteria: Hartford, Conn., Inland Water Resources Division, Connecticut Department of Environmental Protection, 46 p. 

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: Studies in Environmental Science 49, 

New York, Elsevier Science Publishers, 529 p. 

Karl, T.R., and Knight, R.W., 1998, Secular trends of precipitation amount, frequency, and intensity in the USA: 

Bulletin of the American Meteorological Society, v. 79, 

p. 231-241. 

Lins, H.F., and Slack, J.R., 1999, Streamflow trends in the 

United States: Geophysical Research Letters, v. 26, no. 

2, p. 227-230. 

Mazzaferro, D.L., Handman, E.H., and Thomas, M.P., 1979, 

Water resources inventory of Connecticut, part 8, Quinnipiac River Basin: Connecticut Water Resources Bulletin 27, 88 p. 

Quinnipiac River Watershed Partnership, 2000, Preliminary 

assessment of water withdrawals and streamflows in 

the Quinnipiac River watershed: Hartford, Conn., 

Quinnipiac River Watershed Partnership Work Group 

Report, July 2000, Hartford, Conn., 37 p. 

Ries, K.G., Ill, 2000, Methods for estimating low-flow statistics for Massachusetts Streams: U.S. Geological Survey Water-Supply Paper 00-4135, p. 7-9. 

Ritzi, Charles, and Associates, 1987, Computation of 

USFWS aquatic base flow for regulated streams: Winthrop, Maine, 15 p. 

Searcy, J.K., 1959, Flow-duration curves, Manual of hydrology-Part 2, Low-flow techniques: U.S. Geological 

Survey Water-Supply Paper 1542-A, 33 p. 

Smith, R.A., Hirsch, R.N., and Slack, J.R., 1982, A study of 

trends in total phosphorus at NASQAN stations: U.S. 

Geological Survey Water-Supply Paper 2190, 34 p. 

Thomas, M.P., 1966, Effect of glacial geology upon the time 

distribution of streamflow in eastern and southern Connecticut: U.S. Geological Survey Professional Paper 

550-B, p. B209-B212. 

U.S. Fish and Wildlife Service, 1981, Interim regional policy 

for New England streamflow recommendations: Concord, N.H.,U.S. Fish and Wildlife Service, Region 1, 

3 p. 

Weiss, L.A., 1983, Evaluation and design of a streamflowdata network for Connecticut: Connecticut Water 

Resources Bulletin 36, 30 p. 

--1991, Connecticut floods and droughts, with a section on "water management" by C.J. Hughes, in Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, 

D.W. (compilers), National Water Summary 1988-

89-Hydrologic events and floods and droughts: U.S. 

Geological Survey Water-Supply Paper 2375, 

p. 215-222. 

30 Streamflow in the Qulnnipiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
APPENDIXES 

Appendixes 31 

Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum 

and annual maximum, Quinnipiac River at Wallingford, Conn., 1931-2000 

[Kendall's tau val ues range from -Ito I; val ues less than 0 indicate an upward trend, and values grea ter than 0 indicate a downward trend 

p-value, attained significance level of the trend test; p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05; median is the median value for the percentile; Sen slope is the change in streamflow per year, in cubic feet per second; <.less than] 

Percentile Kendall's tau p-value Median Sen slope 

1931-2000 (drought 1929-32) 

0 percentile 0.357 <0.001 40.0 0.436 

(annual minimum daily mean) 

I 51 percentile .281 <.001 46.3 .331 

211d percentile .253 .002 50.2 .313 

5th percentile .201 .014 55.9 .263 

101

h percentile .171 .037 65.0 .235 

20th percentile .151 .066 82.8 .250 

30th percentile .153 .062 102 .358 

40th percentile .145 .077 125 .444 

50th percentile (median) .151 .066 156 .533 

60th percentile .127 .12 1 192 .518 

70th percentile .093 .256 232 .467 

80th percentile .098 .232 292 .583 

90th percentile .133 .105 417 1.21 

I 00111 percentile .253 .002 1,780 16.8 

(annual maximum daily mean) 

1935-2000 

0 percentile .280 <.001 41.0 .333 

(annual minimum daily mean) 

I 51 percentile .194 .021 46.8 .201 

2nd percentile .176 .037 50.9 .204 

5th percentile .136 .107 58.0 .200 

101

h percentile .138 .104 65 .7 .200 

201

h percentile .117 .165 84.3 .205 

30th percentile .117 .168 103 .286 

40th percentile .094 .268 129 .338 

50th percentile (median) .092 .276 160 .333 

60th percentile .073 .388 195 .300 

70th percentile .044 .607 236 .238 

80th percentile .051 .550 299 .324 

90th percentile .103 .223 419 1.02 

I oolh percentile .215 .011 1,860 15.7 

(annual maximum daily mean) 

Appendix 1 33 
Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum 

and annual maximum, Quinnipiac River at Wallingford, Conn., 1931-2000-Continued 

[Kendall's ta u values range from -I to I; va lues less than 0 indicate an upward trend, and values greater than 0 indica te a downward trend 

p-va lue, attained signiftcancc level of the trend test: p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05: medi an is the median va lue for the percentile: Sen slope is the change in streamflow per yea r, in cubic feet per second:<, less than ] 

Percentile Kendall's tau p-va lue Median Sen slope 

1941-2000(droughtl940-45) 

0 percentile 0. 167 0.060 43.0 0.202 

(annual minimum dai ly mean) 

l 51 percentile .145 .102 47.3 . 166 

2nd percentile .145 .104 51.3 . 199 

5th percentile . 159 .073 57.2 .250 

LOth percentile .193 .030 65.0 .314 

20th percentile .200 .024 81.9 .39 1 

301

h percentile .2 14 .016 100 .530 

40th percentile .189 .034 123 .650 

50th percentile (median) . 179 .044 158 .772 

60th percentile .145 . 102 194 .7 16 

701h percent ile .1 18 . 185 232 .709 

s o'" percentile .124 .162 294 882 

901h percentile . 156 .078 4 18 1 84 

1 ooth percentile .24 1 .007 1,860 18.5 

(annual maximum daily mean) 

1945-2000 

0 percentile .125 .174 44.0 .167 

(annual minimum daily mean) 

151 percentil e .114 .216 48.3 .139 

211d percentile . 105 .258 52.2 .176 

5111 percentile .110 .235 59. 1 . 188 

I 01h percentil e .136 .140 66.7 .262 

20th percentil e . 142 .125 84 3 .323 

30th percentile 158 .086 105 .441 

40th percentile . 136 .140 128 .532 

50th percentile (median) .133 .149 160 .619 

6oth percentile .083 .369 195 .368 

70th percentile .045 .631 237 .273 

goth percentile .061 .5! I 298 .41 3 

90th percentile . 107 .246 426 1.34 

I ooth percentile .247 .007 1,980 20.8 

(annual max imum dai ly mean) 

34 Streamflow in the Qulnnlplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum 

and annual maximum, Quinnipiac River at Wallingford, Conn., 1931- 2000-Continued 

[Kendall"s tau values range from -I to I; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend 

p-va lue, attained significance level of the trend test; p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05 ; median is the median value for the percentile; Sen slope is the change in streamflow per year, in cubic feet per second; <, less than] 

Percentile Kendall's tau p-value Median Sen slope 

1951- 2000 

0 percentile 0.158 0.106 44.0 0.227 

(annual minimum daily mean) 

l 51 percentile .179 .068 48.3 .233 

211d percentile .181 .064 52.2 .272 

51h percenti le .1 62 .099 59.1 .289 

I Oth percentile .149 .130 66.7 .329 

20th percentile .145 .139 84.3 .375 

30111 percentile .166 091 105 .5 12 

40th percentile .130 . 186 128 .593 

5oth percentile (median) .122 .213 160 .670 

60u1 percentile 087 .380 194 .524 

70111 percenti le .044 .657 23 7 .321 

80th percenti le .056 .569 298 .436 

901h percentile .091 .353 433 1.31 

I 00U1 percentile .168 .086 2, 100 16.8 

(annual maximum dail y mean) 

1961- 2000 (drought 1961-69) 

0 percentile .176 .113 44.5 .306 

(annual minimum daily mean) 

I st percentile .144 .196 49.4 .278 

2"d percentile .1 2 1 .278 53.9 .256 

slh percenti le .124 .263 60.5 .302 

lOu' percentile .142 .200 67.5 .444 

20U1 percentile .153 .169 85.8 .500 

30tl' percentile .222 .045 109 .912 

401h percentile .203 .067 132 1.18 

soil' percentile (median) .219 .048 160 1.53 

601h percentile .227 .040 194 1.71 

7oth percentile .212 .056 232 1.85 

SOU' percentile .22 1 .046 285 2.53 

90th percentile .245 .027 414 4.42 

IOOU' percentile .179 .105 2,180 22.7 

(ann ual maximum daily mean) 

Appendix 1 35 
Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum 

and annual maximum, Ouinnipiac River at Wallingford, Conn., 1931-2000-Continued 

[Kendall's tau values range from -I to I; va lues less than 0 indicate an upward trend, and values greater tban 0 indicate a downward trend 

p-value, attained significance level of the trend test; p-values in bold and shaded lines are at tbe attained significance level of less tban or equal to 

0.05; medi an is the median value for the percentile; Sen slope is the change in streamflow per year, in cubic feet per second;<. less than) 

Percentile Kendall's tau p-value Median Sen slope 

1971-20<)0 

0 percentile 0039 0.775 48.0 0.111 

(annual minimum daily mean) 

I st percentile -.103 .432 52.1 -.272 

2nd percentile -.140 .284 57.0 -.358 

sth percentile -.133 .309 65 3 -.362 

10th percentile -.133 .309 75.7 -.375 

20th percentile -.138 .292 900 -.583 

30th percentile -.506 .708 112 -.300 

40th percentile -.074 .580 138 -.486 

5oth percentile (median) -.078 .556 171 -.857 

60th percent i I e -.110 .402 205 -.862 

70th percentile -.092 .486 248 -.777 

80th percentile -.030 .830 312 -.800 

90th percentile -.021 .887 452 -.545 

IOoth percentile -.051 .708 2.380 -8.18 

(annual maximum daily mean) 

1931-1960 

0 percentile .566 <.001 33.5 1.29 

(mmual minimum daily mean) 

1st percentile .368 .005 42.0 .920 

2nd percentile .278 .032 45.1 .760 

5th percentile .184 .158 52.5 .385 

I oth percentile .076 .568 61.7 .118 

20~' percentile .253 .858 81.0 <.001 

30th percentile .064 .630 98.5 .350 

40th percentile .097 .464 120 .667 

50111 percentile (median) .113 .392 148 1.07 

60th percentile .154 .239 183 1.50 

70~' percentile .166 .205 231 1.77 

goth percentile .191 .144 298 2.24 

90th percentile .182 .164 417 2.71 

I OOth percentile .101 .443 1,490 12.5 

(annual maximum daily mean) 

36 Streamflow In the Qulnnlpiac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Appendix 1. Trend results of selected percentiles of streamflow between the annual minimum 

and annual maximum, Quinnipiac River at Wallingford, Conn., 1931-2000-Continued 

(Kendall's tau values range from -1 to 1; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend 

p-value, attained significance level of the trend test; p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05; median is the median value for the percentile; Sen slope is the change in streamflow per year, in cubic feet per second;<, less than] 

Percentile Kendall's tau p-value Median Sen slope 

1941-1970 

0 percentile -0.005 0.986 39.0 <0.001 

(annual minimum daily mean) 

1st percentile -.140 .284 42.8 -.263 

2nd percentile -.129 .326 45.6 -.222 

5th percentile -.087 .509 51.6 -.133 

1oth percentile -.021 .886 60.7 -.077 

20th percentile -.007 .972 75.9 <.001 

30th percentile .009 .957 90.8 .087 

40111 percentile -.021 .887 116 -.174 

50th percentile (median) -.064 .630 133 -.364 

6oth percentile -.090 .498 170 -.859 

70th percentile -.094 .475 211 -.975 

80th percentile -.076 .568 263 -1.36 

90th percentile -.113 .392 366 -2.26 

lOOth percentile -.030 .830 1,510 -1.67 

(annual maximum daily mean) 

Appendix 1 37 

Appendix 2. Trend results of annual and monthly precipitation data for selected stations in 

Connecticut, 1930-2000 

[Regional precipitation data from National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Center (data accessed on 

November 26, 2001 on the World Wide Web at URL http:l/lwf.ncdc.noaa.gov/oalclimate/climatedata.html). Kendall's tau values range from -1 

to 1; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend; p-value, attained significance level of the 

trend test; p-values in bold are at the attained significance level of less than or equal to 0.05; median is the median value for the percentile; Sen 

slope is the change in wastewater discharge per year; <, less than) 

Kendall's tau P-value Median Sen slope 

Annual precipitation (NOAA station numbers 65077,64767,67432, 69759, and 66597) 

Annual total 0.047 0.568 50.4 0.028 

Annual median .024 .784 3.80 .001 

Annual mean .031 .732 4.16 .002 

Monthly total precipitation (NOAA station numbers 65077 and 64767) 

January -.048 .588 3.34 -.010 

February -.021 .813 3.10 -.002 

March -.036 .690 4.14 -.006 

April -.025 .779 4.12 -.004 

May -.045 .610 4.07 -.008 

June -.088 .316 3.20 -.014 

July .099 .262 3.85 .016 

August .018 .837 3.48 .003 

September .082 .350 3.56 .014 

October .158 .073 3.38 .026 

November -.020 .828 4.48 -.006 

December -.039 .663 4.27 -.006 

Appendix 2 39 

Appendix 3. Trend results of wastewater-discharge data from municipal wastewater-treatment 

facilities in Meriden, Cheshire, and Southington, Conn., 1985-2001 

[Kendall's tau values range from -I to I; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend; 

p-value, attained significance level of the trend test; p-values in bold and shaded lines are at the attained significance level of less than or equal to 

0.05; median is the median value for the percentile; Sen slope is the percent change of median wastewater discharge per year; <, less than] 

Trend Kendall's tau p-value Median Sen slope 

Meriden wastewater-treatment facility (1985-01; units are total million gallons per month) 

Overall trend (seasonal Kendall) 0.178 0.040 264 2.23 

January .309 .091 318 6.53 

February .456 .012 294 6.86 

March .529 .003 370 11.7 

April .191 .303 378 4.41 

May .140 .458 278 2.60 

June .265 .149 217 2.74 

July .265 .149 207 1.48 

August .044 .837 215 .612 

September .118 .536 204 .775 

October .044 .837 223 .303 

November -.132 .484 245 -1.08 

December -.088 .650 301 -3.20 

Cheshire wastewater-treatment facility (1985-01; units are average million gallons per day) 

Overall trend (seasonal Kendall) .477 <.001 1.93 .046 

January .684 <.001 2.18 .065 

February .691 <.001 1.99 .075 

March .603 <.001 2.15 .080 

Apri.l .485 .007 2.23 .086 

May .441 .015 1.99 .054 

June .463 .010 1.85 .055 

July .485 .007 1.65 .038 

August .353 .052 1.72 .027 

September .463 .010 1.77 .031 

October .382 .036 1.84 .035 

November .375 . . 039 1.91 .030 

December .294 .108 1.98 .029 

Appendix 3 41 
Appendix 3. Trend results of wastewater-discharge data from municipal wastewater-treatment 

facilities in Meriden, Cheshire, and Southington, Conn., 1985-2001-Continued 

I Kendall's tau values range from -I to I; values less than 0 indicate an upward trend, and values greater than 0 indicate a downward trend; 

p-value. attained signif1cance level of the trend test; p-values in bold and shaded lines are at th e attained significance level of less th an or equal to 

0.05 ; median is the median value fo r the percentile; Sen slope is the percent change of median wastewater discharge per year; <, less than] 

Trend Kendall's tau p-value Median Sen slope 

Southington wastewater-treatment facility (198~1; units are total million gallons per month) 

Overall trend (seasonal Kendall) 0.205 <0.001 ll7 1.18 

January .368 .030 124 2.73 

February .380 .025 113 1.76 

March .555 .001 141 2.99 

April .088 .624 152 .985 

May .076 .674 131 .508 

June .193 .263 116 1.33 

July .3 10 .069 11 3 1.17 

August .228 .184 107 1.1 8 

September .041 .834 104 .242 

October .076 .675 105 .591 

November .146 .401 104 .834 

December -.006 1.00 119 -.044 

42 Streamflow in the Quinniplac River Basin, Connecticut-Statistics and Trends, 1931-2000 
Ahearn, 2002-Streamflow in the Quinnipiac River Basin, Connecticut-Statistics and Trends, 1931-20

U.S. Geological Survey WRIR Report 02-4217