Relation Between Ground Water

and Surface Water

in Brandywine Creek Basin

Pennsylvania

GEOLOGICAL SURVEY PROFESSIONAL PAPER 417-A
Relation Between Ground Water

and Surface Water

in Brandywine Creek Basin

Pennsylvania

By F. H. OLMSTED and A. G. HELY

CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 417-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington 25, D.C.
CONTENTS

	Relation between ground-water stage, etc. Continued Page 

Abstract___________________________________________ A 1 Significance of observed relations.________________ A-13

Introduction.__ __ __ ___ ____ Comparison with results of other studies_______ 15

Hydrology of the basin 2 Computation of ground-water parameters with the aid

Physical characteristics 2 of gtreamflow data___________________ 16

Precipitation _ __ ___. 2 Gravity yield of the zone of water-table fluctuation. 16

Runoff.. 2 Computation by method A______________ 16

Ground water __ _____ __________ 3 Computation by method B______________ 17

Water budgets.. . . 8 Evaluation of results..___.__._._..._ 17

Relation between ground-water stage and base flow of Coefficient of transmissibility of rocks in the basin.. 18

Brandywine Creek _ 9 Conclusions_.__________________________ 20

Fluctuations of water table compared to fluctuations References.________________________________________ 21

of base flow________________________________ 9

Ground-water stage in index wells compared to base

flow_________________________. 12

ILLUSTRATIONS

Page

FIGUBE 1. Sketch map of Brandywine Creek basin___________________________-___________-_________--_______._____ A-3

2. Hydrograph of Brandywine Creek at Chadds Ford, Pa., 1952-53_____-_----__-__-_-_--___-_----_------_--- 5

3. Topographic and hydrologic settings of index wells De-3, Ch-14 and Ch-12_______________________________ 7

4. Hydrographs of five wells compared with hydrograph of base flow of Brandywine Creek at Chadds Ford, 1952-53. 10

5. Ground-water stage, base flow, and daily precipitation in Brandywine Creek basin, 1952-53._.__-____-_-_____- 11

6. Composite curve of ground-water-level recession for three index wells______________________________________ 12

7. Relation of monthly average base flow to ground-water stage____________________________________________ 12

8. Base-flow recession curves for Brandywine Creek at Chadds Ford, Pa__-___--_____-_-_-_-------_-__------_. 16

TABLES

Page 

TABLE 1. Description of observation wells in or near Brandywine Creek basin measured in 1952-53-_____________________ A-6

2. Monthly water budget for Brandywine Creek basin, 1928-31____________________________________________ 8

3. Monthly water budget for Brandywine Creek basin, 1952-53_______________-____--__--_________-_-_-_-_- 9

4. Monthly evapotranspiration from ground water in Brandywine Creek basin, 1952-53, as calculated from difference

between actual base flow and base flow at zero evapotranspiration from ground water._______________________ 13

5. Adjusted monthly evapotranspiration from ground water in Brandywine Creek basin, 1952-53____------------ 13

6. Seasonal changes in ground-water stage in 16 wells__________ ___________ ______________________.-. .. 17

m

CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

RELATION BETWEEN GROUND WATER AND SURFACE WATER IN 

BRANDYWINE CREEK BASIN, PENNSYLVANIA

By F. H. OLMSTED and A. G. HELY

ABSTRACT

The relation between ground water and surface water was 

studied in Brandywine Creek basin, an area of 287 square miles 

in the Piedmont physiographic province in southeastern Pennsylvania. Most of the basin is underlain by crystalline rocks that 

yield only small to moderate supplies of water to wells, but the 

creek has an unusually well-sustained base flow. Streamflow 

records for the Chadds Ford, Pa., gaging station were analyzed; 

base flow recession curves and hydrographs of base flow were 

defined for the calendar years 1928-31 and 1952-53. Water 

budgets calculated for these two periods indicate that about 

two-thirds of the runoff of Brandywine Creek is base flow 

a significantly higher proportion of base flow than in streams 

draining most other types of consolidated rocks in the region and 

almost as high as in streams in sandy parts of the Coastal Plain 

province in New Jersey and Delaware.

Ground-water levels in 16 observation wells were compared with the base flow of the creek for 1952-53. The wells 

are assumed to provide a reasonably good sample of average 

fluctuations of the water table and its depth below the land 

surface.

Three of the wells having the most suitable records were 

selected as index wells to use in a more detailed analysis. A 

direct, linear relation between the monthly average groundwater stage in the index wells and the base flow of the creek 

in winter months was found.

The average ground-water discharge in the basin for 1952-63 

was 489 cfs (316 mgd), of which slightly less than one-fourth 

was estimated to be loss by evapotranspiration. However, the 

estimated evapotranspiration from ground water, and consequently the estimated total ground-water discharge, may be 

somewhat high.

The average gravity yield (short-term coefficient of storage) 

of the zone of water-table fluctuation was calculated by two 

methods. The first method, based on the ratio of change in 

ground-water storage as calculated from a winter base-flow 

recession curve to seasonal change in ground-water stage in the 

observation wells, gave values of about 7 percent (using 16 

wells) and 7% percent (using 3 index wells). The second method, in which the change in ground water storage is based on a 

hypothetical base-flow recession curve (derived from the observed linear relation between ground-water stage in the index 

wells and base flow), gave a value of about 10% percent. The 

most probable value of gravity yield is between 7% and 10 percent, but this estimate may require modification when more information on the average magnitude of water-table fluctuation 

and the sources of base flow of the creek becomes available.

Rough estimates were made of the average coefficient of transmissibility of the rocks in the basin by use of the estimated total 

ground-water discharge for the period 1952-53, approximate 

values of length of discharge areas, and average water-table 

gradients adjacent to the discharge areas. The estimated average coefficient of transmissibility for 1952-53 is roughly 1,000 

gpd per foot. The transmissibility is variable, decreasing with 

decreasing ground-water stage.

The seeming inconsistency between the small to moderate 

ground-water yield to wells and the high yield to streams is explained in terms of the deep permeable soils, the relatively high 

gravity yield of the zone of water-table fluctuation, the steep 

water-table gradients toward the streams, the relatively low 

transmissibility of the rocks, and the rapid decrease in gravity 

yield below the lower limit of water-table fluctuation. It is concluded that no simple relation exists between the amount of natural ground-water discharge in an area and the proportion of 

this discharge that can be diverted to wells.

INTRODUCTION

In the Piedmont Upland of the Delaware Eiver region 

the dense crystalline rocks yield only small to moderate 

supplies of water to wells, but ground-water discharge 

sustains unusually high base flow in streams. To explain this seeming inconsistency, the Brandywine Creek 

basin, which lies wholly within the Piedmont Upland 

section of the Piedmont physiographic province 

(Fenneman and others, 1930) in southeastern Pennsylvania, was selected as an area in which to study the relation between ground water and surface water.

The part of the basin studied is an area of 287 square 

miles above the former stream-gaging station at Chadds 

Ford, Pa. The hydrologic and meteorologic records 

that were used in the study included daily discharge of 

Brandywine Creek at Chadds Ford, precipitation at 

6 stations, air temperature at 2 stations, and water levels 

in 16 observation wells (fig. 1). Though not abundant, 

the water-level data are more numerous in this area

A-l
A-2 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

than in other parts of the Delaware River region underlain by consolidated rocks. Fieldwork was restricted 

to a brief reconnaissance of the area and visits to the 

observation wells. The most detailed part of the study 

was made for the calendar years 1952-53, because that 

was the only period for which concurrent records of 

ground-water levels and streamflow were available, but 

a monthly water budget was computed for the period 

1928-31.

The chief purposes of the study were to define the 

base flow of Brandywine Creek, to calculate water 

budgets for the basin and compare these budgets with 

those of other basins in northeastern United States, to 

compare the fluctuations and stages of the water table 

with the base flow of the creek, and, by using both the 

ground-water and the base-flow data, to estimate the 

average gravity yield and coefficient of transmissibility 

of the rocks in the basin. Many of the results are 

rough estimates at best, and the figures given must be 

interpreted with great caution. Nevertheless, important conclusions are reached regarding the overall 

hydrology and especially the ground-water hydrology 

of the area. Such conclusions add significant insight 

into the occurrence and movement of water in the 

weathered and fractured crystalline rocks of the Piedmont Upland of Pennsylvania and help explain the 

apparent inconsistency that the crystalline rocks discharge large amounts of water to streams but only 

small to moderate amounts to wells.

HYDROLOGY OF THE BASIN 

PHYSICAL CHARACTERISTICS

The channel characteristics of Brandywine Creek, 

and the general features of the basin, are described by 

Wolman (1955). Bascom and Stose (1932,1938) have 

described the areal geology, which is generalized in 

figure 1.

The basin is part of a dissected upland which is underlain largely by metamorphic and igneous rocks of 

Precambrian to early Paleozoic age. Chester Valley, 

a long, narrow lowland underlain by dolomite and 

limestone, crosses the middle of the basin in a roughly 

east-west direction. Gneiss and granitic to ultramafic 

rocks of Precambrian age predominate north of 

Chester Valley; schist of early Paleozoic (?) age underlies much of the south half of the basin. Although 

several types of rocks occur within the basin, the hydrologic characteristics of the rocks, with the possible 

exception of the dolomite and limestone, are believed 

to be comparatively uniform for a basin of this size.

A mantle of weathered material of variable thickness 

has formed on all these rocks. The zone of water-table

fluctuation probably lies within the lower part of the 

weathered material or, locally, within the immediately 

underlying fractured rock. At most places and at most 

times the gradient of the water table is toward the 

streams, which therefore act as ground-water drains.

Most of the soils are permeable and well drained. 

According to a classification by the U.S. Department of 

Agriculture (1952) for the basin above Wilmington, 

Del. (an area 27 square miles larger than that of the 

present study), 56 percent of the area is underlain by 

deep, well-drained soils, 21 percent by shallow, welldrained soils, and 23 percent by imperfectly and poorly 

drained soils. Many of the imperfectly and poorly 

drained soils are in swampy areas where ground-water 

discharge occurs.

Land use in the basin is similar to that in most other 

areas in the Piedmont province. About 51 percent of 

the area above Wilmington, Del., is cropland and pastureland, 21 percent is woodland, 21 percent is in miscellaneous use, and 7 percent is occupied by highways, 

roads and streams (U.S. Department of Agriculture, 

1952).

The results of this study probably are applicable to 

other nearby Piedmont Upland basins having similar 

rocks, soils, weathered zones and patterns of land use.

PRECIPITATION

The average precipitation in Brandywine Creek basin 

for 1921-50, 44.1 inches, was computed by the U.S. 

Weather Bureau from an isohyetal map of the Delaware 

Eiver basin and New Jersey (Parker and others, written communication; Hely, Nordenson, and others, 

1961). The areal range in the average annual precipitation within the basin is less than 2 inches.

Daily precipitation in the basin for 1928-31 was calculated as the weighted average of the precipitation at 

Coatesville, Pa. (weight 0.7), and West Chester, Pa. 

(weight 0.3). The monthly and annual values are included in table 2. Daily precipitation for 1952-53 was 

calculated from the records at the six stations shown in 

figure 1. Each station record was weighted on the basis 

of a Thiessen net, as follows: Coatesville, 47.6 percent; 

Morgantown, 19.1 percent; West Chester, 17.1 percent; 

Chadds Ford, 6.6 percent; Devault (1 mile west), 5.0 

percent; and West Grove (1 mile southeast), 4.6 percent. The daily values thus determined are shown in 

figure 5, and the monthly and annual values are included in table 3.

RUNOFF

Runoff data were obtained from the annual series of 

U.S. Geological Survey reports, Surface Water Supply 

of the United States. The effect of regulation is be-
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA.

^ 'I

i Morgantown

 'f}~r/~/ TV I

PENNSYLVANIA

Brandywine 

Creek Basin -

Devault

A-3

Temperature-measuring 

station Gneiss land granitic 

to ultramafic rocks

Bd-10

Schist

Contact, approximately 

located

Stream-gaging station

Precipitation-gaging 

station

0 Ch-3 

Observation well

^-Ch-12 

Index well

Generalized boundary 

of basin

Geology generalized from Bascom 

and Stose(i932, 1938)

FIGDBB 1. Sketch map of Brandy wine Creefc basin, showing generalized geology and location of hydrologte an,4 meteo.rologlc stations used

In report.
A-4 CONTRIBUTIONS TO STREAM-BASIN fiTDROLOGY

lieved to be slight, although measurable at low flows. 

The streamflow for any period longer than a week or 

two is practically equivalent to the runoff (unregulated 

streamflow). Thei difference between the two at low 

flow is shown in figure 2 when the estimated base flow 

is greater than the streamflow for short periods when 

there was no evidence of any direct runoff.

Water is diverted from Coatesville Eeservoir (capacity 320 million gallons) on Eock Eun, a tributary of 

the West Branch, Brandy wine Creek for use by the city 

of Coatesville. Probably a very small part of this water 

is used consumptively, however, so that the consequent 

reduction in streamflow is negligible.

Ground-water withdrawals within the basin probably 

are very small and are considered negligible.

The average runoff in the basin for the period 1921- 

50 was 17.5 inches; the areal range in average within the 

basin was only about 1 inch (Hely, Nordenson, and 

others, 1961; Parker and others, written communication).

Base-flow recession curves were developed and used 

as a partial basis for separation of the daily discharge 

for 1928-31 and 1952-53 into the two components, base 

flow and direct runoff. Base flow is considered to be a 

good estimate of the ground-water discharge to streams 

in this basin because changes in surface storage are 

very small in relation to total flow.

The calendar years 1928-31 were selected for analysis 

because the period contained both wet and dry years 

and average runoff was near the long-term average. 

The base-flow recession curve used for the period was 

derived from hydrographs of daily discharge by a 

graphical method (Parker and others, written communication) and represents primarily summer conditions. The base flow and direct runoff are summarized 

in table 2.

The base-flow recession curves used for 1952-53 were 

derived by H. C. Eiggs (written communication, 1957) 

from tables of daily discharge. The method used was 

slightly different from that used in developing the 

curve used for 1928-31, and separate curves were developed for winter and summer conditions. The summer curve for 1952-53 has a slightly different shape 

from that of the curve used for 1928-31 but a similar 

slope; the winter curve is somewhat flatter. However, 

the differences in slopes of the recession curves generally 

apply for only short periods of record and the resulting 

differences in estimated base flow are probably much 

smaller than the errors involved in estimating the base 

flow during periods of direct runoff.

In separating the discharge into its components, records of daily precipitation, snowfall, and temperature 

were used as guides for interpreting slopes of the hydrograph. Thus there are few sharp rises in the base 

flow hydrograph in winter, as ground-water levels respond to recharge much more slowly when the ground 

is frozen or when there is a heavy snow cover. A few 

minor revisions of the base-flow graph were made on 

the basis of comparison with hydrographs of groundwater stage described in a later section.

The observed daily discharge and the estimated base 

flow are shown in figure 2; the monthly values are summarized in table 3. Direct runoff was computed as the 

difference between total runoff and base flow.

GROUND WATER

Ground-water data for the period 1952-53 consist 

of water-level measurements in 16 observation wells in 

or near the basin (U.S. Geological Survey: Annual 

series on ground-water levels in the United States). 

These wells are described briefly in table 1; their general location is shown in figure 1. They were selected 

for measurement because they were unused, were presumably unaffected or only slightly affected by pumping of nearby wells, and consequently probably indicated the natural levels and fluctuations of the water 

table. All the wells are relatively shallow, ranging in 

depth from 12 to 40 feet, and all but one (Ch-10) are 

of the dug rather than the drilled type. Water-level 

measurements were made by voluntary observers, usually the owners or tenants, except those for wells Ch-4:, 

Ch-7, Ch-13, and Bd-10, which were made by the Geological Survey. A water-stage recorder was installed 

on well De-3 for brief periods.

Not all the 16 observation wells were measured often 

enough to be of value in interpreting short-period fluctuation ; only 6 or 7 were measured at weekly intervals 

or immediately after storms.

Wells De-3, Ch-13, and Ch-14 were selected as index 

wells to use in a more detailed analysis. The selection 

was based on an examination of the water-level records 

of all the observation wells and a comparison of these 

records with the base-flow hydrograph of the creek and 

meteorological data for nearby stations.

In an area of 287 square miles, 16 well records, only 

3 of which were analyzed in detail, may not provide a 

representative sample of water table depths and fluctuations. However, the areal distribution of the 16 wells 

shown in figure 1 is reasonably good. The major rock 

types are represented in roughly the proportions of 

their areal extent in the basin; three wells are in gneiss, 

five in granitic to ultramafic rocks, six in schist, one in 

dolomite, and one in marble. The topographic position 

also is believed to be good; elevations from ridgetops to 

valley bottoms are represented.
j_ 500

AUGUST SEPTEMBER OCTOBER 

1953 

FIQUEH 2. Hydrograph of Brandywine Creek at Chadds Ford, Pa., 1952-53.
A-6 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY 

TABLE 1. Description of observation wells in or near Brandywine Creek basin measured in 1952-53

Well

Bd-10.. ...

Ch-2 . ..

3 .....

4 ._-._

5.........

6... ...... 

7 _ . 

8 .....

9 .....

10.. 

11 ......

12...... ...

13 

14 

15.........

De-3 ._

Well

Bd-10 

Ch-2... ......

3... ......

4 ..... .

5 _ ....

6. ........

7... _ ...

8... _ ...

9 __ . 

10. 

11

12

13 

14..... 

15 

De-3... ......

Owner or tenant (1953)

F. B. Ore 

L. R. Shi 

F. M. An 

C. R. Re 

Richard < 

J. J. Eng] 

D. L. Gil 

John Rot 

C. R. Yo 

R. J. Kle 

J. E. Rys 

T. P. Ha

J. T. Cro 

W. C. AI 

M. S Eb

erth ............. 

)bs.. .............

ling ._ ._

m ______ ...

ert

Approximate 

altitude 

of land 

surface 

(feet)

250 

635 

610 

560 

570 

265 

575 

655 

510 

313 

535 

250 

390 

345 

245 

285

Approximate location (fig. 1)

7 mi W. of Devault. ....... .....

6 mi NW. of West Chester......

7 mi NW. of West Grove.. 

2 mi S. of Coatesville ... .. 

5miN. of West Grove ..........

3 mi W. of West Chester... .

2 mi N. of Chadds Ford . ...

Latitude (N.)

Degrees

39 

40 

40 

40 

40 

40 

39 

39 

39 

39 

39 

39 

39 

39 

39 

39

Topography of well site

Near base of sloi 

Gentle slope

do

About midway i 

Broad ravine bo 

Gentle slope in T 

Summit of ridge

Near top of broa 

Small rise in val

Hillside near flex 

Hillside above si 

Nose of gentle ri 

Ridge above floo 

Small swale abo

ip hillside... __ _

Minutes

47 

06 

03 

06 

05 

00 

53 

59 

57 

54 

53 

57 

56 

53 

53 

50

Seconds

54 

20 

44 

42 

47 

43 

00 

03 

50 

15 

18 

57 

12 

41 

42

Longitude (W.)

Degrees

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

75 

7&

Minutes

34 

51 

48 

40 

44 

41 

54 

52 

49 

48 

48 

39 

43 

40 

36 

34

Water-bearing material

Weatht 

Granoc 

Granoc 

Quartz 

 ..do.

Dolomi 

Schist.

Gneiss 

Schist.

Marble 

Gneiss 

do

sred gabbn 

liorite or qi 

liorite.. . 

monzonite

).._ ... ....

lartz mom

tA

. _ .do.. _ . ____ _ . ____ 

... -do ... ... ... ... 

Seconds

"""20" 

03 

21 

55 

32 

50 

19 

14 

48 

46 

22 

38 

25 

41 

18

Type of 

well

Dug 

...do . 

...do 

...do .... 

...do. 

...do... ... . 

...do .... 

 do.. 

.-do.... ... 

Drilled. 

Dug.... 

_ do _ - 

 do 

... do 

 do 

 do ....

Diameter 

(inches)

42 

36 

36 

48 

30 

60 

18 

36 

30 

6 

24 

30 

24 

36 

42 

42

Frequency of water-level 

measurements

Monthl 

Weekly 

Irregula 

... ..doWeekly 

Irregula 

do

y ....................

 - .do..... 

 . .do..... 

__ do _ - 

Weekly 

Weekly 

Irregula 

Weekly 

Irregula 

Weekly

or monthly... ......

Depth 

(feet)

23 

15 

29.5 

29 

12 

20.5 

39.5 

20.5 

25 

33.5 

20 

33.5 

18.5 

25.5 

38.5 

20

Average 

depth to 

water 

1952-53 

(feet)

12.45 

8.02 

23.26 

23.39 

4.12 

15.06 

32.47 

12.56 

15.80 

11.22 

11.71 

33.64 

14.46 

19.99 

26.33 

14.61

The three index wells are in the southeastern part of 

the area (fig. 1), but their records probably represent 

average water-table fluctuations in the basin fairly accurately. However, some of the shorter period fluctuations, especially those associated with local storms, may 

not be accurately indicated.

The topographic and hydrologic settings of the index wells are shown in figure 3. Each of the wells is 

on a southeastward-facing slope several hundred feet 

from a small perennial stream that presumably is the 

discharge outlet for the ground water moving past the 

well. The water table is sufficiently deep at each well, 

so that ground-water discharge by evapotranspiration 

probably is negligible, although such discharge certainly occurs in the stream valleys several hundred feet 

downslope from the wells. The slope of the water table 

and its highest and lowest positions during 1952-53 are 

indicated for each well in figure 3. The average slope 

of the water table toward the discharge outlet was about 

140 feet per mile at well Ch-12,290 feet per mile at Ch14, and 410 feet per mile at De-3. All three wells are 

in strongly weathered rock, although well Ch-12, which 

is 500 feet southwest of a prominent outcrop, may penetrate relatively unweathered rock near the bottom. The

parent rock is mica schist or gneiss of the Wissahickon 

formation at wells De-3 and Ch-14 and the Baltimore 

gneiss at Ch-12.

Hydrographs for each of the three wells for 1951-55 

and daily precipitation at nearby stations were plotted. 

A ground-water level recession curve was developed 

for each well by synthesizing water-level recessions for 

periods of little or no precipitation in the same manner 

that a base-flow recession curve is derived from a hydrograph of daily discharge. Each well was found to have 

a characteristic recession curve that showed little or no 

seasonal variation, although there were some irregular 

variations.

Average depth to the water table is largely a function 

of topographic position with respect to discharge outlets. Since the sampling of topographic position probably is good, the average depth to water in the observation wells probably represents approximately the average depth to the water table in the basin. The depth 

to water, obtained by averaging the seasonal highs and 

lows listed in table 6, was 17.45 feet in the 16 observation wells and 22.75 feet in the 3 index wells. By comparison, in the crystalline rocks of northern Delaware, 

just south of Brandywine Creek basin, Ward and Eas-
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASEST, PA. A-t

Base from U. S. Geological Survey Umonville 7.5 minute quad

A'

FEET 

300

Water table 

1952-53 

Highest -* 

Lowest -»

250

200

B

FEET 

400

Water table 

1952-53 

Highest 

Lowest

Base from U. S. Geological Survey Umonville 7.5 minute quad

Well Ch-14 

c

350

300

Water table 

1952-53 

Highest 

Lowest

Base from U S. Geological Survey Wilmmgton North 

7.5 minute quadrangle

0 1000 2000 3000 4000 FEET

FEET

300

250

200

400 800 

I

1200 1600 FEET

_I

Well De-3

FIGURE 3. Topographic and hydrologic settings of index -wells De-3, Ch-4, and Cn-12.
CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

mussen (in Rasmussen and others, 1957) reported average depths to water as listed below:

TABLE 2. Monthly water 'budget -for Brandywine Creek "basin,

1928-31

Formation

Wissahickon formation (schist in fig. 1) _ _ 

Gabbro__ ______ ___ _ ______________

Average, 230 wells. ___________

Number of 

wells

18

149 

63

Average depth 

to water 

(feet)

13. 1

23. 6

18. 1

21.7

The magnitude of seasonal water-table fluctuation is 

determined partly by topographic position but largely 

by the nature of the rock; the smaller the specific yield 

(proportion of drainable void space to total volume), 

the greater the fluctuation. It is difficult to evaluate 

the accuracy of an estimate of average seasonal fluctuation of the water table in the basin based on records for 

only 16 wells, and little information in adjacent areas 

is available fof comparison. It is assumed that the 

average seasonal fluctuation for the basin is indicated by 

the average of the 16 wells; this average was 5.75 feet 

for 1952-53 (table 6). The average seasonal fluctuation 

in the 3 index wells was 5.33 feet for the same period. 

These estimates of average seasonal fluctuation of the 

water table may require substantial modification when 

more water-level data become available for the region.

WATER BUDGETS

The data on precipitation and runoff described in the 

preceding sections are summarized in tables 2 and 3, 

which give examples of water budgets for the basin. 

The general budget equation (inflow equals outflow plus 

net increase in storage) is simplified by the assumption 

that net increase in storage for a period of years is negligible. For Brandywine Creek basin above Chadds 

Ford, inflow is assumed to be equal to the precipitation, 

and outflow is assumed to be equal to runoff plus evapotranspiration because underground inflow and outflow 

are probably negligible.

Average annual evapotranspiration, which cannot be 

measured directly for large areas, may be estimated as 

the long-term difference (commonly called water 

loss) between precipitation and runoff. Although 1 

year is too short a period for accurate determination of 

the portion of the water loss that is not appreciably 

affected by change in storage, values of precipitation 

minus runoff for each year of the budget period are 

shown in tables 2 and 3 to indicate the relative magnitude and variability of this quantity. Thus, for the 6 

budget years the maximum difference is about 10.4 

inches as compared to 22.0 inches for precipitation and 

18.3 inches for runoff.

Month

1928

April - 

July 

Year. 

1929

April. _

July 

December. 

Year __ 

1930

April 

July- -

Year-. 

19S1

April.. ___

July -

December. 

Year. ..... 

4-year average . 

Precipitation 

(inches)

3.47

4.32

2.92

5.41

2.62

11.33

6.39

7.73

4.81

69

2.19

1.56

63.44

3.59

4.48

2.78

6.58

4.78

3.10

2.85

4.63

3.84

5.93

3.36

2.76

48.68

2.99

3.03

3.05

2.48

4.11

3.52

2.90

1.77

4.58

.86

1.94

2.56

33.79

2.26 

1.99

4.99

2.68

6.76

3.51

8.20

5.66

1.89

1.46

.97

2.17

42.54

44.61

Precipitation 

minus 

runoff 

(inches)

28.13

30.28

22.40

31.55

28.09

Runoff 

(inches)

1.82 

3.64 

2.31 

2.57 

2.16 

3.06 

2.34 

2.26 

1.77 

1.27 

1.05 

1.06

25.31

1.42 

2.24 

2.70 

2.80 

2.33 

1.29 

.83 

.65 

.43 

1.25 

1.32 

1.14

18.40

1.17 

2.01 

2.07 

1.61 

1.17 

.79 

.58 

.33 

.42 

.29 

.39 

.56

11.39

.92 

.79 

.99 

.91 

1.33 

1.14 

1,71 

1.15 

.60 

.47 

.42 

.56

10.99

16.52

Direct 

runoff 

(inches)

0.33 

1.65 

.30 

1.01 

.31 

1.29 

.77 

.79 

.33 

.04 

.08 

.18

7.08

.58 

1.46 

1.01 

1.13 

.52 

.16 

.19 

.19 

.08 

.63 

.46 

.26

6.67

.19 

.70 

.40 

.29 

.29 

.16 

.16 

.03 

.14 

.02 

.09 

.26

2.72

.52 

.35 

.43 

.29 

.69 

.56 

1.07 

.59 

.11 

.06 

.04 

.13

4.84

5.32

Base flow

(Inches)

1.49 

1.99 

2.01 

1.56 

1.85 

1.77 

1.57 

1.47 

1.44 

1.23 

.97 

.88

18.23

.84 

.78 

1.69 

1.67 

1.81 

1.13 

.64 

.46 

.35 

.62 

.86 

.88

11.73

.98 

1.31 

1.67 

1.32 

.88 

.64 

.42 

.30 

.28 

.27 

.30 

.30

8.67

.40 

.44 

.56 

.62 

.64 

.58 

.64 

.56 

.49 

.41 

.38 

.43

6.15

11.20

(Percentage of 

runoff)

82 

65 

87 

61 

86 

58 

67 

65 

81 

97 

92 

83

72.0

59 

35 

63 

60 

78 

88 

77 

71 

81 

60 

66 

77

63.8

84 

65 

81 

82 

75 

81 

72 

91 

67 

93 

77 

54

76.1

43 

56 

57 

68 

48 

51 

37 

49 

82 

87 

90 

77

56.0

67.8

The direct runoff and base flow also indicate the 

relative magnitude of ground-water discharge to 

streams. The average base flow for the 6 budget years 

is about 67 percent of the total runoff. This compares 

with about 64 percent for North Branch Rancocas 

Creek in the coastal plain of New Jersey (computed for 

this study); 74 percent x for Beaverdam Creek, a small 

stream in the coastal plain of Maryland (Rasmussen

1 Modified slightly in order to account for net changes in storage 

within the budget periods.
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA. A-9

and Andreasen, 1959); 42 percent for Perkiomen 

Creek (Parker and others, written communication), a 

relatively flashy stream in the Triassic lowland in 

Pennsylvania; and 44 percent 1 for the Pomperaug 

River basin (Meinzer and Stearns, 1929), a small stream 

in Connecticut.

TABLE 3. Monthly water budget for Brandywine Creek basin,

1952^53

Month

1952

July...... .......

Year......... ....

1953

February. _ . ......

April 

June _

July.. ___ 

Year..... __

Precipitation 

(inches)

5.05

2.13

5.45

7.53

6

QQ

2.59 

6.29

4.65

.82

5.51

4.36

55.78

5.82

2.64 

6.41

4.34

6.30

2.78

3.52

1.16

2.24 

4.05 

2.28

4.46

46.00

50.89

Precipitation 

minus 

runoff 

(inches)

26.50

21.19

23.85

Runoff 

(inches)

2.94 

2.78 

3.49 

3.89 

3.80 

2.59 

2.02 

1.40 

1.41 

.80 

1.74 

2.42

29.28

3.85 

2.59 

4.29 

3.59 

3.03 

2.17 

1.15 

.66 

.47 

.59 

.79 

1.63

24.81

27.04

Direct 

runoff 

(inches)

1.24 

.67 

1.25 

1.77 

1.30 

.60 

.72 

.38 

.53 

.03 

1.03 

1.08

10.60

1.97 

.57 

1.59 

.84 

1.00 

.64 

.19 

.02 

.04 

.19 

.26 

.89

8.20

9.40

Base flowInches

1.70 

2.11 

2.24 

2.12 

2.50 

1.99 

1.30 

1.02 

.88 

.77 

.71 

1.34

18.68

1.88 

2.02 

2.70 

2.75 

2.03 

1.53 

.96 

.64 

.43 

.40 

.53 

.74

16.61

17.64

Percentage of 

runoff

58 

76 

64 

54 

66 

77 

64 

73 

62 

96 

41 

55

63.8

49 

78 

63 

77 

67 

71 

83 

97 

91 

68 

67 

45

66.9

65.2

The North Branch Rancocas Creek basin is underlain 

by predominantly unconsolidated and semiconsolidated 

sandy sediments; the Beaverdam Creek basin by rather 

uniformly sandy unconsolidated deposits; the Parkiomen Creek basin in large part by shale, argillite, sandstone, and diabase; and the Pomperaug River basin by 

crystalline rocks and some sandstone, conglomerate, and 

shale, all of which are covered by a discontinuous mantle of glacial deposits. The base flow characteristics of 

Brandywine Creek are more characteristic of streams 

draining the unconsolidated deposits of the Coastal 

Plain than of most other streams in areas underlain by 

consolidated rocks.

Factors other than the geology of a basin affect the 

amount of base flow. Differences in topography, soils, 

and vegetation affect infiltration capacity, which in part 

determines the amount of base flow; some of these factors can be modified considerably by land-management 

techniques such as those reported extensively in the 

literature of the Forest Service and Soil Conservation

'See footnote page 8. 

618101 62 3

Service of the U.S. Department of Agriculture. A 

summary and evaluation of most of these techniques is 

presented by Colman (1953).

However, the writers believe that the great differences in runoff characteristics between the Brandywine 

Creek basin and the other two basins in consolidated 

rock must be due chiefly to geological differences.

RELATION BETWEEN GROUND-WATER STAGE AND 

BASE FLOW OF BRANDYWINE CREEK

FLUCTUATIONS OF WATER TABLE COMPARED TO 

FLUCTUATIONS OF BASE FLOW

The hydrographs of five wells that were measured 

weekly or immediately after storms are compared with 

the hydrograph of base flow of Brandywine Creek in 

figure 4. The depths to the water table in these wells 

represent the full range in all 16 observation wells, and 

the fluctuations in water level in the 5 wells indicate 

the known variation in fluctuation of the water table 

throughout the basin.

The general similarity of the fluctuations in both the 

wells and the creek is apparent. All the hydrographs 

rise after the first heavy precipitation in the fall and 

decline during the growing season from late spring to 

fall. The hydrographs differ in detail, however. Except for the hydrograph of well Ch-5, the short-period 

fluctuations decrease in number and amplitude with 

increasing depth to water. In general, this is to be 

expected because increasing the distance from land surface to water table increases the time of transit and 

the volume of water in transit (gravity water).

Factors other than depth to water also influence the 

fluctuations. The permeability of the materials in the 

zone between the water table and the land surface in 

part determine the transit time of recharge to the water 

table; variations in permeability below the water table, 

and the relation of the site to nearby discharge outlets 

are also important. For example, well Ch-5, which has 

the shallowest depth to water of any of the five wells 

in figure 4, is in the bottom of a small ravine only a 

few feet upslope from a wet-weather spring, and the 

hydrograph reflects the local conditions in the zone 

supplying the spring discharge. The water level in 

the well remains very nearly at the level of the discharge outlet (the spring) as long as the discharge 

continues (which was much of the year in both 1952 and 

1953), but recedes rapidly after the spring ceases to 

flow.

The hydrographs of the three index wells were averaged to form a composite hydrograph, which is compared with the base-flow hydrograph of the creek in 

figure 5. In order to simplify the computations of
A-10 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

gravity yield (discussed on pages A-16-A-18), average 

ground-water stage is shown in inches above an arbitrary datum 28 feet below the land surface. Scales for 

ground-water stage and base flow were selected to produce similar amplitudes of seasonal fluctuation of the 

two curves.

The principal differences between the hydrographs 

are the relatively greater amplitude and greater number of short-period fluctuations of the base flow, especially during winter and spring storm periods, and the

delay of as much as a week between the occurrence of the 

peaks and troughs of the ground-water stage and the 

equivalent peaks and troughs of the base flow. A less 

obvious, but equally important, difference is the slower 

winter recovery in water level in the wells compared to 

the more rapid increase in base flow in the creek.

As emphasized by Hurst and Brater (1941), the water table gradient adjacent to a stream is one of the factors that determines the rate of ground-water discharge 

to the stream (which approximately equals base flow) ;

10

20

25

30

40

Ch-5

De-3

Ch-14

Ch-12

Brandywine Creek

\

JAN | FEB | MAR' APR | MAY |JUN? JULY [ AUG | SEPT OCT | NOV | DEC JAN | FEB | MAR APR | MAY |JUN? JULY | AUG JSEPT OCT | NOV | DEC

1952 1953

FIGURE 4. Hydrographs of five wells compared with hydrograph of base flow of Brandywine Creek at Chadds Ford, 1952-53.
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA. A-ll

consequently, the base-flow hydrograph should most 

closely resemble the hydrographs of wells adjacent to 

the creek, where the water table is relatively shallow. 

However, the index wells are several hundred feet upslope from streams where the water table is much 

deeper: because of this greater depth, recharge water 

takes a longer time to reach the water table at the index 

wells than it does near the streams. Consequently the 

ground-water stage at the index wells does not respond

to precipitation as rapidly as it does near the stream 

where its effect on base flow is rapid.

Evapotranspiration from ground water affects the 

base-flow hydrograph but does not materially affect the 

ground-water stage where the depth to the capillary 

fringe above the water table is greater than the maximum depth of plant roots. In Brandywine Creek basin 

evapotranspiration from ground water probably is limited almost entirely to stream valleys. Part of the

1952

A/V

Average ground-water stage in index 

wells De-3. Cn-12, and Ch-14

Base flow ot Brandywine Creek 

Snow Snow and Rain

1953 

FIGURE 5. (Ground-water stage, base flow, and daily precipitation in Brandywine Creek basin, 1952-53.

300 ^
A-12 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

ground water moving toward the stream from higher 

areas is intercepted and transpired by riparian plants. 

Small summer showers may replenish soil moisture and 

lessen the demand on ground water, thereby allowing 

more of the ground-water discharge to reach the stream, 

even though no recharge to ground water has occurred. 

This probably accounts for some of the increases in base 

flow that commonly occur after storms when rainfall is 

insufficient to produce recharge to ground water.

Some of the differences indicated by the comparison 

of the hydrographs may not be real because of uncertainities in the delineation of base flow during periods 

of direct runoff.

GROUND-WATER STAGE IN INDEX WELLS COMPARED 

TO BASE FLOW

Recession curves of ground-water levels in the three 

index wells (De 3, Ch 12, Ch 14) were combined 

to form a composite curve (fig. 6) which is considered 

representative of the entire basin.

ts> 60

0 20 '40 60 80 100 120 140 160 

TIME, IN DAYS

PIGUEHJ 6. Composite curve of ground-water-level recession for three 

index wells.

A relation between base flow and ground-water stage 

is shown in figure 7. This figure shows the monthly base 

flow (table 3) plotted against monthly average groundwater stage in the three index wells (determined from 

the hydrographs in fig. 5), and a line representing the 

relation between them for the condition of no evapotranspiration from ground water. The points for the 

24 months represented form a flat loop (actually two 

similar loops one for each year) sloping down to the

left. The points for the winter months lie very near a 

straight line marking the lower right side of the loop. 

The points for the summer months also lie near a 

straight line having a similar slope but displaced to the 

left. The fall and spring months lie between these 

extremes.

JUNE.). 

MAYQ

9 30

JUNEQ

JULY.).

JULYO

+ AUG

0 100 200 300 400 500 600 700 800 

MONTHLY AVERAGE BASE FLOW, IN CUBIC FEET PER SECOND (Rg)

FIGURE 7. Kelation of monthly average base flow to ground-water stage.

Qualitatively this relation suggests not only that the 

higher ground-water stages are associated with higher 

base flows, as would be expected, but also that the base 

flow corresponding to no evapotranspiration from 

ground water (or total ground-water discharge) is directly proportional to ground-water stage. On the 

basis of this plot, it is assumed that the total monthly 

ground-water discharge is a direct linear function of 

monthly average ground-water stage, and that the displacement of the monthly points to the left of a relation line representing no loss is the measure of loss by 

evapotranspiration. For example, if the monthly average ground-water stage were the same for July and 

January, but the base flow in July averaged 250 cfs less 

than that in January, the evapotranspiration from 

ground water would be 0.87 cfs per square mile, or 1.00 

inch more in July than in January.

The slope of the relation line was determined in the 

following manner. Pairs of months having about the 

same average potential evapotranspiration were selected on the basis of data for Seabrook, N".J. (Thornthwaite and Mather, 1955, table 2.1). The ratio of 

change in base flow to change in ground-water stage 

for each pair of months is shown on facing page:
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA. A-13

Pairs of months

February-December.- ____ __ _

March November. _____ __ _ ___

April-October ___ ___ ____ __ _

May-September __ __ __ _ _ __

June-August __ ____ _ __ ____ _

1952

9.5

8.8

8.4

10. 8

10.3

9.6

Ratio

1953

9.2

9.2

9.3

8.5

8. 1

8.9

Average

9.4

9.0

8.8

9.6

9.2

9.2

TABLE 5. Adjusted, monthly evapotranspiration from ground 

water in Brandywine Greek 'basin, 195&-5S

The average ratio, 9.2, is the slope of the relation line.

A period much longer than 2 years, and points differing appreciably in stage and flow for the same months 

rather than different months would have been preferable, but the computed slope probably is reasonably close 

to the correct value.

A trial location of the line was determined on the basis 

of losses for January and. July 1952. The equation of 

the line in this location (fig. 7) is

in which Rg is the average ground-water discharge to 

streams assumed equal to base flow in cubic feet per 

second, and Hg is the average ground-water stage in 

inches. Table 4 shows the computation of losses for 

each month based on this line.

TABLE 4. Monthly evapotranspiration from ground water in 

Brandywine Greek basin, 1952-53, as calculated from difference between base flow computed from hydrograph and base 

flow at zero evapotranspiration from ground water

Month

195%

April. . 

July.. .... ..... 

Year. 

19SS

July... ...........

Year..... ...........

Weighted 

average groundwater stage 

(inches)

58.0 

73.9 

79.3 

86.7 

90.1 

88.5 

74.1 

63. 5 

53.5 

44.5 

36.7 

49.9

65.7 

77.1 

85.4 

95.3 

85.6 

77.7 

62.5 

48.6 

39.2 

30.2 

27.3 

36.7

Base flow 

computed 

for zero 

evapotranspiration 

from ground 

water 

(cfs)

435

582 

632 

700 

731 

716 

584 

486 

394 

311 

240 

361

506 

611 

688 

779 

690 

617 

477 

349 

263 

180 

153 

240

489

Base flow 

computed 

from 

hydrograph 

(cfs)

423 

561 

557 

545 

623 

511 

323 

254 

226 

191 

183 

334

468 

557 

672 

707 

505 

394 

240 

159 

111 

99 

137 

185

Evapotranspiration 

from ground water 

(Difference between 

computed base flows)

(cfs)

12 

21 

75 

155 

108 

205 

261 

232 

168 

120 

57 

27

38

54 

16 

72 

185 

223 

237 

190 

152 

81 

16 

55

(inches)

0.05 

.08 

.30 

.60 

.43 

.80 

1.05 

.93 

.65 

.47 

.22 

.11

5.69

.15 

.20 

.06 

.28 

.74 

.87 

.95 

.76 

.59 

.33 

.06 

.22

5.21

Month

1952

April 

July.. 

Year .

1953

July ... ..... .

Year. .

A

Average 

temperature (°C)

1.8

2.0

3.8

11.8

15.3

22.2

25.0

22.7

19.0

10.8

6.5

1.9

2.1

2.8

5.8

10.6

18.0

21.5

24.0

22.8

19.5

14.0

7.0

2.8

B

Percentage of 

annual 

daylight

6.69

6.93

8.26

8.91

9.95

10.05

10.20

9.56

8.39

7.79

6.76

6.51

100.00

6.71

6.69

8.28

8.93

9.98

10.07

10.23

9.58

8.41

7.81

6.78

6.53

100.00

AXB

12

14

31

105

152

223

255

217

159

84

44

12

1,308

14

19

48

95

180

217

246

218

165

109

47

18

1,376

2,684

Percentage of 2- 

year total 

AXB

0.45

.52

1.15

3.91

5.66

8.31

9.50

8.09

5.92

3.13

1.64

.45

48.73

.52

.71

1.79

3.54

6.71

8.08

9.17

8.12

6.15

4.06

1.75

.67

51.27

100.00

Adjusted 

evapotranspiration 

from 

ground 

water 

(inches)

0.05

.06

.12

.43

.62

.91

1.04

.88

.64

.34

.18

.05

5.32

.06

.08

.19

.39

.73

.88

1.00

.88

.67

.44

.19

.07

5.58

10.90

Evapotranspiration from 

ground 

water (from 

table 4) 

(inches)

0.05

.08

.30

.60

.43

.80

1.05

.93

.65

.47

.22

.11

5.69

.15

.20

.06

.28

.74

.87

.95

.76

.59

.33

.06

.22

5.21

10.90

Computation of adjustments to the computed losses 

is shown in table 5. The product of monthly temperature (obtained from publications of the U.S. Weather 

Bureau) and percentage of annual daylight (computed 

from standard ephemeris tables) is shown for each 

month and year and for the 2-year period. The percentage of this 2-year total that occurred during each 

month is multiplied by the 2-year total loss (from table 

4) to obtain the ad justed loss.

Comparison of the adjusted losses (table 5) with 

those derived from the relation in figure 7 indicates 

that the trial location of the relation line is satisfactory. 

This line is used in one method of estimating the gravity yield of the zone of water-table fluctuation, described in a following section. The total ground-water 

discharge for the 2-year budget period was calculated 

as the sum of the total loss and the total base flow, and 

the yearly average for this period was used to calculate 

the approximate coefficient of transmissibility of the 

rocks of the basin. The average discharge for the 2- 

year period was 489 cfs (table 4), or 316 mgd. Slightly 

less than one-fourth of this total was loss by evapotranspiration ; the remainder was base flow.

SIGNIFICANCE OP OBSERVED KELATIONS

Three index wells provide a very small sample of 

water-table fluctuations in an area of 287 square miles,
A-14 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

and water levels in other wells might relate to the flow 

of the creek quite differently from those in the three index wells. A different period of analysis particularly 

one that included relatively dry years might have 

altered not only the quantitative relations of the water 

levels in the index wells to base flow but the qualitative 

conclusions as well. For example, Harrold (1934) in 

his study of a small basin in the Piedmont province in 

Maryland, found a difference in the relations between 

ground-water stage and base flow in wet and dry seasons. No such difference is apparent in the records for 

Brandywine Creek basin in 1952-53, but both 1952 and 

1953 were relatively wet years. A dry year or even a 

normal year might have shown a difference. However, 

at least part of the difference in the stage-flow relations 

noted by Harrold was due to the difference in evapotranspiration from ground water. This factor is accounted for in the Brandywine data because stage-flow 

relations are estimated for conditions of no evapotranspiration from ground water.

The linear relation (fig. 7) between ground-water 

stage in the index wells and base flow could be accidental. If this relation were based on ground-water 

stage from the composite recession curve in figure 6 and 

base flow at the corresponding time from the winter 

recession curve in figure 8, it would not be linear but 

curved, the slope increasing with decreasing stage.

A hypothetical base-flow recession curve that represents no loss by evapotranspiration from ground water 

was computed from the ground-water-level recession 

curve (fig. 6) and the relation (fig. 7) corresponding to 

no loss. Thus from figure 6, at a time of 80 days the 

ground-water stage is 47.5 inches, which, when inserted 

in the equation of the relation line (fig. 7), gives (9.2) 

(47.5) 98=338 cfs as the corresponding base flow. 

This hypothetical recession is shown in figure 8 and was 

used in a later section in the computation of gravity 

yield by method B.

The hypothetical recession curve is flatter than the 

winter recession curve at the upper end and steeper at 

the lower end (fig. 8), partly because the winter recession curve includes effects of some evapotranspiration 

from ground water. Probably the greater part of the 

discrepancy results from selection of the index wells to 

indicate average water-table fluctuations throughout 

the basin rather than the water-table gradients adjacent 

to the streams (ground-water discharge outlets). When 

the water table is relatively deep, as at index well Ch-12, 

the response to precipitation is delayed as much as 

several weeks. Thus, if wells located nearer streams and 

having shallower depths to water had been used, the 

recharge from precipitation might have occurred more 

quickly, and would have been sustained from increased

flow in the aquifer, with the result that the water-table 

fluctuations would have more nearly resembled those of 

the creek. In particular, the rise in water table during 

the winter would have been more rapid, the average 

stages for the winter months, especially December, 

January and February, would have been relatively 

higher than those in figure 7, and consequently the relation line between ground-water stage and base flow for 

the condition of no evapotranspiration from ground 

water would be curved and would increase in slope at 

lower stages. The estimated evapotranspiration from 

ground water would then be less than that estimated in 

tables 4 and 5. A base-flow recession curve derived 

from such a curved relation between ground-water 

stage and base flow would resemble the winter recession 

curve more nearly than the hypothetical recession curve 

in figure 8.

A comparison of local relief and depth to the water 

table in wells shows that, at most of the wells at least, 

the water table is considerably higher than the nearby 

stream where ground-water discharge occurs, even in 

the fall when the water table is lowest. If the winter 

relation between base-flow and ground-water stage for 

the three index wells and the creek (fig. 7) held true 

beyond the lower limits observed for 1952-53, zero flow 

would occur at a ground-water stage 10.7 inches above 

the 28-foot depth datum. Yet the water table corresponding to this stage in each of the three wells would 

still be substantially above the nearby stream, particularly in well De 3 (fig. 3). This condition cannot, 

of course, really exist so long as any drainable water 

remains in the rocks. Therefore, the slope of the relation line in figure 7 would have to steepen sharply below the limits of the 1952-53 data (stage of about 27 

inches). However, a more gradual steepening appears 

more reasonable, and would be consistent with the 

curved stage-flow relation line suggested above. It is 

concluded that the drainable void space in the rocks 

decreases rapidly, perhaps abruptly, below the lower 

limit of water-table fluctuation.

Meinzer and Stearns (1929, p. 129) discuss the significance of the ground-water stage corresponding to 

zero flow. They state:

As the water table conies near this point the ground-water 

runoff approaches zero, and therefore the actual arrival of the 

water table at the point of zero flow would be indefinitely delayed were it not for the fact that evaporation of ground water 

would continue and might easily carry the water table below 

this point a condition common in arid regions.

Owing to the lack of suitable data for a drought period the stage-flow relations could not be defined for 

flows approaching zero.
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA. A-15

Some of the difference between the winter recession 

curve and the hypothetical curve in figure 8 may be 

caused by an error in defining the winter curve. The 

upper and lower parts are extrapolated from Riggs' 

data (written communication, 1957) and therefore are 

less certain than the central part. Also, the upper part 

might be too steep and the lower part too flat because 

most of the higher base flows occur in the spring when 

evapotranspiration is increasing rapidly, and most of 

the lower base flows occur in the fall when evapotranspiration is decreasing rapidly.

During periods of little or no rain in October and 

November the discharge of Brandywine Creek, and 

many other streams in the region, may decrease at much 

lower rates than normal, remain practically constant, 

or even rise slightly because of the decreases in evapotranspiration from ground water.

The slope of the recession curve is steeper and the 

effects of changing rates of evapotranspiration from 

ground water are less apparent at the higher base flows 

that prevail in the spring months than in the fall but 

these factors probably are of equal magnitude in spring 

and fall and cause the curves defined by conventional 

methods to be steeper than a recession curve for a constant rate of loss.

In summary, it is concluded that if index wells had 

been selected to indicate the average water-table gradients adjacent to streams rather than average fluctuation of water table in the basin, the relation line between 

ground-water stage and base flow of the creek probably 

would be curved rather than straight as shown in figure 

7. However, the curvature probably would not be as 

great as that in a relation line derived from the winter 

base-flow recession and figure 7 because of the probable 

effects of evapotranspiration from ground water on the 

winter recession curve and the uncertainties in definition of the upper and lower parts of that curve. The 

evapotranspiration from ground water calculated from 

a curved relation line would be less than that calculated 

from the straight line shown in figure 7, and the average 

ground-water discharge for 1952-53 would be correspondingly less than 489 cfs (316 mgd) given in table 4.

COMPARISON WITH RESULTS OF OTHER STUDIES

Comparison of relations between ground-water stage 

and base flow in the Brandywine Creek basin with those 

reported in other areas reveals not only similarities but 

also significant differences.

In their detailed study of the Beaverdam Creek basin, 

Maryland, Easmussen and Andreasen (1959, fig. 6) 

found that the slope of the stage-flow relation increases 

markedly with decrease in stage, and that the points 

representing monthly averages do not define a loop.

Part of the curvature may be due to the changes in 

evapotranspiration in spring and fall, as described 

above. The absence of a loop is probably due to the fact 

that evapotranspiration from ground water occurs over 

the entire Beaverdam Creek basin instead of being restricted largely to the stream valleys as in the Brandywine Creek basin. Thus, in the Beaverdam Creek basin, 

increased evapotranspiration in the growing season 

would tend to lower ground-water levels almost uniformly throughout the basin instead of only in the zones 

adjacent to the streams, and the relation of average 

ground-water stage to base flow would be nearly the 

same at all times. The average ground-water stage 

recession rate at a given stage would vary with the season instead of being nearly constant as in the Brandywine Creek basin.

In the Dilldown Creek basin, a 1,530-acre area in the 

Delaware-Leliigh Experimental Forest (Pennsylvania 

Dept. of Forests and Waters, 1951) the relation of 

ground-water stage in two index wells to base flow is 

similar to that determined, by Rasmussen and Andreasen 

(1959), except that the stage-flow relation is even more 

strongly curved, and significant seasonal differences in 

the stage-flow relation were observed. Two curves are 

shown by the Pennsylvania Department of Forests and 

Waters (1951, fig. 16): one for the growing season, May 

to September; the other for the nongrowing season, 

October to April.

Albertson 2 (1942), who studied the Rapid Creek 

watershed, a small basin in Iowa, found a logarithmic 

relation between base flow and ground-water stage 

which is similar in a qualitative way to the relations 

observed in the Beaverdam Creek and Dilldown Creek 

basins. Albertson found a different stage-flow relation from year to year, however.

In the Pomperaug Eiver basin, Conn., Meinzer and 

Stearns (1929) found a curved relation similar to but 

having decidedly less curvature than the relations for 

the Beaverdam Creek, Dilldown Creek, and Kapid 

Creek basins. Meinzer and Stearns also found seasonal changes in the stage-flow relation which they attributed to ground-water evaporation (evapotranspiration 

as presently defined).

Whelan (1950) in a study of the Baker River basin, 

an area of 143 square miles in the White Mountains of 

New Hampshire, noted markedly different groundwater depletion (base-flow recession) curves for different months; the differences apparently were due to seasonal changes in evapotranspiration from ground water.

Not all investigators have observed the curved stageflow relation described above. Clark (1956, fig. 4c)

a Albertson, M. L., 1942, Ground-water flow In Kapid Creek watershed : Iowa Univ. M.S. thesis.
A-16 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

determined a nearly linear 

stage in one index well and 

Okla., after lie had removed 

piration from ground water 

streamflow loss) by using an 

relation is very similar to 

Brandy wine Creek basin (fig.

relation of ground-water 

base flow in Pond Creek, 

the effects of evapotrans- 

(which he called index of 

empirical formula. This 

that determined in the

COMPUTATION OF GROUND-WATER PARAMETERS WITH 

THE AID OF STREAMFLOW DATA

Two hydrologic parameters 3 that commonly are most 

useful in appraising the water-yielding potential of 

rocks or earth materials are: (a) coefficient of storage 

and (b) coefficient of transmissibility. For the watertable conditions that probably prevail in most of Brandywine Creek basin, the coefficient of storage is about 

equal to the specific yield or long-term gravity yield.

GRAVITY YIELD OF THE ZONE OF WATER-TABLE 

FLUCTUATION

As defined by Rasmussen and Andreasen (1959, p. 

83) : "The gravity yield of a rock or soil after saturation or partial saturation is the ratio of (a) the volume 

of water it will yield by gravity to (b) its own volume 

during the period of ground-water recession." Conversely, gravity yield of the material after drainage or 

partial drainage may be considered as the ratio of the 

volume of water it will take into storage to its own 

volume during the period of ground-water rise. In a 

strict sense this is a short-term coefficient of storage, 

which for convenience in this paper is referred to as 

gravity yield. Gravity yield, thus defined, ordinarily 

is less than specific yield, which is attained only after 

long periods of recession or rise, when gravity drainage 

or refilling is complete. For periods of several months 

the gravity yield probably is not significantly less than 

the specific yield.

In the following analyses, it is assumed that the observation wells and index wells indicate the average 

fluctuations in the water table throughout the basin and 

that almost all the ground-water discharge to streams 

is from the water-table (unconfined) zone.

COMPUTATION BY METHOD A

Gravity yield may be calculated from the simple 

equation

g~AHg

in which Tff is gravity yield (a dimensionless ratio), 

&Sg is the increase in ground-water storage in a specified

3 Parameter, in this report, refers to a characteristic that can be 

measured, in contrast to a characteristic that can only be described.

period (expressed in inches of water over the area), and 

AHff is the corresponding increase in ground-water stage 

(expressed in inches). The change in stage may be 

measured in observation wells, but the change in storage 

must be determined indirectly from base-flow data.

During periods of no evapotranspiration and no 

recharge to ground water the change in storage is equal 

to the runoff from ground water, which may be substituted for increase in storage in the equation above. 

(Withdrawal of water from wells is considered negligible.) These ideal conditions seldom, if ever, occur. 

However, the base-flow recession curve for winter (fig. 

8) represents conditions when the evapotranspiration 

loss from ground water is relatively small. The runoff 

from natural storage derived by integration of the 

recession curve is considered a fair estimate of the runoff 

from ground water and hence of change in storage. 

The actual change in storage is somewhat greater 

than the runoff from ground water because of a small 

amount of evapotranspiration from ground water.

300

on an interpretation of 

stream flow records by 

H. C. Riggs (dashed parts 

extrapolated)

0 20 40 60 80 100 120 140 160 

TIME, IN DAYS

FIGUEB 8. Base-flow recession curves for Brandywlne Creek at Chadds 

Ford, Pa. Hypothetical curve computed from figures 6 and 7 ; see text.

The part of the base-flow recession curve between 

about 410 cfs and 120 cfs is plotted from data furnished 

by H. C. Riggs (written communication, 1957) The 

extrapolated portions above and below these limits, 

shown by dashed lines in figure 8, are needed to cover 

the range in data for the study period.

Three seasonal periods were analyzed: two recession periods, one from late April or early May to mid-
RELATION BETWEEN GROUND AND S0RFAC& WATERS, BRANDYWINE CREEK BASIN, PA. A-17

November 1952, and the other from middle or late 

March to late October 1953; and one period of rise, 

from mid-November 1952 to middle or late March 1953. 

The ground-water stages and seasonal changes in stage

at each of the 16 observation wells and the averages 

for these periods are listed in table 6. The corresponding base flows are obtained from the base flow hydrograph in figure 5.

TABLE 6. Seasonal changes in ground-water stage in 16 wells, 1952-53

Well

Bd-10 _ . _

Ch-2..- _ .......... .............

3... ___ . ___ ... _____ .

5 _ ...._.

6- _ - __ . _ . ____ . __

7

8 . _. 

9- ____ ..... _______ . ...

10 . _. -..._____

11 ... _ . ___________ .

12... ___ .. __ .......... .

13 ..........................

14 ............ __ . ___ ..

15 __ ... __________ ..

De-3 - __ ... _ . _____ . .....

Average.. __________

Spring 1952

Date

May 2 

Apr. 29 

May 2

Apr. 29

May 2 

Apr. 27 

Apr. 29

May 2

 do 

...do ..

...do.....

May 4

Depth to 

water (feet)

10.30 

4.20 

18.48 

19.45 

1.55 

12.19 

28.05 

6.86 

12.53 

8.46 

10.36 

30.90 

12.62 

17.42 

22.38 

12.09

14.24

Change in 

stage, spring 

to fall 1952 

(feet)

<? 7Q

-6.50 

-7.35 

-6.75 

-4.66 

-4. 67

7 SI

-9.24 

-5.89 

-5.17 

-2.40 

-5.40 

-3.43 

-4.98 

-7.64 

-4.60

-5.64

Fall 1952

Date

Nov. 19 

Nov. 18 

Nov. 19 

Nov. 16 

Nov. 20 

Nov. 19 

 do 

Nov. 16 

Nov. 19 

 do 

Nov. 17 

Nov. 16 

Nov. 19 

 do . 

.Nov. 17 

Nov. 21

Depth to 

water (feet)

'14,09 

10.70 

.125.83 

26.20 

6.21 

U6.86 

»35.86 

16.10 

i 18. 42 

1 13. 63 

12.76 

36.30 

i 16. 05 

122.40 

a 29. 92 

16.69

19.88

Change in 

stage, fall 1952 

to spring 1953 

(feet)

2.74 

4.97 

6.21 

5.70 

4.46 

3.82 

6.48 

7.10 

6.83 

4.91 

2.54 

5.70 

3.65 

5.05 

8.67 

4.79

5.23

Spring 1953

Date

Mar. 31 

Mar. 24 

Mar. 20 

...doMar. 28 

Mar. 20 

...do 

 do 

... do 

 do 

... do .

Mari 22 

Mar. 20 

Mar. 30 

Mar. 20 

Mar. 30

Depth to 

water (feet)

11.35 

5.73 

19.62 

20.50 

1.75 

13.04 

29.38 

9.00 

11.59 

8.72 

10.24 

30.60 

12.40 

17.35 

21. 25 

11.90

14.65

Change in 

stage, spring 

to fall 1953 

(feet)

-2.72 

-5.74 

-9.51

-5.22 

-5.11 

-7.20

-9.30 

-9.07 

-5.34 

-3.23 

-6.15 

-4.38

-5.43 

-10. 73

5

Q7

-6.37

Fall 1953

Date

Oct. 30 

Oct. 25 

Oct. 28 

Oct. 23 

Oct. 28 

Oct. 23 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

... do 

Oct. 29

Depth to 

 water (feet)

14.07 

11.47 

i 29. 13 

27.40 

6.97 

18.15 

36.68 

18.30 

20.66 

14.06 

13.47 

36.75 

16.78 

22.78 

31.98 

17.77

21.02

i Estimated.

a Depth reported, 24.92 ft, probably 5 ft in error.

The calculation of the gravity yield for each of the 

three periods is shown below ; the base flow at the ends 

of the periods are taken from figure 2 and the change in 

stage is taken from table 6.

Spring to fall 1952

Sg=integral of winter base-flow recession curve in figure 8 from

750 cf s to 162 cfs=33,100 cf s-days=->4.29 in. 

Hg=-5.64 ft.= 67.7 in.

¥g=

 o7.7 in.- =0.063, or 6.3 percent

Fall 1952 to spring 1953

Sg=integral from 162 cfs to 806 cfs=35,900 cfs-days=4.65 in.

Hg=5.23ft.=62.8in.

xr 4.65 in.

62.8 in.=0.074, or 7.4 percent

Spring to falll958

Sg=integral from 806 cfs to 95 cfs=42,000 efs-days= 5.44 in. 

Hg=-6.37 ft.=-76.4 in.

 ^ AA in

°-071> °r 7'1 Percent

Arithmetic sum of seasonal fluctuations

Sg=14.38in. 

Hg=206.9 in. 

v 14.38 in. JLa - =0.070, or 7.0 percent 206.9 in.

If the calculation is based on the 3 index wells instead 

of 16 wells, the change in storage is practically the same 

as above, but the total fluctuation in ground-water stage 

is reduced to 191.9 inches, and

y _ 14.38 in. » * - ff ~f9T9ln:~0-0r4' orr-4 Percent 

This result is more nearly comparable with the gravity 

yield obtained by method B from the index wells.

COMPUTATION BY METHOD B

The effects of evapotranspiration from ground water 

(which are not entirely absent in the winter base-flow 

recession curve) can be eliminated by using the hypothetical recession curve (fig. 8). This advantage may 

be offset, at least in part, by the greater chance for error 

in the more indirect method described below.

For any period the ground-water discharge to streams 

corresponding to no loss by evapotranspiration from 

ground water (which is equal to change in storage) may 

be obtained by integrating the hypothetical recession 

curve in figure 8 between the limits corresponding to the 

beginning and end of the period. Using these values 

for increase in storage and the same procedure as used 

in method A the gravity yield is computed as 10.7 

percent.

Because of the linear relation between ground-water 

stage and base flow, values of gravity yield computed 

by this method are the same at all stages. However, as 

previously noted, the available evidence indicates that 

gravity yield probably decreases sharply at depths 

below the lower limit of water-table fluctuation.

EVALUATION OP RESULTS

The difference in values of gravity yield computed 

by the two methods is due to the differences in the baseflow recession curves, both of which are subject to error.
A-18 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

Earlier discussion indicated that the winter recession 

curve may be too flat at the lower end, and that the 

hypothetical curve representing no loss probably should 

resemble the winter recession curve more closely than 

it does. Although part of the difference between the 

curves is due to inclusion in the winter curve of effects 

of evapotranspiration from ground water, probably a 

large part of the difference is due to errors in definition 

of the curves.

The accuracy of the estimates of gravity yield by both 

methods depends in part on whether the index wells and 

observation wells provide a representative sample of 

average water-table fluctuation in the basin, and in part 

on whether essentially all the ground-water discharge 

to streams is from the water-table (unconfined) zone. 

For example, if the average fluctuation of the water 

table in the basin were double that in the index wells or 

observation wells, the gravity yield would be only half 

as much as the values estimated in the study. If a significant amount of ground-water discharge occurs from 

perched or semiperched water bodies above the true 

water table, or from confined or semiconfined water 

bodies below the water-table zone, the estimates of 

gravity yield are too high. Ward 4 made the following 

pertinent observation:

In some places two or more water-bearing zones may be 

encountered in deep excavations. The first occurs in the 

weathered zone and the water there can probably be drained 

off with tiles. Below the zone of weathering massive rock may 

be encountered that will yield no water. Upon continued excavation, zones of shearing or closely spaced joints may be 

encountered. These are usually saturated, and flows up to 

several gallons per minute may be encountered.

This suggests that, at some places at least, more than 

one water body exists in the crystalline rocks, and that 

appreciable ground-water discharge to streams may 

take place from fractures below, and not freely connected with, the water-table zone.

In summary, the average gravity yield of the zone of 

water-table fluctuation probably is between about 7% 

and 10 percent. However, this estimate may require 

modification if information becomes available to show 

that the magnitude of average water-table fluctuation 

in the basin is appreciably different from that indicated 

by the wells used in the study, or that a significant 

amount of natural ground-water discharge occurs from 

zones above or below the water-table zone tapped by the 

wells.

By comparison, Herpers and Barksdale (1951, p. 27) 

estimated that the average specific yield (long-term 

gravity yield) of the zone within 300 feet of the land

* Ward, R. P., 1966, The geology of the Wissahickon formation of Delaware : Unpublished report prepared in cooperation between Delaware 

State Highway Dept., Delaware Geol. Survey, and U.S. Geol. Surrey.

surface in the Brunswick formation (red shale and 

sandstone of Triassic age) in the Newark, N.J., area is 

on the order of 1 or 2 percent. The average gravity 

yield of the unconsolidated sandy sediments in the zone 

of water-table fluctuation in Beaverdam Creek basin, 

in the Coastal Plain province in Maryland, was estimated to be about 11 percent by Rasmussen and Andreasen (1959).

COEFFICIENT OF TRANSJOSSIBILITY OF ROCKS IN 

THE BASIN

Coefficient of transmissibility is ordinarily measured 

by means of a pumping test made with a pumped well 

and one or more observation wells. The methods have 

become standardized and are described by Wenzel 

(1942) and in more recent papers. The standard test 

procedures, although yielding reasonably accurate results, may be interpreted only in terms of the relatively 

small sample of material in the vicinity of the pumped 

well. Moreover, in many areas, including Brandywine 

Creek basin, pumping-test data are not available.

Methods of approximating the coefficient of transmissibility from numerical analysis of water-level data 

are described by Stallman (1956). For hypothetical 

cases where a very thick uniform aquifer discharging to 

a stream or other body of surface water is assumed, the 

coefficient of transmissibility may be calculated from 

formulas devised by Jacob (1943) .

In Brandywine Creek basin, both the water-level 

data required for Stallman's method and the thick, 

uniform aquifer assumed by Jacob are lacking. However, a very rough estimate of the average coefficient 

of transmissibility may be made, using the total groundwater discharge (assumed equal to base flow plus evapotranspiration from ground water) estimated in the 

preceding section, the average hydraulic gradient 

(water-table slope) adjacent to the discharge areas, 

and the length of the discharge areas. The equation 

is

..... in which T is the coefficient of transmissibility in gallons per day per foot, Ra is the base flow in gallons per 

day, ETg is evapotranspiration from ground water in 

gallons per day, 7 is the average water-table slope adjacent to discharge areas in feet per mile, and L is the 

length of discharge areas in miles (as indicated by 

length of blue lines representing streams on standard 

topographic quadrangle maps) .

The average ground- water discharge (including 

evapotranspiration from ground water) for the 2-year 

period 1952-53 (col. 3, table 4) was 489 cfs or
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASIN, PA. A-19

The average water-table slope was computed as the 

average slope of the water table between the observation wells and the discharge outlets. The elevations 

of the natural discharge outlets (usually small stream 

valleys) downgradient from the wells in a direction 

perpendicular to the landslope were obtained from 

topographic maps. The average depths to water were 

calculated as the arithmetic average of the seasonal 

extremes for the calendar years 1952-53 (last col. table 

1). This depth was subtracted from the elevation of 

the land surface at each of the wells to obtain the elevation of the water table. The difference in elevation 

was divided by the horizontal distance from each well 

to the discharge outlet to obtain the average watertable slope in feet per mile. The slopes at the three 

index wells are illustrated in figure 3; the values for 

15 observation wells are listed below (well Bd-10 was 

omitted because it was too near the discharge outlet 

to permit an accurate estimate of the slope).

Well

Ch- 2.. ______________

3 _ _

4_ ________ _

5. _ .__. __ ..__

6_ ____ _________

7. ____ _________

8. _ _ ____ ____

9____.___ __ ____

10____ _ _ _ _____

11 _ ---. __ _____

12___-__. __ _____

13 _ __

14_---_--___-_____

15__ _ ----_._____

De-3__ _ 

Average. __ ___

Distance to 

discharge outlet (miles)

0.40

.25

. 18

.23

. 18

.20

. 16

.21

.18

.20

.08

. 16

.07

. 13

. 17

Average difference 

in elevation of water 

table (feet)

17

19

59

46

5

10

12

44

5

38

11

70

20

3Q

70

Average slope 

of water table 

(feet per mile)

42

76

330

200

28

65

75

210

28

190

140

140

290

300

410

190

The discharge areas in the basin are arbitrarily assumed to coincide with the streams shown by blue lines 

on the 1:62,500 topographic maps of the basin, although 

these are not accurate measures of lengths of discharge areas, which vary with season and water-table 

conditions. The total length of streams, calculated 

from Langbein's figure for stream density of 2.26 miles 

per square mile (Langbein and others, 1947), is 649 

miles. Ground water moves toward the discharge outlet 

from both sides of each stream, so that the total length 

of transmitting section is 2X649 miles=1,298 miles.

The coefficient of transmissibility is then calculated 

as follows:

T=- 316X106 (gal)(day)- -,=1,300 (gal)(day)-1 (ft)-1

"190 (ft)(mi)-1 X2X649 mi

Another rough estimate of average water-table 

gradient was made by using stream density, average

landslope, and an assumed average depth to the water 

table under drainage divides. In this estimate it is assumed for the sake of simplicity that the average slope 

of the water table adjacent to the discharge areas is the 

same as that from drainage divides to streams. Stream 

density is 2.26 miles per square mile; average interstream distance is the reciprocal of stream density; and 

the average distance from streams to divides is half the 

average interstream distance

1

;=0.22 mi 2X2.26(mi)(mi)-2 " 

Average landslope is 450 feet per mile (Hely and Olmsted, written communication, 1961); the average elevation of divides above the streams (the average relief) is

450 feet per mile X 0.22 mile=100 feet

The average depth to the water table beneath divides is 

assumed to be 35 feet slightly more than the deepest 

levels in the 16 observation wells. The average slope of 

the water table from divides to streams is therefore 

(100-35) feet+0.22 mile=300 feet per mile. Substituting this value for the value of 190 feet per mile 

used in the previous estimate, the coefficient of transmissionibility is 800 gpd per foot, instead of 1,300.

The estimates of the coefficient of transmissibility 

probably are less accurate than those of gravity yield. 

The calculated water-table gradients and length of discharge areas are the most uncertain quantities used in 

the estimates; the value for total ground-water discharge is considerably more accurate. The assumption 

of a uniform, continuous water-table gradient adjacent 

to the discharge areas, as obtains in homogeneous granular material, is not applicable to parts of Brandywine 

Creek basin. In fractured crystalline rock, the zone of 

saturation is discontinuous, and the water table may 

stand at different levels in different sets of fractures. 

The gradients adjacent to the discharge areas almost 

certainly are not the same as the average gradients from 

ground-water divides to streams, as assumed in the second estimate.

In permeable materials extending to great depth the 

amplitude of water-table fluctuation is small in proportion to the total thickness of water-bearing material, 

so that the thickness of that portion of the material 

transmitting water varies only slightly as the water 

table fluctuates. In the weathered and fractured crystalline rocks of Brandywine Creek basin however, the 

drainable void space decreases rapidly with depth below 

the water table so that, as the water table declines, the 

effective thickness, and consequently the transmissibility, of the water-bearing zone likewise decreases 

rapidly. Therefore, a coefficient of transmissibility determined for a wet period, like 1952-53, would be signifi-
A-20 CONTRIBUTIONS TO STREAM-BASIN HYDROLOGY

cantly higher than that determined for a drier period 

when the water table was lower.

It is concluded that the average coefficient of transmissibility of the rocks in the basin for the calendar 

years 1952-53 is on the order of 1,000 gpd per foot. In 

comparison, the coefficient of transmissibility estimated 

from average specific capacity of wells in the Piedmont 

Upland of the Delaware River region is about 1,500 

gpd per foot (Hely and Olmsted, written communication, 1961). Callahan and Stewart (1959) report that 

pumping-test data from the weathered crystalline rocks 

in the Piedmont province in Georgia indicate a coefficient of transmissibility ranging from 500 to 4,000 gpd 

per foot. The rocks there are similar to those in 

Brandywine Creek basin, but the weathered zone is 

thicker and the degree of weathering possibly is more 

intense.

CONCLUSIONS

As an average for periods of several years, about twothirds of the total runoff of Brandywine Creek is base 

flow (chiefly ground-water discharge to the streams). 

This proportion is significantly higher than in streams 

draining areas underlain by most other types of consolidated rocks in the region and is nearly as high as 

the base flows of streams draining areas underlain by 

permeable sandy sediments in the Coastal Plain.

A direct, linear relation exists between the monthly 

average ground-water stage in three index wells (which 

are assumed to have indicated average water-table conditions in Brandywine Creek basin) and base flow of 

the creek in winter months, when evapotranspiration 

from ground water is very small. However, if index 

wells had been selected so as to indicate average gradient of the water table adjacent to areas of groundwater discharge rather than average fluctuation of the 

water table in the basin, the relation line between 

ground-water stage and base flow would have been 

curved rather than straight, and the slope of such a 

relation line would steepen with decreasing stage.

The average ground-water discharge in the basin 

for the calendar years 1952-53 was 489 cfs (316 mgd), 

of which slightly less than one-fourth was estimated to 

be loss by evapotranspiration (based on the linear relation between ground-water stage and base flow). However, loss estimated from the curvilinear relation described above would be less, and the total discharge correspondingly less.

The average gravity yield of the zone of water-table 

fluctuation in the basin probably is between about 7^ 

and 10 percent surprisingly high for such aquifers. 

This estimate may require modification if future information indicates that the wells used in the study 

do not accurately reflect the average magnitude of

water-table fluctuation in the basin, or that a significant 

amount of ground water is discharged from zones above 

or below the water-table zone tapped by the wells.

Even if allowance is made for possible errors in the 

estimate, the gravity yield is several times as great as 

that of some shale and sandstone of the Brunswick formation of Triassic age, and may not be much less than 

that of some of the unconsolidated sandy sediments of 

the Coastal Plain. However, the gravity yield in 

Brandywine Creek basin probably decreases rapidly, 

perhaps abruptly, below the lower limit of water-table 

fluctuation.

The average coefficient of transmissibility of the 

rocks in the basin is relatively low and decreases with 

decreasing ground-water stage. The average value for 

the period 1952-53 is on the order of 1,000 gpd per foot. 

It agrees generally with an estimate made from average 

specific capacity of wells in the region and is within 

the limits obtained from pumping tests of wells in 

similar rocks in Georgia.

Average gradients of the water table in the basin are 

rather steep one estimate is 190 feet per mile, and another 300 feet per mile. Most of the drainable water in 

the rocks is above the stream channels, which are the 

major discharge outlets.

The seeming inconsistency of low to moderate groundwater yield to wells and high yield to streams in the 

crystalline rocks of the Piedmont Upland was cited in 

the introduction. Several of the conclusions stated 

above help to explain this paradox.

The most plausible explanation concerns the nature 

of the bedrock, the overlying weathered zone and soil, 

and the topography. Most of the water-bearing openings in crystalline rocks are in the weathered zone and 

in the immediately underlying fractured rock. Normally the water table lies within the lower part of the 

weathered zone, or locally in the upper part of the fractured zone. The gravity yield and consequent groundwater storage capacity of the zone of water-table fluctuation are high, and the deep, permeable soils permit 

much of the precipitation to infiltrate to the water table 

to be stored for later ground-water discharge to streams 

as base flow. The rocks have comparatively low transmissibility and their gravity yield or drainable void 

space decreases rapidly with depth. Because of the low 

transmissibility, the rapid decrease of gravity yield with 

depth, and the fairly rugged topography, water-table 

gradients are steep. Consequently, relatively high 

ground-water discharge to streams is maintained by 

drainage from the substantial ground-water reservoir, 

despite the fairly low transmissibility of the rocks.

On the other hand, well yields in the basin are restricted by the small thickness and fairly low transmissi-
RELATION BETWEEN GROUND AND SURFACE WATERS, BRANDYWINE CREEK BASEST, PA. A-21

bility of the water-bearing material. A well has a 

small length and area of contact with water-bearing 

material in comparison with the stream system, and 

the yield of most wells increases slightly, if at all, 

when they are deepened below the zone of weathering 

and fracturing or when the pumping drawdown is 

increased and lowers the water level much below that 

zone.

The above conclusions apply to problems of estimating the potential productivity of aquifers. In general, 

no simple relation exists between the amount of groundwater discharge in an area and the proportion of this 

discharge that can be diverted to wells. Natural 

ground-water recharge and discharge are almost as 

large in the Brandywine Creek basin as in the sandy 

parts of the Coastal Plain, but the productivity of wells 

in crystalline rocks in Brandywine Creek basin is 

generally much less than that of wells in the sandy 

aquifers of the Coastal Plain.

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U.S. GOVERNMENT PRINTING OFFICE: 1962