In cooperation with the Chester County Water Resources Authority

Drought-Trigger Ground-Water Levels in Chester County, 

Pennsylvania, for the Period of Record Ending May 2006

U.S. Department of the Interior

U.S. Geological Survey

Data Series 292
Cover. View of Tributary to Trout Creek, Chester County, Pennsylvania, summer 2007. 

(Photograph by Kirk E. White, U.S. Geological Survey). 
Drought-Trigger Ground-Water Levels in 

Chester County, Pennsylvania, for the 

Period of Record Ending May 2006

By Peter J. Cinotto 

In cooperation with the Chester County Water Resources Authority 

Data Series 292

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

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

Telephone: 1-888-ASK-USGS

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

by the U.S. Government.

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

to reproduce any copyrighted material contained within this report.

Suggested citation:

Cinotto, P.J., 2007, Drought-trigger ground-water levels in Chester County, Pennsylvania, for the period of 

record ending May 2006: U.S. Geological Survey Data Series 292, 9 p. 
 iii

Contents

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

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Overview of the Chester County Observation-Well Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 2

Methods for Determination of Drought-Trigger Water Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Use of Drought-Trigger Water Levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Drought-Trigger Water Levels for Wells in the Chester County Observation-Well Network. . . . . . . . . . . . . . . . . . . 5

Comparison to Previous Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

References Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figures

1. Map showing location of wells in the Chester County observation-well network , Pennsylvania . . . . .3

2. Sketch showing hydrologic technician making a water-level measurement in an 

observation well. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Tables

1. Wells in the Chester County, Pennsylvania, observation-well network as of May 2006 . . . . . . . . . . . . . .2

2. Wells in the Chester County observation-well network, Pennsylvania, for which 

drought-trigger water levels could not be determined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

3. Monthly median water levels (50th percentile) for wells with at least 15 years of 

measurements in the Chester County observation-well network, Pennsylvania, 

period of record ending May 2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

4. Monthly drought-watch water levels (75th percentile) for wells with at least 15 years 

of measurements in the Chester County observation-well network, Pennsylvania, 

period of record ending May 2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

5. Monthly drought-warning water levels (90th percentile) for wells with at least 15 years 

of measurements in the Chester County observation-well network, Pennsylvania, 

period of record ending May 2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

6. Monthly drought-emergency water levels (95th percentile) for wells with at least 15 years 

of measurements in the Chester County observation-well network, Pennsylvania, 

period of record ending May 2006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

7. Comparison of historical percentiles and methods to current determinations of the 

annual-mean 75th-percentile and 90th-percentile water levels in the Chester County 

observation-well network, Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
iv 

Conversion Factors

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)
Drought-Trigger Ground-Water Levels in Chester County, 

Pennsylvania, for the Period of Record Ending May 2006

By Peter J. Cinotto

Abstract

This report presents the results of a study by the U.S. Geological Survey (USGS), in cooperation with the Chester County 

Water Resources Authority (CCWRA), to update the droughttrigger water levels for the Chester County observation-well 

network. The Chester County observation-well network was 

established in 1973 through a cooperative agreement between 

the CCWRA and the USGS to monitor local ground-water levels and trends and to determine drought conditions. 

In 1990 and again in 1997, drought-warning and droughtemergency water-level triggers were determined for the majority of wells in the existing Chester County observation-well network of 23 wells. Since 1997, the Chester County observationwell network expanded to 29 wells, some of the original wells 

were destroyed, and additional monthly water-level observations were made to allow for better statistical relations. Because 

of these changes, new statistics for water-level triggers were 

required.

For this study, 19 of the 29 wells in the observation-well 

network were used to compute drought-trigger water levels. An 

additional “drought-watch water-level trigger” category was 

developed to make the Chester County drought-trigger waterlevel categories consistent with those implemented by the Pennsylvania Department of Environmental Protection (PaDEP). 

The three drought-trigger water-level categories, as defined by 

PaDEP are 1) “drought watch” when at the 75th-percentile 

level; 2) “drought warning” when at the 90th-percentile level; 

and 3) “drought emergency” when at the 95th-percentile level.

A revised methodology, resulting from longer periods of 

record representing ground-water and climatic conditions and 

changes in local water use, has resulted in some observed differences in drought-trigger water levels. A comparison of current drought-trigger water levels to those calculated in 1997 

shows the largest mean annual change in percentile values was 

in northeastern Chester County. In this northeastern region, the 

average 90th-percentile water levels changed by as much as 

1.84 feet (17.5 percent) from those determined in 1997. Other 

observation wells in the county generally exhibited a smaller 

mean annual change in percentile values; a typical change was 

less than 0.5 foot. While most revisions in the drought-trigger 

water levels generally can be attributed to different methodology and (or) the additional years of data collected under existing 

climatic conditions, anthropogenic influences, such as the cessation of local pumping stresses, also were shown to cause 

changes in the drought-trigger water levels determined in this 

study.

Introduction

In 1990, a set of monthly drought-trigger water levels were 

developed for wells in the Chester County observation-well network with the exception of wells CH-89 and CH-2561, which 

were affected by nearby quarries (Sloto, 1991). These same 

monthly drought-trigger water levels were updated in 1997 

(using data collected through December 1996) so that more 

recent climatic influences would be reflected in the defining statistics (Schreffler, 1997). This study by the U.S. Geological 

Survey (USGS), in cooperation with the Chester County Water 

Resources Authority (CCWRA), began in 2006 to update the 

drought-trigger water levels for the Chester County observation-well network with data through May 2006.

Purpose and Scope

This study updates previously developed drought-trigger 

water levels, further strengthens the statistical relations by the 

addition of recent data (through May 2006), and computes additional drought-trigger water-level categories to coincide with 

those implemented by the Pennsylvania Department of Environmental Protection (PaDEP). Wells throughout Chester 

County meeting established criteria were included in the analysis. 
2 Drought-Trigger Ground-Water Levels in Chester County, Pennsylvania, for the Period of Record Ending May 2006

Overview of the Chester County Observation-Well 

Network

The Chester County observation-well network was established in 1973 and currently (2007) consists of 29 wells (table 1 

and fig. 1). These 29 wells are distributed throughout the county 

in various geologic units and are used to assess natural and (or) 

anthropogenic influences that cause fluctuations in groundwater levels. Water levels are measured monthly, except at well 

CH-10, which is also part of a statewide ground-water observation-well network, where water levels are recorded on an hourly 

basis. 

Table 1. Wells in the Chester County, Pennsylvania, observation-well network as of May 2006. 

Well number Type of bedrock in which wells 

are completed

Well depth, 

in feet

Year 

measurements 

started 

Period of record 

for analysis

CH-2

CH-10

CH-12

CH-38

CH-89

CH-210

CH-254

CH-1201

CH-1229

CH-1247

CH-1387

CH-1571

CH-1921

CH-2273

CH-2313

CH-2328

CH-2456

CH-2457

CH-2561

CH-2584

CH-2663

CH-3289

CH-5422

CH-6513

CH-6516

CH-6517

CH-6518

CH-6519

CH-7007

Gneiss

Carbonate rock

Gneiss

Schist, phyllite, and quartzite

Carbonate rock

Carbonate rock

Schist, phyllite, and quartzite

Carbonate rock

Gneiss

Gneiss

Gneiss

Sandstone and shale

Schist, phyllite, and quartzite

Gneiss

Carbonate rock

Gneiss

Gneiss

Schist, phyllite, and quartzite

Carbonate rock

Carbonate rock

Carbonate rock

Carbonate rock

Schist, phyllite, and quartzite

Sandstone and shale

Gneiss

Quartzite

Schist, phyllite, and quartzite

Schist, phyllite, and quartzite

Gneiss

15

34

38

18

265

600

250

83

165

75

159

16

65

298

507

323

225

285

240

Not available

150

202

49

Not available

100

Not available

37

400

Not available

1973

1951

1973

1974

1988

1978

1987

1973

1974

1973

1974

1974

1974

1975

1978

1975

1982

1982

1984

2002

1984

1988

2000

2002

2001

2001

2001

2001

2001

10/73-5/06

8/51-5/06

10/73-5/06

9/74-5/06

No analysis

6/78-5/06

1/87-5/06

10/73-5/06

2/74-5/06

12/73-5/06

7/74-5/06

6/74-5/06

18/74-5/06

3/75-5/06

4/78-5/06

4/75-5/06

2/82-5/06

11/87-5/06

No analysis

No analysis

1/84-5/06

4/88-5/06

No analysis

No analysis

No analysis

No analysis

No analysis

No analysis

No analysis

1

Only post-pumping data used.
Introduction 3

CH-2313

CH-6516

CH-1921

CH-2313

CH-2663

CH-2

CH-12

CH-2584 CH-2584

CH-38

CH-254

CH-1201 CH-1201

CH-1229 CH-1229

CH-1247

CH-1571 CH-1571

CH-2273

CH-2328 CH-2328

CH-2456 CH-2456

CH-2457 CH-2457

CH-3289 CH-3289

CH-10

CH-2561 CH-2561 CH-89 CH-210

CH-1387

1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

0

76° 75°45’ 75°30’

39°

45’

40°

CH-5422

CH-6518

CH-6516

CH-6513

CH-6517

CH-7007

CH-6519

CHESTER COUNTY

SANDSTONE AND SHALE

CARBONATE ROCK

SCHIST, PHYLLITE, AND QUARTZITE

GNEISS

TOWNSHIP BOUNDARY

LOCATION AND IDENTIFICATION NUMBER OF WELL 

 WITH DROUGHT TRIGGER

LOCATION AND IDENTIFICATION NUMBER OF WELL 

 WITHOUT DROUGHT TRIGGER

CH-2

CH-7007

EXPLANATION PENNSYLVANIA

Figure 1. Location of wells in the Chester County observation-well network, Pennsylvania. 
4 Drought-Trigger Ground-Water Levels in Chester County, Pennsylvania, for the Period of Record Ending May 2006

Water-level measurements from these wells provide 

water-resource managers with the ability to apply ground-water 

data to decisions regarding drought conditions, to monitor longterm trends in ground-water levels, and to gauge the effects of 

anthropogenic influences on ground-water levels. Most wells in 

the network assess natural fluctuations in water levels; however, wells CH-89 and CH-2561 are monitored to gauge the 

effects of pumping by nearby quarries. 

 With the exception of well CH-10, as noted previously, 

ground-water levels usually are measured during the middle of 

each month over a 1- to 2-day period, allowing for comparison 

of water levels among wells in the observation-well network. 

To measure the water level in wells, a measuring tape that 

detects the water surface is lowered down the well (fig. 2). The 

water level is the distance between the water surface and the 

land surface (depth from land surface) and is reported in feet 

below land surface. Artesian conditions are measured in the 

same manner but are noted by a negative (-) sign. 

Methods for Determination of DroughtTrigger Water Levels 

The monthly drought-trigger water levels used in 1990 by 

Sloto and again in 1997 by Schreffler included drought-warning 

and drought-emergency water-level triggers; by definition, 

these drought triggers were the same, respectively, as the 

drought-watch (75th percentile) and drought-warning (90th percentile) triggers used in the statewide ground-water observation-well network by the PaDEP (Pennsylvania Department of 

Environmental Protection, 2006). For this study, the definitions 

for monthly drought-trigger water levels established for the 

Chester County observation-well network will be identical to 

those defined for the PaDEP statewide observation-well network and will include the median, drought-watch (75th percentile), drought-warning (90th percentile), and drought-emergency (95th percentile) water-level triggers.

The previous studies conducted by Sloto (1991) and Schreffler (1997) were limited due to the large percentage of observation wells with short periods of record. This lack of data 

resulted in some data sets that were not well suited for direct 

analysis and, consequently, required that water-level data in 

certain observation wells be related to a long-term index well in 

order to estimate drought-trigger water levels. Under certain 

circumstances, this regression method, while often required, 

may lead to errors. A primary reason for the potential error in 

this regression method is the potential for differences in hydrologic response between wells in different hydrogeologic settings; in effect, some wells respond to recharge more quickly 

than others because the surrounding rocks, soil, and surface 

cover transmit water to and through the aquifer system more 

efficiently. This difference in hydrologic response may be consistent enough to provide a fairly strong relation between two 

wells when based only on discrete monthly measurements; 

however, the slope of the regression line that relates the two 

wells may not be totally accurate at any point in time because of 

the relative lag in response of a given well. Small changes in the 

slope of the regression line relating two wells can have the 

effect of under- or over-estimating peak values and can cause 

potential errors in the determination of percentiles; this can, 

consequently, cause errors in the estimated drought-trigger 

water levels. 

To determine monthly drought-trigger water levels for 

specific wells in the Chester County observation-well network 

for this study, it was determined that at least 15 years of 

monthly water-level measurements must have been collected 

from a well. During the required 15-year period, the hydrologic 

properties of the well must have remained stable, and the well 

must not have been affected by outside factors, such as pumping 

stress. Nineteen of 29 possible wells in the Chester County 

observation-well network met these requirements; these wells 

are CH-2, CH-10, CH-12, CH-38, CH-210, CH-254, CH-1201, 

CH-1229, CH-1247, CH-1387, CH-1571, CH-1921, CH-2273, 

CH-2313, CH-2328, CH-2456, CH-2457, CH-2663, and 

CH-3289. Wells within the Chester County observation-well 

15 FEET

GROUND

SURFACE

WATER-LEVEL

SURFACE

Figure 2. Hydrologic technician making a water-level 

measurement in an observation well (from Schreffler, 

1997). 
Use of Drought-Trigger Water Levels 5

network for which drought-trigger water levels could not be 

determined are presented in table 2. 

For each of the 19 selected wells, the monthly water-level 

measurements were entered into a Microsoft Excel 2002 

spreadsheet with which the selected percentages equal to the 

drought thresholds were determined. Data from wells with periods of continuous record were edited to include only one waterlevel measurement per month to eliminate any analytical bias 

resulting from more data representing drier or wetter periods. 

The water level collected closest to the 20th of the month was 

selected for the analysis. In the case of CH-10, when continuous 

record was available, the mean daily value on the 20th of the 

month was used. The median water level, drought-watch water 

level, drought-warning water level, and drought-emergency 

water level were then chosen to be the 50th percentile, 75th percentile, 90th percentile, and 95th percentile, respectively, of the 

water-level measurements. These percentiles relate the percentage of the time the water level measured in a specific well was 

above a certain level during the period of record. For example, 

the 75th percentile of ranked measurements means that over the 

entire period of record the water level was higher than that level 

75 percent of the time and, conversely, was lower than that level 

25 percent of the time.

Use of Drought-Trigger Water Levels 

Drought-trigger water levels provide Chester County 

water-resource managers with an effective alert system for 

impending drought conditions and also with an effective way to 

monitor the duration and severity of an existing drought. This 

information allows water-resource managers to be proactive in 

their management approach and to begin to conserve water and 

take appropriate actions prior to any reduction in the availability 

of water for critical uses. For example, the restricted use of 

water for nonessential uses (such as washing cars) early in a 

drought can help ensure ample supplies of water for essential 

uses (such as drinking water) should the drought continue. 

Additionally, internet distribution of county water levels and 

associated drought triggers serves as a mechanism for educating 

and alerting the public about the condition of ground-water 

resources. 

Drought-Trigger Water Levels for Wells in 

the Chester County Observation-Well 

Network

The monthly median, drought-watch, drought-warning, 

and drought-emergency water levels for the 19 wells in the 

Chester County observation-well network are shown in tables 3 

through 6, respectively. For example, if the water level in well 

CH-1387 measures 35.9 ft below land surface on July 15th of an 

arbitrary year, this value would then classify the well as being 

within the range of a “drought-warning.” It must be noted that 

water-resource managers consider not just ground-water levels 

but the response of multiple indicators that include streamflow, 

precipitation, reservoir storage, and soil moisture in the determination of a drought condition.

Table 2. Wells in the Chester County observation-well network, Pennsylvania, for which drought-trigger water levels could not be 

determined. 

[bls, below land surface]

Well 

number Reason for exclusion from drought-trigger determination

CH-28

CH-89

CH-249

CH-2561

CH-2584

CH-5422

CH-6513

CH-6516

CH-6517

CH-6518

CH-6519

CH-7007

CH-28 was destroyed and consequently removed from observation-well network

Quarry pumping affects water levels in CH-89

CH-249 was destroyed and consequently removed from observation-well network

Quarry pumping affects water levels in CH-2561

Insufficient data; CH-2584 added to network in 2002

Insufficient data; CH-5422 added to network in 2000

Insufficient data; CH-6513 added to network in 2002

Well is artesian with insufficient data; CH-6516 added to network in 2001

Insufficient data; CH-6517 added to network in 2001

Insufficient data; CH-6518 added to network in 2001

Fracture opened at approximately 72 feet bls during 2004, changing the hydrologic character of the well

Insufficient data; CH-7007 added to network in 2005

 
6 Drought-Trigger Ground-Water Levels in Chester County, Pennsylvania, for the Period of Record Ending May 2006

Table 3. Monthly median water levels (50th percentile) for wells with at least 

observation-well network, Pennsylvania, period of record ending May 2006. 

[A negative value indicates a water level that is above the land surface]

15 years of measurements in the Chester County 

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

CH-2

CH-10

CH-12

CH-38

CH-210

CH-254

CH-1201

CH-1229

CH-1247

CH-1387

CH-1571

CH-1921

CH-2273

CH-2313

CH-2328

CH-2456

CH-2457

CH-2663

CH-3289

8.63

12.67

37.08

7.34

20.45

23.89

5.16

37.64

30.46

33.61

6.40

45.71

-.36

9.09

2.19

19.36

19.42

10.04

20.20

8.31

12.28

36.31

6.78

19.50

23.07

5.07

36.65

29.22

33.11

6.03

44.72

-.88

6.83

1.96

19.29

17.71

9.97

20.00

8.12

11.62

35.45

6.80

19.30

22.23

4.87

35.09

28.45

32.44

5.72

44.58

-1.52

5.06

1.59

19.10

17.24

9.94

19.41

8.01

11.58

33.57

6.03

18.84

21.00

4.72

34.33

27.77

32.29

5.77

42.11

-2.02

4.14

1.62

19.00

16.83

9.90

18.69

8.48

12.51

33.85

7.30

19.74

20.95

5.13

34.92

28.10

32.09

6.34

40.88

-1.37

4.86

1.74

19.24

18.08

10.05

20.95

8.89

12.98

34.56

7.51

21.00

21.00

5.33

35.84

28.49

32.95

7.25

40.29

-.97

5.54

2.07

19.45

19.02

10.39

21.93

9.47

13.52

35.46

9.05

22.07

21.66

5.78

37.10

29.22

34.06

8.90

40.73

-.24

7.50

2.68

19.71

20.64

10.58

23.65

10.09

13.93

36.06

10.07

22.98

22.36

6.01

38.50

29.86

34.53

9.96

41.89

.51

9.11

3.04

19.89

21.60

10.68

24.76

10.65

14.32

36.50

10.38

23.20

22.44

6.18

39.48

30.70

35.19

10.25

42.95

1.01

10.12

3.69

19.94

21.92

10.67

25.24

10.24

14.44

37.30

11.24

23.83

22.17

6.22

39.86

31.30

35.39

9.95

43.84

1.29

11.22

3.37

19.99

22.09

10.60

24.51

10.18

14.13

38.25

10.79

23.15

23.13

6.13

38.95

32.35

35.31

8.27

44.63

.80

11.36

2.93

19.70

21.59

10.44

23.23

8.54

13.07

37.50

9.00

21.24

23.81

5.66

37.95

31.22

34.88

6.50

44.98

-.09

9.50

2.46

19.56

19.52

10.03

20.23

Table 4. Monthly drought-watch water levels (75th percentile) for wells with at least 15 years of measurements in the Chester County 

observation-well network, Pennsylvania, period of record ending May 2006. 

[A negative value indicates a water level that is above the land surface]

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

CH-2

CH-10

CH-12

CH-38

CH-210

CH-254

CH-1201

CH-1229

CH-1247

CH-1387

CH-1571

CH-1921

CH-2273

CH-2313

CH-2328

CH-2456

CH-2457

CH-2663

CH-3289

9.64

13.68

1

38.50

10.17

21.72

25.30

6.09

39.52

31.79

35.81

7.00

47.05

.98

10.85

3.10

19.77

20.59

10.45

21.04

9.27

13.12

1

38.50

8.62

20.96

25.11

5.73

37.26

31.50

34.64

6.81

47.03

.23

9.06

2.45

19.52

18.99

10.32

21.00

8.57

12.53

36.92

7.35

20.26

24.71

5.44

38.10

29.28

34.18

6.09

45.80

-.83

7.35

2.03

19.39

18.37

10.05

20.50

8.52

12.38

36.25

7.70

20.01

24.04

5.35

36.23

28.80

33.94

6.06

44.11

-.91

6.10

1.96

19.27

18.12

10.14

20.35

9.06

13.49

36.11

8.10

20.87

23.44

5.60

36.64

28.47

33.67

6.70

43.75

-.55

7.42

2.26

19.39

19.22

10.39

21.41

9.57

14.12

35.76

8.84

21.63

23.95

6.00

38.05

29.25

34.66

8.64

44.65

-.37

6.87

2.61

19.63

20.14

10.45

22.50

10.21

14.40

36.43

10.20

22.42

23.40

6.29

38.85

30.22

35.05

10.04

44.80

.79

9.07

3.14

19.95

21.22

10.74

23.99

10.71

14.80

36.95

11.41

23.80

23.96

6.57

39.87

31.23

35.46

10.44

45.15

1.75

10.68

3.98

20.14

22.12

10.82

26.81

11.50

14.99

37.52

12.56

25.07

24.24

6.79

40.16

32.23

36.13

10.56

45.89

2.19

11.91

4.54

20.35

22.72

10.91

27.93

11.23

15.10

1

38.50

12.35

25.10

24.84

6.65

40.87

32.47

36.25

10.51

46.35

2.65

12.57

4.50

20.29

22.60

10.82

26.06

11.08

14.80

1

38.50

12.16

24.15

25.36

6.67

41.01

33.16

36.32

9.80

46.58

2.47

12.71

4.25

20.06

22.36

10.74

24.67

9.88

14.20

138.50

11.12

21.94

25.37

6.12

39.84

32.45

36.01

7.79

47.06

1.37

11.52

3.00

19.98

21.09

10.68

22.22

1Well dry. 
Drought-Trigger Water Levels for Wells in the Chester County Observation-Well Network 7

Table 5. Monthly drought-warning water levels (90th percentile) for wells with at least 15 years of measurements in the Chester County 

observation-well network, Pennsylvania, period of record ending May 2006. 

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

CH-2

CH-10

CH-12

CH-38

CH-210

CH-254

CH-1201

CH-1229

CH-1247

CH-1387

CH-1571

CH-1921

CH-2273

CH-2313

CH-2328

CH-2456

CH-2457

CH-2663

CH-3289

10.65

14.57

1

38.50

12.93

24.77

27.51

7.00

41.18

34.02

36.45

8.78

47.95

3.75

12.41

4.04

19.95

21.29

10.89

21.94

10.16

13.72

1

38.50

11.53

22.58

26.55

6.29

40.22

31.99

35.78

7.27

47.96

2.60

11.82

3.64

19.86

20.35

10.45

22.29

9.44

13.45

1

38.50

10.13

22.37

26.27

6.04

39.51

31.50

35.81

6.47

47.56

2.00

11.07

3.27

19.67

19.20

10.29

21.37

9.39

13.40

38.11

8.60

22.17

25.30

5.83

38.64

31.15

35.07

6.64

46.73

1.05

9.32

2.58

19.50

19.48

10.61

21.65

9.70

14.11

37.29

9.49

21.86

25.09

6.02

38.29

30.60

35.46

7.12

45.99

1.20

9.35

2.87

19.59

19.97

10.55

21.63

10.70

14.75

37.84

10.29

22.99

24.79

6.28

39.72

30.52

35.71

8.84

46.43

1.54

10.47

3.62

19.79

21.15

10.72

23.16

11.29

15.03

1

38.50

11.06

24.26

25.25

6.57

40.60

31.42

35.76

10.12

46.81

1.56

11.70

3.83

20.19

21.82

10.92

25.56

12.10

15.22

138.50

12.20

24.84

26.12

7.20

41.09

32.43

36.63

10.60

47.61

2.95

13.62

4.75

20.38

22.91

11.08

29.22

12.07

15.45

1

38.50

12.79

25.79

26.50

7.30

41.07

33.29

36.63

10.75

48.47

3.31

14.17

5.09

20.59

23.10

11.07

30.46

12.42

15.39

1

38.50

13.52

25.99

27.12

7.05

41.69

33.89

37.58

10.82

48.80

3.96

14.73

5.38

20.61

22.96

10.95

30.32

12.05

15.44

1

38.50

14.13

25.21

27.56

6.91

41.62

33.80

37.03

10.74

48.00

3.62

15.08

4.78

20.27

23.46

10.99

27.03

11.93

14.82

1

38.50

14.45

25.06

26.63

6.76

41.43

33.33

36.76

10.00

48.26

3.56

14.47

5.03

20.25

22.56

10.94

26.75

1Well dry. 

Table 6. Monthly drought-emergency water levels (95th percentile) for wells with at least 15 years of measurements in the Chester County 

observation-well network, Pennsylvania, period of record ending May 2006. 

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

CH-2

CH-10

CH-12

CH-38

CH-210

CH-254

CH-1201

CH-1229

CH-1247

CH-1387

CH-1571

CH-1921

CH-2273

CH-2313

CH-2328

CH-2456

CH-2457

CH-2663

CH-3289

11.89

15.10

1

38.50

14.24

26.29

28.52

7.20

41.62

34.50

37.27

10.74

48.85

4.36

12.76

4.94

21.36

22.01

11.20

23.58

11.15

14.10

138.50

14.20

25.02

27.58

6.44

41.63

32.91

37.13

9.37

48.31

3.84

12.75

3.96

20.56

20.92

10.92

23.37

10.05

14.03

1

38.50

12.84

24.27

27.16

6.40

40.75

33.53

36.72

6.83

47.83

3.20

11.68

3.94

19.79

19.52

10.44

22.06

9.89

14.05

1

38.50

11.54

23.10

26.43

6.35

40.29

32.60

36.37

7.01

47.29

2.37

10.66

3.80

19.50

19.78

10.71

22.09

9.86

14.70

37.95

10.75

22.18

25.82

6.73

39.14

31.26

35.69

7.35

46.82

1.87

9.70

3.72

19.64

20.21

10.67

21.88

10.83

14.90

1

38.50

11.25

23.90

26.22

6.90

39.95

31.03

36.38

9.06

48.02

1.99

11.18

4.02

19.80

21.30

10.83

24.35

12.02

15.29

1

38.50

11.91

25.29

26.57

7.23

41.63

32.30

36.29

10.26

47.89

2.09

12.43

4.64

20.24

22.11

11.20

26.42

12.52

15.61

138.50

12.27

26.17

27.23

7.41

42.15

33.13

37.47

10.68

48.47

2.97

14.88

5.33

20.51

22.96

11.20

29.66

12.97

15.82

1

38.50

13.26

26.19

27.55

7.80

41.57

33.69

37.57

11.09

48.68

4.43

15.47

5.70

20.65

23.55

11.25

31.39

12.89

15.55

1

38.50

13.86

26.24

28.04

7.08

42.26

33.96

38.50

11.11

49.91

4.29

15.68

5.47

20.69

23.17

11.03

30.95

12.52

15.63

138.50

14.29

26.60

28.08

6.92

42.04

34.64

37.17

11.04

48.82

4.18

15.89

4.97

20.39

23.57

11.12

30.32

12.86

15.00

1

38.50

14.98

27.14

28.60

6.83

41.71

34.16

37.14

10.94

49.37

4.33

15.96

5.24

20.53

23.56

11.07

30.15

1

Well dry. 
8 Drought-Trigger Ground-Water Levels in Chester County, Pennsylvania, for the Period of Record Ending May 2006

Comparison to Previous Studies 

Differences in methodology, local water use, and changes 

in climatic conditions resulted in differences between the 

drought-trigger water levels determined by Schreffler (1997) 

and those determined by this study; also, Schreffler did not 

determine the 95th-percentile statistic in 1997. Annual mean 

water levels for the 75th and 90th percentiles, which are considered general indicators of water level, were determined for each 

of the 18 wells included in both Schreffler (1997) and this study 

for comparison of observed changes throughout the Chester 

County observation-well network. On the basis of these mean 

annual percentiles, the largest changes were mainly in the northeastern part of Chester County; relative changes were smaller 

elsewhere (table 7). Differences in the annual mean 75th-percentile water level ranged from a decline of 0.65 ft (7.3 percent) 

in well CH-2313 to a rise of 1.00 ft (4.6 percent) in CH-2457. 

Differences in the annual mean 90th-percentile water level 

ranged from a decline of 1.84 ft (17.5 percent) in well CH-2313 

to a rise of 1.05 ft (4.7 percent) in well CH-2457. The large relative change in well CH-2457 is primarily because of the cessation of nearby pumping in 1986, which caused a rise in local 

water levels; the relative change in well CH-2313 is primarily 

because of differences in methodology. A rise in the annual 

mean water level indicates that the water level in a well is at a 

higher elevation for a specific percentile than was originally 

estimated and that exceedence of that drought threshold would 

have occurred earlier during a period of declining ground-water 

levels and more frequently than previous studies would suggest. 

A decline in annual mean water level indicates that water levels 

are lower for a specific percentile and that exceedence of that 

drought threshold would have occurred later during a period of 

declining ground-water levels and less frequently than previously estimated. 

Annual mean water levels in table 7 generally appear to 

have risen between 1997 and 2006, which may be due to wetter 

conditions or a change in the methodology used to determine 

drought triggers. However, these values are intended for comparative purposes and additional statistical tests (such as the 

seasonal Kendall trend test) should be used to determine actual 

trends in local water levels.

Table 7. Comparison of historical percentiles and methods to 

current determinations of the annual mean 75th-percentile and 

90th-percentile water levels in the Chester County observationwell network, Pennsylvania.

Well

Method used 

by Schreffler 

(1997)

Mean annual 

change in 75thpercentile 

water level 

(feet)1

Mean annual 

change in 90thpercentile 

water level 

(feet)1

CH-2 S2 0.00 0.10

CH-10 S .09 .07

CH-12 S .38 .33

CH-38 S .04 .38

CH-210 R3 -.04 -.87

CH-1201 S .14 .31

CH-1229 S .22 .44

CH-1247 S .19 .32

CH-1387 S .27 .44

CH-1571 S .03 .13

CH-1921 S .14 .03

CH-2273 S .11 .33

CH-2313 R -.67 -1.84

CH-2328 S .04 .08

CH-2456 R .13 .08

CH-2457 R 1.00 1.05

CH-2663 R .01 -.06

CH-3289 R .25 -.45

1

A negative value indicates a decline (lower) in the respective water-level 

percentile; a positive value indicates a rise (higher) in the water-level percentile.

2

Percentiles determined in 1997 by statistical analysis of direct measurements.

3

Percentiles determined in 1997 by regression analysis with a local index 

well (CH-10); these fields are shaded.
Summary 9

Summary 

Monthly drought-trigger water levels were determined for 

19 of the 29 wells in the Chester County observation-well network. The drought-trigger water-level categories of drought 

watch, drought warning, and drought emergency are consistent 

with the PaDEP statewide drought-management water-level 

categories and are used by Chester County and PaDEP officials 

to assess and manage water resources throughout the year. Differences in methodology, local water use, and changes in climatic conditions resulted in variations between the droughttrigger water levels that were previously published and those 

determined by this study. Annual means for the 75th and 90th

percentiles generally increased between 1997 and 2006; however, these values are intended for comparative purposes and 

additional statistical test should be used to determine actual 

trends in local water levels.

References Cited

Pennsylvania Department of Environmental Protection, 2006, 

Drought Information Center: accessed August 22, 2007, at 

http://www.depweb.state.pa.us/watershedmgmt/cwp/

view.asp?a=1435&q=523309&watershedmgmtNav=|. 

Schreffler, C.L., 1997, Drought-trigger ground-water levels and 

analysis of historical trends in Chester County, Pennsylvania: 

U.S. Geological Survey Water-Resources Investigations 

Report 97-4113, 6 p.

Sloto, R.A., 1991, Drought management based on water-level 

data, Chester County, Pennsylvania in Conference on Water 

Management of River Systems, 27th, New Orleans, La., 

1991, Proceedings: New Orleans, La., American Water 

Resources Association, p. 439-440.