Streamflow Characteristics of the 

Waccamaw River at Freeland, 

North Carolina, 1940-94

By Jerad D. Bales and Benjamin F. Pope

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 96-4093

Prepared in cooperation with the

North Carolina Department of Environment,

Health, and Natural Resources,

Division of Water Resources

Raleigh, North Carolina 

1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

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

For additional information write to:

District Chief 

U.S. Geological Survey 

3916 Sunset Ridge Road 

Raleigh, NC 27607

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Federal Center 

Denver, CO 80225-0286
CONTENTS

Abstract..............................................................................^ 1

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

Purpose and scope........................................................................................................................................................... 3

Study area and available data........................................................................................................................................... 3

Approach......................................................................................................................................................................... 4

Streamflow characteristics of the Waccamaw River.................................................................................................................. 6

Precipitation......................................................................................................................................^ 6

Annual yields................................................................................................................................................................... 9

Monthly streamflow characteristics............................................................................................................................... 10

Rescaled cumulative departures........................................................................................................................... 11

LOWESS-smoothed records................................................................................................................................ 14

Flow distributions................................................................................................................................................ 16

Double mass curve............................................................................................................................................... 16

Daily flow durations...................................................................................................................................................... 17

Low flows...................................................................................................................................................................... 19

Highflows........................................................^

Base-flow separation...................................................................................................................................................... 26

Base flows............................................................................................................................................................ 26

Rimoff..........................................................................................................................................................^

Flow variability.............................................................................................................................................................. 30

Conclusions ............................................................................^^ 33

References............................................................................................................................................................................... 35

FIGURES

1. Map showing locations of streamflow and precipitation stations in the Waccamaw River study area,

North Carolina.............................................................................................................................................................. 2

2-6. Graphs showing:

2. Annual departures from 1940-94 mean rainfall at Elizabethtown, N.C.............................................................. 7

3. Five- and 10-year moving averages of annual rainfall amounts at Elizabethtown, N.C., 1940-94..................... 7

4. Differences between annual rainfall amounts and 5-year moving average of differences at

(A) Elizabethtown and Laurinburg, and (B) Elizabethtown and Lumberton ...................................................... 8

5. Ratio of annual streamflow to annual rainf all at the Waccamaw and Lumber Rivers, 1940-94.......................... 9

6. Mean monthly percentage of 1940-94 mean precipitation at Elizabethtown and 1940-94 mean streamflow 

at the Waccamaw River at Freeland................................................................................................................... 11

7-10. Hydrographs showing:

7. Five-year moving average of monthly mean streamflow at the Waccamaw River at Freeland and

the Lumber River at Boardman, 1940-94.......................................................................................................... 12

8. Monthly mean streamflow for the Waccamaw and Lumber Rivers, 1940-94................................................... 13

9. Rescaled cumulative departures (RCD's) of monthly mean streamflow for the Waccamaw and

Lumber Rivers and Drowning Creek, 1940-94.................................................................................................. 14

10. LOWESS-smoothed plots of monthly mean streamflow during 1940-94 for the Waccamaw

and Lumber Rivers using smoothness factor (/) of (A) 0.5 and (B) 0.2............................................................ 15

11. Boxplots of monthly distributions of flow in the Waccamaw River for five 10-year periods

between 1945 and 1994.............................................................................................................................................. 17

12-22. Graphs showing:

12. Double mass curve of accumulated differences between observed Waccamaw River monthly flows and

Waccamaw River monthly flows predicted from Lumber River monthly flows, 1940-94................................ 18

Contents III
13. Daily flow duration curves for the Waccamaw and Lumber Rivers, 1940-94................................................... 19

14. Daily flow duration curves for the Waccamaw River for the periods (A) 1945-54, 1975-84, and 1985-94

and (B) 1949-58, 1970-79, and 1985-94............................................................................................................20

15. LOWESS-smoothed plots of annual 1-day, 7-day, and 30-day low flows for the Waccamaw and

Lumber Rivers, 1940-94 ....................................................................................................................................22

16. LOWESS-smoothed plots of annual 1-day, 7-day, and 30-day high flows for the Waccamaw and

Lumber Rivers, 1940-94 ....................................................................................................................................24

17. Difference between annual (A) 1-day and (B) 7-day high flows for the Waccamaw and Lumber Rivers......... 25

18. Ratio of total annual base flow to total annual flow for the Waccamaw and Lumber Rivers, 1940-94............. 27

19. Relation of base-flow index to ratio of total annual base flow to annual mean flow for the Waccamaw

and Lumber Rivers.............................................................................................................................................28

20. LOWESS-smoothed monthly mean base flow and runoff for the Waccamaw and Lumber Rivers, 1940-94 ... 29

21. Three-year moving average of the ratio of base flow to runoff for the Waccamaw and

Lumber Rivers, 1940-94 ....................................................................................................................................30

22. LOWESS-smoothed curves of 12-month moving range of (A) total flow, (B) runoff, and (C) base flow

for the Waccamaw and Lumber Rivers, 1940-94............................................................................................... 31

23. Boxplots of the Waccamaw River distributions of monthly 12-month moving ranges of (A) total flow,

(B) runoff, and (C) base flow for the periods 1949-58, 1970-79, 1975-84, and 1985-94........................................... 32

24. Graph showing percentage of time 10-day moving range of daily flow values were equaled or exceeded for

the Waccamaw River, 1940-94.................................................................................................................................... 33

TABLES

1. Streamflow gaging stations used in data analysis........................................................................................................4

2. Precipitation measurement stations used in data analysis........................................................................................... 4

3. Monthly and annual precipitation statistics for selected sites in the study area.......................................................... 6

4. Difference between measured temperatures at Whiteville, 1984-93, and 1951-80 average temperatures................ 10

5. Results of rank sum tests comparing distributions of monthly flow for the Waccamaw and

Lumber Rivers during selected 10-year periods........................................................................................................ 16

6. Selected low-flow statistics for the Waccamaw and Lumber Rivers and Drowning Creek, 1940-94........................ 21

7. 7Q10 low flows computed from data for selected 10-year periods ...........................................................................21

8. Selected peak-flow statistics for the Waccamaw and Lumber Rivers and Drowning Creek, 1940-94...................... 23

9. Results of rank sum tests comparing monthly distributions of runoff and base flow for the Waccamaw and

Lumber Rivers during selected 10-year periods........................................................................................................ 29

IV Contents
Streamflow Characteristics of the Waccamaw River 

at Freeland, North Carolina, 1940-94

ByJerad D. Bales and Benjamin F. Pope

ABSTRACT

Streamflow characteristics of the 

Waccamaw River at Freeland, North Carolina, for 

the period 1940-94 were described and compared 

to streamflows in the adjacent Lumber River 

Basin. Precipitation in the two basins was about 

equal for the study period. During 1940-63, 

streamflows in the Waccamaw and Lumber Rivers 

were essentially identical relative to average conditions. The flow regime from the late 1950's to 

the early 1980's was distinctly wetter than the 

flow regimes which immediately preceded and 

followed this period. Following 1963, droughts in 

the Waccamaw Basin seem to have been less 

severe than in the Lumber Basin, and the annual 

1-, 7-, and 30-day low flows exhibited a slightly 

increasing trend in the Waccamaw River. Mean 

daily flow in the Waccamaw River at the 

90 percent exceedance level (low flows) during 

1985-94, a relatively dry period, was very nearly 

equal to flows at the same exceedance level for 

1970-79, the wettest 10-year period between 1940 

and 1994. Prior to the 1980's, flows per unit 

drainage area in the Waccamaw Basin were 

generally less than those in the Lumber Basin, but 

after 1980, the opposite was true. There is an 

increasing trend in the difference between 

Waccamaw River and Lumber River high flows, 

primarily as a result of increases in Waccamaw 

River high flows. On average, Streamflow in the 

Waccamaw River consisted of 53.3 percent base 

flow, but base flow accounted for 70.6 percent of 

the total flow in the Lumber River, which is more 

typical of Coastal Plain streams. The ratio of base 

flow to runoff in the Waccamaw River may have 

changed relative to that in the Lumber River in the

late 1970's. There was greater variability in 

Waccamaw River Streamflow than in Lumber 

River flow, and flow variability in the Waccamaw 

River may have increased slightly during 

1985-94.

INTRODUCTON

The Waccamaw River, which originates at Lake 

Waccamaw, drains an area of 1,257 mi2 (square miles) 

in extreme southeastern North Carolina (fig. 1). In 

1990, 63 percent of the basin was covered with forest, 

27 percent was cropland, and 2.3 percent was urban or 

developed (North Carolina Division of Environmental 

Management, 1993). The 1990 population of the basin 

was 48,586, compared to 42,691 in 1970 (North 

Carolina Division of Environmental Management, 

1993). A U.S. Geological Survey stream gage has been 

in continuous operation on the Waccamaw River at 

Freeland (fig. 1) since August 1939.

Citizens in Brunswick and Columbus Counties 

have expressed concerns about the Waccamaw River, 

including changes in Streamflow characteristics, 

effects of land-use activities on nonpoint-source 

pollutants entering the river, and generally poor waterquality and biological conditions in the river. As one of 

a series of studies funded by the North Carolina 

General Assembly to address these concerns, the U.S. 

Geological Survey, in cooperation with the North 

Carolina Department of Environment, Health, and 

Natural Resources, Division of Water Resources, 

initiated an investigation in 1994 of the Streamflow 

characteristics of the Waccamaw River. The objectives 

of the investigation were to (1) characterize streamflow in the Waccamaw River at Freeland for the period 

1940-94, and (2) compare Waccamaw River flow 

characteristics to flow characteristics of nearby 

streams.

Introduction 1
PIEDMONT COASTAL PLAIN

Area shown 

below

GENERALIZED PHYSIOGRAPHIC PROVINCES IN NORTH CAROLINA

PIEDMONT 79° COASTAL PLAIN

35°

34°

 f "\ - '' Drowning Cr.

L x I i '" /' -

EXPLANATION

GENERALIZED PHYSIOGRAPHIC 

PROVINCE LINE Waccamaw River

BASIN BOUNDARY 

SUBBASIN BOUNDARY

STREAMFLOW GAGING STATION 

AND SITE NUMBER

PRECIPITATION MEASUREMENT STATION 

AND SITE NUMBER

50 MILES

Waccamaw j

River X' .

50 KILOMETERS

Figure 1. Locations of streamflow and precipitation stations in the Waccamaw River study area, North Carolina.

2 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
Purpose and Scope

The purpose of this report is to describe the 

streamflow characteristics of the Waccamaw River at 

Freeland, North Carolina, for the period 1940-94. 

Flows in the Waccamaw River are compared to those 

in the Lumber River at Boardman and Drowning 

Creek near Hoffman for the same period. Precipitation 

for the study period is characterized and is used in the 

interpretation of flow characteristics. Monthly streamflow statistics, including departures from normal (or 

average) conditions, distributions of flows, and trends 

are presented. Flow durations, flood frequencies, lowflow statistics, and base-flow conditions are evaluated. 

Differences among Waccamaw River, Lumber River, 

and Drowning Creek flow characteristics are identified 

and changes in flow conditions are noted. The relation 

between changes or trends in Waccamaw River flow 

characteristics identified in this study and human 

activities is very difficult to determine because of the 

absence of quantitative information on changes in 

basin land use, irrigation, and drainage practices 

during 1940-94.

Study Area and Available Data

The Lumber and Waccamaw River Basins are 

similar in many respects. However, population in the 

entire Lumber River Basin (drainage area of 1,630 mi2 

at the South Carolina State line) increased from 

124,219 in 1970 to 181,064 in 1990 (North Carolina 

Division of Environmental Management, 1993), which 

is a 45-percent increase compared to a 14-percent 

increase in the Waccamaw River Basin during the 

same period. Most of the growth in the Lumber River 

Basin occurred in the Drowning Creek subbasin 

(fig. 1).

Current land use in the Lumber River Basin is 

similar to that in the Waccamaw Basin 50 percent of 

the basin is forest, 36 percent is cropland, and 4 percent is urban or developed areas. Historical land-use 

information generally is not available for either basin. 

Some ditching and draining associated with silviculture and agriculture has occurred in the Waccamaw 

Basin, but there is no quantitative information on the 

extent of these practices nor on changes over time. 

Flows in the Waccamaw River (fig. 1, site 1), Drowning Creek (site 2), and the Lumber River (site 3) are 

not significantly affected by regulation; the town of 

Southern Pines withdrew an average of 2.9 ft /s (cubic

feet per second) from Drowning Creek in 1994. It is 

likely that there are a number of unmonitored withdrawals for irrigation in both basins.

The primary difference between the two basins 

is that the Waccamaw River lies entirely in the Coastal 

Plain Province, but the Lumber River drains both the 

Sand Hills region and the Coastal Plain Province. The 

boundary between the Sand Hills and Coastal Plain is 

near the origin of the Lumber River, at the confluence 

of Drowning Creek and Buffalo Creek.

Lake Waccamaw, from which the Waccamaw 

River originates, receives drainage from an area of 

103 mi2 , or about 15 percent of the drainage area of 

the Waccamaw River at Freeland. The lake has a 

surface area of 8,950 acres and a volume of about 

44,000 acre-feet (North Carolina Department of 

Environment, Health, and Natural Resources, 1992). 

Releases of water from the lake into the Waccamaw 

River are controlled by an outlet structure; configuration of the structure has been changed from time to 

time, but these changes are generally undocumented.

Local topographic gradients in the Coastal Plain 

are less than about 2 ft/mi (foot per mile). As a consequence, hydraulic gradients also are low, resulting in 

less potential to move water from the land to streams 

than in areas with greater relief. In contrast, topographic gradients in the Sand Hills range from 50 to 

200 ft/mi. Low flows are generally much greater in the 

Sand Hills than in other parts of the Coastal Plain 

having similar soils (Giese and Mason, 1993).

Low-flow hydrologic areas (HA) for North 

Carolina were identified by Giese and Mason (1993). 

Most of the Waccamaw Basin upstream from site 1 

(fig. 1) lies in HA1, which is in the Coastal Plain and is 

characterized by clay soils. The median 7Q10 (the 

annual minimum 7-day consecutive low flow that, on 

average, occurs once every 10 years) for streams in 

HA1 is 0.0 (ft3/s)/mi2 (cubic foot per second per 

square mile) of drainage area (Giese and Mason, 

1993). The Drowning Creek subbasin lies entirely in 

HA3 (Sand Hills region, sandy soils). Most of the 

Lumber Basin upstream from site 3 is in either HA2 

(Coastal Plain, sandy soils) or HA3, and is underlain 

by sandy soils with a small percentage of clay-sand 

mix and clay soils in the lower part of the basin. 

Median 7Q10 for HA2 streams is 0.006 (ft3/s)/mi2 and 

is 0.318 (ft3/s)/mi2 for streams in HA3 (Giese and 

Mason, 1993). In general, a higher percentage of 

precipitation on high-permeability sandy soils is

Introduction
Table 1 . Streamflow gaging stations used in data analysis

[USGS, U.S. Geological Survey; mi2, square miles]

Site 

number

(fig- 1)

1

2

3

USGS 

station 

number

02109500

02133500

02134500

Station name

Waccamaw River at Freeland

Drowning Creek near Hoffman

Lumber River at Boardman

Latitude

34°05'55"

35°03'38"

34°26'32"

Longitude

78°32'55"

79°29'39"

78°57'38"

Drainage 

area (mi2)

680

183

1,228

Period of 

record

1939-94

1939-94

1929-94

Table 2. Precipitation measurement stations used in data 

analysis

Site 

number 

(fi9- 1)

4

5

6

Station 

location

Elizabethtown

Laurinburg

Lumberton

Year 

installed

1911

1941

1898

Latitude

34033-

34°45'

34°42'

Longitude

78°35'

79°27'

79°04'

stored in the shallow ground-water system than on 

low-permeability clay soils, resulting in more sustained low flows, as the stored water is released during 

periods of low rainfall.

Records from three streamflow gaging stations 

(table 1; fig. 1) and three precipitation measurement 

stations (table 2; fig. 1) were compiled and analyzed. 

Concurrent records from two nearby gages in the 

Lumber River Basin were analyzed to determine if 

streamflow characteristics at the Waccamaw site 

(site 1) were localized or were consistent with regional 

variations.

At least 11 precipitation stations are located in, 

or very near, the Lumber and Waccamaw River Basins. 

Of the 11 stations, six have records which predate 

1950, and three have records prior to 1939. Records 

from stations at Elizabethtown (site 4), Laurinburg 

(site 5), and Lumberton (site 6) were selected for 

analysis because these stations provide good geographic coverage of the basins, and the stations have 

long periods of record with little missing data.

Approach

Although collection of records at the Lumber 

River site (site 3) began prior to the measurement of 

streamflow on the Waccamaw River in August 1939, 

only concurrent data collected after October 1939 were

included in this analysis. Specific steps in the analysis 

were as follows:

 Comparison of precipitation distributions 

The distribution of annual precipitation amounts from 

the three long-term rainfall sites (table 2; fig. 1) were 

compared to determine if data from the rainfall sites 

are statistically different. Because annual precipitation 

amounts typically are not normally distributed, a nonparametric test (the Mann-Whitney test) was used. 

Differences in precipitation distributions, if they exist, 

may explain differences in streamflow characteristics 

at the three streamflow sites.

 Evaluation of precipitation characteristics and 

trends at the rainfall sites Trends in precipitation at 

each of the three rainfall sites were evaluated by using 

5- and 10-year moving averages and the LOWESSsmoothing procedure, which is a weighted local 

regression filter (Cleveland, 1979). LOWESS, which 

uses an iteratively weighted least-squares technique, is 

useful in identifying trends in highly variable timeseries data. The degree of smoothing can be controlled 

by altering the value of the smoothness factor,/ The 

factor is chosen somewhat subjectively and reflects the 

size of the window used to compute a value on the 

LOWESS curve (Helsel and Hirsch, 1992). The 

smoothness factor can range from 0.01 to 0.99, with 

higher values resulting in greater smoothing because 

of a larger window size. Greater smoothing also can be 

interpreted as representing longer-term trends relative 

to lower values of the smoothness factor. Differences 

between annual rainfall amounts at Elizabethtown 

(site 4) near the headwaters of the Waccamaw River 

Basin and at the other two rainfall sites in the Lumber 

Basin were evaluated.

 Determination of annual yields The annual 

ratio of streamflow to precipitation (yield) was determined for the Waccamaw and Lumber River Basins. 

Trends in annual yield were identified.

Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
 Evaluation of monthly streamflow characteristics A linear regression of logarithmically 

transformed flow versus time was developed for the 

Waccamaw and Lumber River flows, and the slopes of 

the regressions were evaluated. The residuals 

(difference between predicted and actual flows) were 

analyzed to determine if there was a trend with time.

The rescaled cumulative departures of the 

monthly mean streamflow at each streamflow site 

were plotted and interpreted. The rescaled cumulative 

departures of the monthly mean streamflow were 

computed as the cumulative sum of the differences 

between the monthly value and the monthly mean for 

the period of record, divided by the monthly standard 

deviation for the period of record (Garbrecht and 

Fernandez, 1994); the values are dimensionless. This 

procedure allows fluctuations and trends in flow to be 

visualized and was used to identify extended wet and 

dry periods and to compare general trends among the 

streamflow sites. Extended periods of greater-thanaverage and less-than-average streamflow and trends 

in streamflow were identified.

Periods of negative slope (sloping downward 

from left to right) indicate time spans of monthly 

streamflow which are less than the mean; periods of 

positive slope indicate greater-than-average streamflow conditions. The steepness of the slope indicates 

the magnitude of the deviations from average 

conditions. Periods of relatively small changes in the 

rescaled cumulative departures, whether the actual 

values are positive or negative, indicate periods of 

near-average flows. Changes or trends near the 

beginning and end of the record are difficult to identify 

from plots of the rescaled cumulative departures. A 

unidirectional trend in streamflow would appear as a 

parabolic trend in a plot of the rescaled cumulative 

departures against time.

Streamflow departures were compared with 

precipitation departures. This analysis provided 

preliminary information on trends in streamflow at 

streamflow sites, the relation of the streamflow trend 

to precipitation, and the regional variability of the 

trend.

The LOWESS technique was used to smooth 

the monthly flows and to further evaluate possible 

trends. The distributions of the monthly mean flows 

from each of the streamflow sites were compared 

to determine if the distributions are statistically 

different. The nonparametric Wilcoxon rank sum test 

was applied to the data. A double mass curve of

monthly streamflow in cubic feet per second per 

square mile was constructed as another method to 

compare streamflow in the Waccamaw and Lumber 

Rivers.

 Computation of routine flow statistics The 

following information was determined from each of 

the three sets of streamflow records: (1) flow duration, 

(2) low-flow statistics, and (3) high-flow statistics 

(including flood frequencies). Low-flow statistics 

included the 7Q10, 7Q2, and 30Q2. These statistics 

were computed using the standard log-Pearson Type 

III distribution (Riggs, 1973). Flood frequencies were 

computed using methods described by the Interagency 

Advisory Committee on Water Data (1982). Flows 

were converted to cubic feet per second per square 

mile for these computations. The statistics were 

computed for the period 1939-94, and for 10-year 

periods at 5-year intervals (for example, 1940-49, 

1945-54, 1950-59, and so on), and for other selected 

10-year periods.

 Evaluation of annual series of low and high 

flows The annual series of 1-day, 7-day, and 30-day 

(«-day) low and high flows were determined. The 

n-day low/high flow is the n-consecutive day period 

with the lowest/highest mean flow during the year. The 

data were smoothed using the LOWESS technique. 

The Mann-Kendall test (Helsel and Hirsch, 1992) was 

used to determine whether a statistically significant 

monotonic trend was present in each of the annual 

series of n-day flows. The test was applied to the full 

1940-94 period. General trends in annual n-day low 

and high flows identified from LOWESS-smoothed 

plots were evaluated and compared among streamflow 

sites.

 Separation of base flow from runoff and 

analysis of each separately In some cases, changes 

in the hydrologic regime can be more clearly manifested as changes in the runoff component of streamflow. Base flow was separated from runoff using the 

method of the Institute of Hydrology (1980a, 1980b). 

Base flow and runoff were then analyzed separately. 

Time series of the base flow and runoff and the ratio of 

base flow to runoff were plotted. The LOWESS 

technique was applied to each of the three (base flow, 

runoff, and base flow to runoff ratio) data series. 

Regressions of log-transformed base flow and runoff 

against time were computed and analyzed.

 Evaluation of flow variability Twelve-month 

and 10-day moving range values were determined 

(Barringer and others, 1994) as a measure of flow

Approach
variability. The range is defined as the difference 

between the highest and the lowest flow within a 

period. The 12-month moving range for a given 

month, analogous to the moving average, is computed 

for the month of interest and the 11 successive months 

following. Likewise, the 10-day moving range is 

computed for the daily mean flow on the day of 

interest and the 9 successive days. Ranges for total 

streamflow, base flow, and runoff were determined. 

Values were smoothed using the LOWESS technique.

STREAMFLOW CHARACTERISTICS OF 

THE WACCAMAW RIVER

Long-term precipitation characteristics of the 

Waccamaw River at Freeland for the period 1940-94 

are first described to provide a context for the characterization of streamflow conditions. Flow durations, 

flood-frequency distributions, low-flow statistics, and 

base-flow characteristics are then presented for the 

Waccamaw River. Streamflow characteristics of the 

Waccamaw River are compared with those at the 

Lumber River at Boardman (site 3) and Drowning 

Creek near Hoffman (site 2).

Precipitation

Long-term mean and median precipitation 

amounts were generally the same at sites 4, 5, and 6 

(table 3). The driest year on record at each site was 

1951, but the wettest year was different for each 

rainfall site. High annual precipitation amounts were 

generally the result of excessive rainfall in the summer

or early fall months. Rainfall amounts during these 

months can vary greatly over short distances, which 

may account for the different years of maximum 

precipitation at the three rainfall sites. The distributions of annual rainfall totals were not normally 

distributed for any of the three sites for 1940-94. The 

nonparametric Mann-Whitney signed rank test 

indicated that the distribution of annual rainfall at 

Elizabethtown was not statistically different from the 

distributions at Lumberton and Laurinburg.

Periods of four or more consecutive years of 

less-than-average precipitation at Elizabethtown 

occurred four times during 1940-94 (fig. 2): 1940-43, 

1951-54, 1965-68, and 1978-81. The greatest multiyear precipitation deficit occurred during 1951-54. 

Periods of four or more years of greater-than-average 

precipitation occurred during 1955-60 and 1991-94 

(fig- 2).

Five and 10-year moving averages of the annual 

rainfall totals suggest a slight increase in annual rainfall amounts at Elizabethtown for the period 1940-94 

(fig. 3). Precipitation amounts were generally quite 

low between 1940 and 1954. These low annual totals 

affect the moving averages and partially cause the 

appearance of an increasing trend.

During any given year, annual rainfall amounts 

at the three rainfall sites could differ significantly 

(fig. 4). Annual differences between site 4 (Elizabethtown) and site 5 (Laurinburg) ranged from -18.20 in. 

(inches) (site 5 rainfall exceeded that at site 4) in 1985 

to 15.42 in. in 1977. In 1947, rainfall at Laurinburg 

exceeded that at Elizabethtown by 13.32 in. Differences between sites 4 and 6 ranged from -14.40 in. 

(1947) to 10.54 in. (1950). The comparably large 

differences between site 4, Elizabethtown, and the

Table 3. Monthly and annual precipitation statistics for selected sites in the study area

Monthly mean, in inches

Site 

(fig-D

Elizabethtown 

(site 4)

Laurinburg 

(site 5)

Lumberton 

(site 6)

Period 

of 

record

1940-94

1947-94

1940-89, 

1992-94

£ & 

ra § .c <J

S^StB-SiS

3£E«E33<

3.42 3.28 4.16 2.88 3.49 4.51 6.10 5.74

3.77 3.63 4.50 3.09 3.55 5.05 5.91 4.94

3.42 3.37 4.14 3.08 3.58 4.84 5.85 5.25

September

4.25

4.15

4.09

October November December

2.97 2.73 3.05

3.27 3.12 3.19

2.84 2.81 3.09

Annual mean, 

innches

46.70

47.99

46.39

Annual median, 

innches

46.58

47.60

46.37

Maximum annual, 

innches (year)

60.26 

(1974)

61.32 

(1959)

62.25 

(1945)

Minimum annual, 

innches (year)

31.64 

(1951)

30.76 

(1951)

28.41 

(1951)

6 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
15

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-15

-20

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1994

Figure 2. Annual departures from 1940-94 mean rainfall at Elizabethtown, N.C.

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1940-94 average

ANNUAL RAINFALL TOTAL 

10-YEAR MOVING AVERAGE 

5-YEAR MOVING AVERAGE

194O 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1994 

Figure 3. Five- and 10-year moving averages of annual rainfall amounts at Elizabethtown, N.C., 1940-94.

Streamflow Characteristics of the Waccamaw River
20

15

No 

data

DIFFERENCE BETWEEN ELIZABETHTOWN AND

LAURINBURG ANNUAL RAINFALL 

5-YEAR MOVING AVERAGE OF DIFFERENCE 

LINE OF ZERO DIFFERENCE BETWEEN ELIZABETHTOWN

AND LAURINBURG ANNUAL RAINFALL

LU

o

LU

o:

LU

15

Q 10

-5

-10

-15

-20

DIFFERENCE BETWEEN ELIZABETHTOWN AND

LUMBERTON ANNUAL RAINFALL 

5-YEAR MOVING AVERAGE OF DIFFERENCE 

LINE OF ZERO DIFFERENCE BETWEEN ELIZABETHTOWN

AND LUMBERTON ANNUAL RAINFALL

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1994

Figure 4. Differences between annual rainfall amounts and 5-year moving average of differences at 

(A) Elizabethtown and Laurinburg, and (B) Elizabethtown and Lumberton.

8 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
other sites for 1947 suggest that rainfall for site 4 may 

have been under-reported. Precipitation amounts for 

other rainfall sites in the region support this conclusion. Total annual precipitation for 1947 at Whiteville 

(Columbus County), Willard (Fender County), Clinton 

(Sampson County), and Southport (Brunswick 

County) was 61.79 in., 60.57 in., 53.20 in., and 69.58 

in., respectively, compared to the reported 44.39 in. at 

Elizabethtown.

The 5-year moving average of the differences 

between annual rainfall amounts at Elizabethtown 

(site 4) and the other two sites shows an increasing 

trend from generally negative differences (site 4 

rainfall is less than rainfall at other sites) to near-zero 

or slightly positive differences (fig. 4). These trends 

support the conclusion of a general increase in 

reported rainfall amounts at site 4 during the period 

1940-94. However, neither 5- and 10-year moving 

averages nor LOWESS-smoothed plots (not shown) 

indicated a trend in annual rainfall amounts at sites 5

and 6 (fig. 1). Consequently, the apparent trend in 

reported rainfall amounts at site 4 is likely associated 

with the under-reporting of rainfall in 1947, and 

possibly other years, rather than changes in rainfall 

patterns.

Annual Yields

The mean ratios of annual streamflow to annual 

rainfall (yield) for the Waccamaw River and the 

Lumber River were the same for the period 1940-93 

(fig. 5). In both basins, 30 percent of the rainfall, on 

average, passed the respective gaging stations as 

streamflow.

The same general patterns in the streamflowrainfall ratio existed at both sites except during the late 

1940's. The high streamflow-rainfall ratios for the 

Waccamaw River in 1947 and 1948 (fig. 5) are likely a 

result of under-reported rainfall at Elizabethtown

- WACCAMAW RIVER ANNUAL STREAMFLOW DIVIDED 

BY ELIZABETHTOWN ANNUAL RAINFALL

- LUMBER RIVER ANNUAL STREAMFLOW DIVIDED 

BY LUMBER RIVER ANNUAL RAINFALL

- 1940-93 AVERAGE

1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1994

Figure 5. Ratio of annual streamflow to annual rainfall at the Waccamaw and Lumber Rivers, 1940-94.

Streamflow Characteristics of the Waccamaw River 9
during those years (fig. 4). Following 1950, there is no 

trend in the difference between the Waccamaw and 

Lumber streamflow-rainfall ratios. However, during 

1950-93, the Waccamaw ratio was less than the 

Lumber ratio about two-thirds of the time. This is 

probably explained by the fact that the Lumber River 

drains part of the Sand Hills region, where yields are 

higher than in other parts of the Coastal Plain. For 

example, Drowning Creek, which is located in the 

Sand Hills, has a long-term average (1940-94) streamflow of 1.38 (ft3/s)/mi2, whereas the long-term average 

streamflow for the Waccamaw River during the same 

period was 1.03 (ft3/s)/mi2.

During 1958-83, the streamflow-rainfall ratio 

was 0.331 for the Waccamaw Basin and 0.339 for the 

Lumber Basin, or about 10 percent greater than the 

long-term average of 0.3. Rainfall at Elizabethtown 

during this period was about 4 percent greater than 

average. During 1984-93, however, the streamflowrainfall ratio was 0.261 for the Waccamaw Basin and 

0.276 for the Lumber Basin, and rainfall at Elizabethtown was about 3 percent less than average. Hence, 

although 1984-93 was not a period of prolonged, 

severe drought, the percentage of precipitation which 

became streamflow in the Waccamaw and Lumber 

Basins was significantly lower than average, and much 

lower than the previous 26 years. In fact, annual 

precipitation at Elizabethtown was higher than average 

during much of 1984-93 (fig. 2). Higher-than-average 

summer temperatures during the period (table 4) 

probably were partially responsible for lower yields 

during this time despite the higher-than-average 

rainfall.

Table 4. Difference between measured temperatures at 

Whiteville, 1984-93, and 1951-80 average temperatures

[Positive value indicates mean for period was greater than long-term mean 

temperature; negative value indicates mean for period was less than longterm temperature; , not available]

Year

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

May

0.2

1.2

1.4

.6

-.4

-1.7

-

4.3

-3.0

2.6

June

1.3

1.3

2.6

2.0

-2.1

3.7

-

1.6

-1.2

2.6

Month

July

-1.0

.6

3.4

.9

.5

.9

-

-

4.3

5.0

Aug.

0.3

-.4

-.5

2.4

2.5

-.9

-

1.0

-.2

.8

Sept.

-2.5

.5

.7

1.7

-1.9

,_

-

.3

1.4

2.1

Annual

0.7

1.9

2.2

.1

-.2

_

-

-.2

1.0

1.3

Monthly Streamflow Characteristics

Monthly mean flow in the Waccamaw River is, 

on average, greatest during March (fig. 6). More than 

half (57 percent) of the total annual flow occurs during 

the four months of January to April. Conversely, the 

four months with the greatest precipitation at Elizabethtown are June to September, when 44 percent of 

the total annual precipitation occurs. Despite the 

higher rainfall amounts, streamflow is lower during 

June to September than during January to March 

because evapotranspiration is greater during the 

summer months than during winter months.

Four major droughts occurred in North Carolina 

between 1940 and 1988 (Zembrzuski and others, 

1991). The 1950-57 drought, which had a recurrence 

interval of greater than 25 years throughout most of 

the State, was the most persistent drought on record in 

North Carolina. The drought was, however, briefly 

interrupted in the fall of 1954 and summer of 1955 by 

rains from the passage of four hurricanes, with two of 

the hurricanes having some effect on the Waccamaw 

Basin. Hurricane Hazel (October 15, 1954) passed 

directly over the Waccamaw Basin, and Hurricane 

Diane (August 17, 1955) passed just to the north and 

east of the basin. Hurricane Hazel had little effect on 

precipitation and flow in the Waccamaw Basin (figs. 2 

and 7). Rainfall associated with Hurricane Diane 

resulted in the second highest recorded monthly mean 

streamflow in the Waccamaw River at Freeland 

(fig. 8). During the remainder of 1955, however, flows 

in the Waccamaw remained quite low. Other droughts 

identified by Zembrzuski and others (1991), all of 

which had recurrence intervals of between 10 and 

25 years, occurred during 1966-71, 1980-82, and 

1985-88.

Monthly mean flows for the Waccamaw and 

Lumber Rivers were logarithmically transformed and 

regressions of the log-transformed flows against time 

were computed. The slope of the regression line for 

the Waccamaw River was statistically different from 

0 at the 90 percent confidence level, indicating that 

there is a 90 percent chance the Waccamaw River 

monthly mean flows exhibited a change (or slight 

trend) over the period of record. The slope of the 

Lumber River regression was not statistically different 

from zero. Residuals from the two regressions were 

computed (difference between actual and predicted 

values), untransformed, corrected for logarithmic bias, 

and plotted against time. There was no trend in the

10 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
18 I r

16

14

UJ 19

CD

<

CL 

UJ

CL 

UJ

10

S

LL

o

Ul 

O 

CL

UJ 

Q.

ELIZABETHTOWN RAINFALL (Site 4) 

WACCAMAW RIVER FLOW (Site 1) 

(Sites located in figure 1)

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

Figure 6. Mean monthly percentage of 1940-94 mean precipitation at Elizabethtown and 1940-94 mean streamflow at the 

Waccamaw River at Freeland.

residuals for either river, and the residuals were 

normally distributed.

Five-year moving averages of monthly mean 

streamflow (fig. 7) are similar for the Waccamaw and 

Lumber Rivers. Periods of sustained lower-thanaverage flows include the early 1940's, the early 

1950's, and the mid-1980's through the mid-1990's. 

The period from the late 1950's through the early 

1980's was one of generally greater-than-average 

flows in the Waccamaw and Lumber Rivers, as well as 

greater-than-average yields (fig. 5). During much of 

the latter period, however, Waccamaw Basin flows in 

cubic feet per second per square mile were lower than 

those from the Lumber Basin. Since about 1980, 

however, Waccamaw Basin flows in cubic feet per 

second per square mile usually exceeded those of the 

Lumber Basin. The general patterns seen in the 

monthly mean flows (fig. 8) and moving averages 

(fig. 7) are more clearly evident in the plots of rescaled 

cumulative departures.

Rescaled Cumulative Departures

The rescaled cumulative departures (RCD's) of 

monthly flows for the Waccamaw River, Lumber 

River, and Drowning Creek each exhibit the same 

general patterns for the period of record (fig. 9). In 

particular, the Lumber and Waccamaw Rivers have 

almost identical RCD's from 1940 through about mid1964. Although the RCD's for Drowning Creekfollow 

the same general trends as the other two sites, there 

was a slight difference Drowning Creek is located in 

the Sand Hills region and drains a substantially 

smaller area than the other two sites. Consequently, 

most of the subsequent comparisons of flow conditions in this report will be between the Waccamaw and 

Lumber Rivers.

Although not identified by Zembrzuski and 

others (1991) as a significant drought, the period 

1940-43 was one of less-than-average rainfall (fig. 2) 

and streamflow (fig. 9). In fact, this drought appears to

Monthly Streamflow Characteristics 11
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 

5-year moving average of monthly mean flows

Lumber River 1940-94 

mean flow

Waccamaw River 1940-94 

mean flow

WACCAMAW RIVER (Site 1) 

LUMBER RIVER (Site 3) 

(Sites located in figure 1)

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Figure 7. Five-year moving average of monthly mean streamflow at the Waccamaw River at Freeland and the Lumber 

River at Boardman, 1940-94.

have been the second or third most severe on record 

(1940-94) for this basin. However, from mid-1945 

through late 1949, flows were significantly greater 

than average at all three sites.

The 1950-57 drought was the most severe on 

record. Rainfall was extremely low during 1951-54 

(fig. 2), and was greater than average in 1955 only 

because of the high rainfall associated with Hurricane 

Diane (12.40 in. at Elizabethtown in August 1954). 

Extrapolating the trends from 1950-54 shown in 

figure 9, the 1950-57 drought would have been much 

more severe without the rainfall of Hurricane Diane 

and associated high flows. The effects of the hurricane 

on flows were substantially greater in the Waccamaw 

Basin than in the Lumber Basin (rainfall at Lumberton 

was 9.96 in. in August 1954).

Flows in the Waccamaw and Lumber Basin 

were average or greater than average during 1958-65. 

From September 1964 to June 1966, flows in the 

Lumber River (and Drowning Creek) were much

greater than average (as indicated by the large positive 

slope in the RCD's, fig. 9), but were near or slightly 

greater than average in the Waccamaw River. Moreover, the 1966-71 drought identified by Zembrzuski 

and others (1991) was much less severe in the 

Waccamaw Basin than in the Lumber Basin. The 

drought appears to have actually ended in both basins 

in the spring of 1969.

Flows in the Waccamaw and Lumber Rivers 

were generally greater than average from the spring of 

1969 through April 1980. The general patterns of the 

respective RCD's for the Waccamaw and Lumber 

Rivers were the same for the period. The slopes of the 

RCD lines for the two sites also were similar during 

the period; the RCD's at both sites increased about 

30 standard deviations during the period.

The 1980-81 drought was much more severe 

and prolonged in the Lumber Basin than in the 

Waccamaw Basin. The RCD's for the Waccamaw 

River declined about 10 standard deviations during the

12 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
10,000

WACCAMAW RIVER (Site 1) 

LUMBER RIVER (Site 3) 

(Sites located in figure 1)

1940 1945 1950 1955 1960 1965 1970 1975 

Figure 8. Monthly mean streamflow for the Waccamaw and Lumber Rivers, 1940-94.

1980 1985 1990 1995

period, but the decline in the Lumber River was about 

16 standard deviations. In addition, flows in the 

Waccamaw River were greater relative to average 

conditions than in the Lumber River during 1982-83, 

as indicated by the larger increase in RCD for the 

Waccamaw River than for the Lumber River during 

the period.

The 1985-88 drought (Zembrzuski and others, 

1991) appears to have actually begun in the Waccamaw Basin in mid-1983 and in the Lumber River 

(and Drowning Creek) Basin in mid-1984. As with the 

1980-81 drought, the 1983-88 drought was more 

severe in the Lumber Basin than in the Waccamaw 

Basin. Flows were greater than average in 1989, but 

less-than-average flows resumed in 1990 and 

continued through 1992. Again, flows were lower 

relative to average conditions in the Lumber River 

than in the Waccamaw River during much of 1990-92.

Tree rings from living bald cypress trees were 

used by Stable and others (1988) to reconstruct longperiod climatic conditions in eastern North Carolina. 

Tree ring samples were collected from bald cypress 

trees along the Black River, which is a tributary to the 

Cape Fear River and is located about 60 miles north of 

Freeland. As an indication of climatic conditions, the 

Palmer drought severity index (PDSI) for June was 

reconstructed for a 1,614-year period from A.D. 372 to 

1985. Reconstructed PDSI's for 1887 to 1985 

compared favorably with observed PDSI values for the 

same period.

The reconstructed PDSI values indicate that 

statistically significant changes in average climatic 

conditions occur on an approximately 30-year time 

scale. The statistically different regimes averaged 

34 years in length for the full analysis period and 

ranged in length from 21 to 63 years. In 1988, Stable 

and others suggested that "the record June droughts in

Monthly Streamflow Characteristics 13
50 i r-rco

2

o

40

111 

Q 

Q

tr 

<

Q

CO 

LL. 

O

cr 111 

m

WACCAMAW RIVER (Site 1) 

LUMBER RIVER (Site 3) 

DROWNING CREEK (Site 2) 

(Sites located in figure 1)

30

20 Line of zero 

cumulative departure

10

CO111 

cr

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Figure 9. Rescaled cumulative departures (RCD's) of monthly mean streamflow for the Waccamaw and Lumber Rivers and 

Drowning Creek, 1940-94.

1985 and 1986, for example, could signal the end of 

the relatively wet regime that began in 1956, but this 

possibility remains highly speculative in the absence 

of a physical explanation for the changes."

The RCD's presented in figure 9 independently 

support and extend the conclusions drawn from the 

tree-ring data. Flows in the Waccamaw River generally were greater-than-average from 1957 to 1983 

(fig. 9), a period of 27 years. And, as suggested by 

Stahle and others (1988), it appears that the "relatively 

wet regime" has indeed ended, because flow remained 

generally less than average from 1983 through 1994.

LOWESS-Smoothed Records

The 1940-94 records of monthly mean flows for 

the Waccamaw and Lumber Rivers were smoothed 

using the LOWESS technique to compare trends in

monthly streamflows. Values of the smoothness factor 

/of 0.2 and 0.5 were used in this application (fig. 10). 

For/= 0.5 (fig. 10A), trends in Waccamaw and 

Lumber River monthly flows appear to be very 

similar, with a generally increasing trend in flow from 

1939 through about 1966 and a generally declining 

trend from 1966 through 1994. Except for the period 

from 1952 through about 1966, which included the 

severe 1950-57 drought, the LOWESS-smoothed 

curves for the Waccamaw and Lumber Rivers are 

generally parallel, indicating no long-term changes in 

flows at one site relative to the other. However, the 

difference between the two curves is greater 

(0.17 (ft3/s)/mi2 or more) prior to 1966 than after 

1966, when the difference was about 0.11 (ft3/s)/mi2. 

As with the RCD's, the smoothed Drowning Creek

14 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
1.6

1.4

1.2

1.0

m 0.8

en

<

O to

IT 

LU

Q 0.6

z

O 

O

LU 

tO

IT 

LU

0- 0.4

LU 1 D LU l-o 

U. 

O

m 

o 1.6

I

u! 1-4

z

LU

I"

 - I I i i i 

LOWESS-smoothed monthly mean flows,/= 0.5

 WACCAMAW RIVER (Site 1)

 LUMBER RIVER (Site 3)

 DROWNING CREEK (Site 2) 

(Sites located in figure 1)

i . . . . i , , , . i , , , . l

O

1.0

0.8

0.6

0.4

0.2

LOWESS-smoothed monthly mean flows,/= 0.2

WACCAMAW RIVER (Site 1) 

LUMBER RIVER (Site 3) 

DROWNING CREEK (Site 2) 

(Sites located in figure 1)

i

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1994

Figure 10. LOWESS-smoothed plots of monthly mean streamflow during 1940-94 for the Waccamaw and Lumber 

Rivers using smoothness factor (f) of (A) 0.5 and (B) 0.2.

LOWESS-Smoothed Records 15
flows are substantially different from the Waccamaw 

and Lumber River flows.

Short-term trends in monthly flows are depicted 

by using a smoothing factor of 0.2 (fig. 10B). As with 

/= 0.5, Lumber River flows in cubic feet per second 

per square mile are generally greater than those from 

the Waccamaw River, and the Waccamaw and Lumber 

River LOWESS-smoothed monthly flows have the 

same general trends, with the exception of the period 

1979-94. During this period, and particularly following about 1983, the smoothed flows (including 

Drowning Creek) show distinctly different trends. 

These differences are investigated further using other 

techniques.

Flow Distributions

Waccamaw and Lumber monthly streamflow 

records were subdivided into five 10-year periods 

(table 5). The nonparametric Wilcoxon rank sum test 

was used to statistically compare distributions of 

monthly mean flow for the two sites. The /^-statistic 

computed for this test indicates the probability that 

flows from two separate 10-year periods are from the 

same distribution. Values of p less than 0.1 were considered significant; in other words, if p was less than 

0.1, then the two flow distributions were said to be 

statistically different.

According to the test, the distribution of 

monthly mean flows for the Waccamaw River during 

the period 1985-94 differed from those during the 

periods 1955-64, 1965-74, and 1975-84 (table 5). 

However, the distribution of monthly mean flows for 

1985-94 was not statistically different from the 

1945-54 distribution. This provides further support for 

the conclusion reached from the RCD analysis that the 

flow regime from the late 1950's to the early 1980's 

was distinctly wetter than the flow regimes which 

immediately preceded and followed this period. The 

distribution of monthly mean flows in the Waccamaw 

River exhibited less variability and a lower median 

during 1985-94 than during the four other 10-year 

periods (fig. 11). As previously shown, yields during

1984-93 were lower than average (fig. 5).

Similar results were observed for the five distributions of Lumber River monthly mean flows. The

1985-94 distribution of monthly flows was statistically 

different from the 1955-64, 1965-74, and 1975-84, as 

well as for the 1945-54 distribution (table 5). The 

/7-statistics computed for the comparison of 1985-94 

distribution with the other distributions were smaller

for the Lumber River than for respective Waccamaw 

River comparisons. A smaller ^-statistic indicates a 

greater probability that the distributions are different.

Table 5. Results of rank sum tests comparing distributions 

of monthly flow for the Waccamaw and Lumber Rivers during 

selected 10-year periods

[ , not applicable; bold values are those considered statistically significant, having a p-statistic less than 0.1]

10-year 

period

1945-54

1955-64

1965-74

1975-84

1985-94

Waccamaw River p-statistic for 10-year period

1945-54

...

0.377

.134

.351

.140

1955-64

...

...

0.187

.486

.057

1965-74 1975-84

...

...

...

0.176

.006 0.069

Lumber River p-statistic for 10-year period

1945-54

1955-64

1965-74

1975-84

1985-94

...

0.065

.028

.274

.078

...

0.331

.181

.001

...

...

0.108

.001 0.035

Double Mass Curve

A double mass curve was constructed to directly 

compare flows in the Waccamaw River with those in 

the Lumber River during 1940-94. The monthly mean 

arithmetic flows were transformed into logarithmic 

values because the monthly mean flows are approximately lognormally distributed. A simple linear 

regression model was used to determine a relation 

between the log-transformed Waccamaw River 

monthly mean flows and the log-transformed Lumber 

River flows. The relation had an r2 value (coefficient 

of determination) of 0.6 (60 percent of the variance in 

the Waccamaw River monthly mean flows is described 

by Lumber River flows). The regression equation was 

used to predict Waccamaw River monthly mean flows 

for the period 1940-94 from the corresponding 

Lumber River flows. The predicted Waccamaw River 

flows, which were logarithmic values, were untransformed, correcting for logarithmic bias. The monthly 

residual flows, or difference between the predicted and 

observed monthly mean flows, were then computed 

and accumulated. The double mass curve is the plot of

16 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
o 

o

LU 

CO

oc

LJ 8

R6

w MM' . 

 LJLJiHii

EXPLANATION

OUTLIERS

95 PERCENTILE

75 PERCENTILE

MEDIAN

25 PERCENTILE

5 PERCENTILE

1945-54 1955-64 1965-74 1975-84 1985-94

Figure 11. Boxplots of monthly distributions of flow in the Waccamaw River for five 10-year periods between 1945 and 1994.

accumulated monthly residual flows with time 

(fig. 12).

If the relation of Waccamaw River flows to 

Lumber River flows remains unchanged, the double 

mass curve plots, more or less, as a straight line. If the 

slope of the double mass curve increases relative to the 

slope of this line, then Waccamaw River flows are 

increasing relative to Lumber River flows. Short-term 

variations in the curve are the result of the imperfect 

relation between Lumber River and Waccamaw River 

flows.

From about 1940 through 1943 when a drought 

was occurring (fig. 9), Waccamaw River flows 

decreased relative to those in the Lumber River (the 

slope of the double mass curve decreased). As is 

explained in the following section, low flows in the 

Lumber River are more sustained, on a per drainage 

area basis, than those in the Waccamaw River. Hence, 

declines in Waccamaw River flows relative to Lumber 

River flows during droughts are not unexpected. 

Similar declines occurred between 1950 and 1957, 

1967 and 1968, and from about the mid-1980's to the 

end of the record. The slope of the double mass curve 

during the period 1967-80, which was a period with 

generally greater-than-average flows, remained 

essentially unchanged with very few deviations. There 

is no indication from the double mass curve for the 

period prior to 1983 that there has been a major, 

sustained change in the relation between Waccamaw 

River and Lumber River monthly mean flows during 

the period 1940-94, but there have been fluctuations in

the relation between flows in the two rivers. If earlier 

patterns persist, the slope of the double mass curve for 

the period after 1983 will return to the long-term mean 

slope after the period of lower-than-average flows has 

ended. However, if the relation between Waccamaw 

and Lumber River flows has changed, the slope of the 

double mass curve after 1983 reflects that new 

relation.

Daily Flow Durations

Flow duration curves of daily mean flow depict 

the percentage of time a given daily mean flow is 

equaled or exceeded during a selected time period. 

The shape of the flow duration curve is highly dependent on the period of record used to construct the 

curve. Flow duration curves were constructed from the 

Waccamaw and Lumber River daily mean flow data 

for the period 1940-94 (fig. 13). Differences between 

the curves at the higher flows is primarily a result of 

the difference in drainage area for the two basins. With 

a smaller drainage area, the Waccamaw Basin is more 

likely to be affected by intense rainfall events which 

cover the entire basin, resulting in higher flows per 

unit drainage area. The low-permeability clay soils 

predominant in the Waccamaw Basin also result in 

more rapid runoff of precipitation than the more 

permeable sandy soils of the Lumber Basin. The larger 

drainage area and sandy soils of the Lumber Basin 

provide greater storage than soils in the Waccamaw

Daily Flow Durations 17
700

Double mass curve of accumulated differences 

between observed Waccamaw River monthly 

mean flow and Waccamaw River monthly 

mean flow predicted from Lumber River flow

Line of constant 

slope

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Figure 12. Double mass curve of accumulated differences between observed Waccamaw River monthly flows and 

Waccamaw River monthly flows predicted from Lumber River monthly flows, 1940-94.

River Basin, so that peak flows in the Lumber River 

are somewhat dampened (on a cubic foot per second 

per square mile basis) relative to those in the Waccamaw River. The greater storage in the Lumber Basin 

results in more sustained medium-to-low flows than in 

the Waccamaw River. Drainage from the Sand Hills 

part of the Lumber Basin likely further augments low 

flows, resulting in substantially greater yields than the 

Waccamaw Basin at exceedance values greater than 

30 percent (fig. 13).

To examine possible changes in flow distributions during the study period, Waccamaw and Lumber 

River daily flow records were subdivided into the same 

five 10-year periods used in the monthly mean flow 

analysis. The lowest median daily flow for the five 

periods occurred during 1985-94 in the Waccamaw 

(fig. 11) and Lumber Rivers. The smallest interquartile 

range (difference between 25th and 75th percentile 

flows) of daily mean flows in both rivers also occurred 

during this 10-year period. In both rivers the greatest

median daily flow of the five periods was during 

1965-74, and the largest interquartile range was during 

1975-84.

Flow duration curves for three 10-year periods 

were constructed for the Waccamaw River (fig. 14A) 

to compare 1985-94 flow durations with the relatively 

high-flow of 1975-84 and with the relatively low-flow 

of 1945-54. Daily mean flows at all exceed-ance levels 

that were less than about 80 percent were lower during 

1985-94 than during 1945-54, which included about 

half of the 1951-57 drought, and 1975-84. However, 

the lowest 10 percent of the flows during 1975-84 

were less than the lowest 10 percent of the flows 

during the relatively dry period of 1985-94. Hence, 

although flows were generally lower during 1985-94 

than during 1975-84, the lowest flows during 1985-94 

were more sustained than during the previous 

10 years.

The lowest 10-year average flow (based on 

monthly means) was during 1949-58 (509 ft3/s or

18 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
LLJ

10-4

Figure 13. Daily 

flow duration curves 

for the Waccamaw 

and Lumber Rivers, 

1940-94.

2 5 10 20 30 50 70 80 90 95 98 

PROBABILITY FLOW EQUALED OR EXCEEDED

99.5

iy n

0.749 (ft/s)/mi ), and the second lowest period that 

did not include any of the 1949-58 period was during 

1985-94 (589 ft3/s or 0.866 (ft3/s)/mi2). The highest 

10-year average flow was during 1970-79, when the 

mean monthly flow for the period was 840 ft3/s 

(1.24 (ft3/s)/mi2). Flow duration curves were constructed for these three periods to compare extreme 

dry and wet conditions with the most recent 10-year 

period (fig. 14B).

At exceedance levels less than about 60 percent, 

daily mean flows during 1985-94 were not greatly 

different from those during the 1949-58 dry period. 

However, flows equaled or exceeded about 80 percent 

of the time during 1985-94 were about double flows at 

the same exceedance level during 1949-58. At extreme 

low flows (exceedance levels greater than 90 percent) 

flows during 1985-94 approached those that occurred 

during 1970-79. Hence, during at least 60 percent of 

the time in 1985-94, flows were comparable to those 

that occurred during the extremely dry period of 

1949-58. However, even during this relatively dry

period, extremely low flows (exceedance levels of 

90 percent or more) during 1985-94 were no less than 

half of those during the wet period of 1970-79 and 

were more than three times greater than those that 

occurred during 1949-58.

Low Flows

Low-flow statistics were computed for each of 

the three gaging stations (table 6). The low-flow 

statistics were computed for the period 1940-94, and 

for ten 10-year periods: 1940-49, 1945-54, 1950-59, 

and so on through 1985-94.

Although the Drowning Creek drainage area at 

site 2 is less than one-third of the Waccamaw River 

drainage area at site 1, low-flow statistics for 

Drowning Creek are substantially greater than those in 

the Waccamaw River at least an order of magnitude 

greater. Low flow statistics per unit drainage area for

Low Flows 19
10

LLJ

o

CO

cc

LU

0. 1Q-3

a

z 

o 

o

LLJ 

CO 

CC

1945-54 

1975-84 

1985-94

LU 10

LL 

O

m

D

o

5"

o

LU 

CC

co

10-2

ID'3

io-4

1949-58 

1970-79 

1985-94

0.5 2 5 10 20 30 50 70 80 90 95 98 99.5 

PROBABILITY FLOW EQUALED OR EXCEEDED

Figure 14. Daily flow duration curves for the Waccamaw River for the periods 

(A)1945-54,1975-84, and 1985-94 and (B) 1949-58,1970-79, and 1985-94.

20 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
Table 6. Selected low-flow statistics for the Waccamaw and Lumber Rivers and Drowning Creek, 1940-94

Site

(fig-1)

1

2

3

Station

Waccamaw River

Drowning Creek

Lumber River

Low flow, in cubic feet per second

7Q2

22.0

66.1

236

7Q10

2.06

35.2

119

30Q2

33.4

87.5

306

Low flow, in cubic feet per second 

per square mile

7Q2

0.032

.361

.192

7Q10

0.003

.192

.097

30Q2

0.049

.478

.249

the Lumber River also are several times greater than 

those for the Waccamaw River.

The lowest mean daily flow, 7-day low flow, and 

30-day low flow were identified for each year during 

the period of record for each station. Values for each 

year were plotted (fig. 15), and data were smoothed 

using the LOWESS procedure with/= 0.5. Throughout the period of record, 1-day, 7-day, and 30-day low 

flows per unit drainage area in the Lumber River were 

always higher than corresponding flows in the Waccamaw River. Lumber River 1-day, 7-day, and 30-day 

low flows each exhibited the same general trend during 

1940-94 (fig. 15), with an increase during the 1960's, a 

steep decline during the 1970's, and a more gradual 

decline in the 1980's and early 1990's. Drowning 

Creek annual low flows exhibited the same trends. The 

«-day low-flow trends for Drowning Creek and the 

Lumber River are similar to the trends in monthly 

mean flows (fig. 10A).

Waccamaw River annual low flows, however, 

were different from those observed in the Lumber 

River (fig. 15). In general, annual 1-, 7-, and 30-day 

low flows increased until about the mid-1960's, 

declined slightly until about 1980, and then increased 

through 1994. The strength of the increasing trend in 

the 1980's increased with the duration of the low-flow 

period (30-day low flows increased more than 1-day 

and 7-day). LOWESS-smoothed annual 7-day low 

flows in the Waccamaw increased at a rate of about 0.3 

cubic feet per second per year during 1981-94, while

the Lumber River experienced a decline of about 3 

ft3/s/year during the same period.

The Mann-Kendall trend test was used to 

determine if a statistically significant monotonic trend 

was present in the observed «-day low flows for the 

Waccamaw and Lumber Rivers. There was a 

statistically significant (95-percent confidence level) 

decrease in 1-day and 7-day low flows in the Lumber 

River during 1940-94. The decrease in 30-day low 

flows for the Lumber River was not statistically 

significant at the 95-percent level, but the p-statistic 

was 0.11. There was no statistically significant trend 

in Waccamaw River observed «-day low flows; 

p-statistics were between 0.21 and 0.37.

As was previously shown, monthly mean flows 

during 1981-94 (figs. 8 and 9), and particularly 

1985-94 (fig. 14), were generally quite low compared 

to the entire period of record. Despite this, the lowest 

flows in the Waccamaw River were generally as great 

or greater than low flows during other times between 

1940 and 1981. This is suggested by the LOWESSsmoothed curves in figure 15, and is evident from the 

flow duration curves (fig. 14B) in which flows greater 

than the 90-percent exceedance level for 1985-94 

(a dry period) approached those for 1970-79 (a wet 

period). In fact, the 7Q10 for the Waccamaw River 

computed using 1985-94 flows was the second highest 

7Q10 computed for the ten 10-year periods (table 7), 

with the highest being for the period 1965-74. 

However, the 7Q10 computed for the Lumber River

Table 7. 7Q10 low flows computed from data for selected 10-year periods

7Q10 low flow, in cubic feet per second, for stated 10-year period

Site

Waccamaw 

River

Lumber River

1940-49

1.15

165

1945-54

0.35

118

1950-59

0.32

121

1955-64

3.73

220

1960-69

5.56

129

1965-74

13.2

131

1970-79

8.09

157

1975-84

2.46

111

1980-89

3.07

91.0

1985-94

11.9

91.3

Low Flows 21
LOWESS-SMOOTHED 1-DAY LOW FLOW, WACCAMAW RIVER 

LOWESS-SMOOTHED 1-DAY LOW FLOW, LUMBER RIVER 

WACCAMAW RIVER 1-DAY LOW FLOW 

LUMBER RIVER 1-DAY LOW FLOW

LOWESS-SMOOTHED 7-DAY LOW FLOW, WACCAMAW RIVER 

LOWESS-SMOOTHED 7-DAY LOW FLOW, LUMBER RIVER 

WACCAMAW RIVER 7-DAY LOW FLOW 

LUMBER RIVER 7-DAY LOW FLOW

.

LOWESS-SMOOTHED 30-DAY LOW FLOW, WACCAMAW RIVER 

LOWESS-SMOOTHED 30-DAY LOW FLOW. LUMBER RIVER

* WACCAMAW RIVER 30-DAY LOW FLOW

 LUMBER RIVER 30-DAY LOW FLOW

1940 1945 1950 1955 1960 1965 1970 1975 1980 19B5 1990 1995

Figure 15. LOWESS-smoothed plots of annual 1-day, 7-day, and 30-day low 

flows for the Waccamaw and Lumber Rivers, 1940-94.

22 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
from 1985-94 flow records was the second lowest 

Lumber River 7Q10 computed for the 10 periods. 

Channelization and artificial drainage in the 

Waccamaw Basin could be responsible for these 

apparent increases in low flows. It has been demonstrated that both channelization and artificial drainage 

lead to an increase in base, or low, flows. Mason and 

others (1990) found that channelization of a small 

stream in the central Coastal Plain of North Carolina 

increased base flow almost 10-fold over pre-channelization conditions. Heath (1975) reported similar 

results for channelization of a small stream in the 

northern Coastal Plain of North Carolina, and 

suggested that artificial drainage would have a similar 

effect.

High Flows

Peak flows having recurrence intervals of 2, 5, 

10, 25, 50, and 100 years were computed from the 

1940-94 series of annual peak flows (table 8). 

Although the Waccamaw River drainage area is only 

slightly more than half that of the Lumber River, 

Waccamaw peak flows for all recurrence intervals 

were very nearly equal to those for the Lumber River 

(table 8). As with the low flows, peak flows in cubic 

feet per second per square mile for Drowning Creek 

were much greater than those for the Waccamaw and 

Lumber Rivers.

The annual series of the greatest mean daily 

flow, 7-day high flow, and 30-day high flow were 

developed for the Waccamaw and Lumber Rivers for 

the period 1940-94 (fig. 16). With few exceptions,

Waccamaw River 1-day, 7-day, and 30-day high flows 

in cubic feet per second per square mile exceeded 

corresponding values for the Lumber River (fig. 16). 

Differences were greatest for years having the higher 

flows (for example, 1983 and 1955, which had the two 

highest monthly mean flows on record for the 

Waccamaw River, fig. 8). All occurrences of Lumber 

River n-day high flows exceeding corresponding flows 

for the Waccamaw River were prior to 1970 (except 

the 30-day high flow for 1989). Results from the 

Mann-Kendall test performed on the observed n-day 

values indicated that there were no statistically 

significant trends in the n-day high flows for either the 

Waccamaw or the Lumber Rivers.

LOWESS-smoothed (with/= 0.5) curves of 

n-day high flows for the two rivers exhibited the same 

general trends, with an increase in n-day high flows 

until the early (Lumber) to mid (Waccamaw) 1970's, 

followed by a decline (fig. 16). This trend also is 

similar to the trend in smoothed monthly mean flows 

(fig. 10A). Differences between Waccamaw and 

Lumber River n-day high flows decreased as n 

increased (greater difference for 1-day than 30-day 

high flows). However, the difference between the two 

LOWESS-smoothed curves is not constant during the 

period, but shows a generally increasing difference 

throughout the period of record for the n-day high 

flows.

The annual series of differences between the 

Waccamaw 1-day and corresponding Lumber River 

1-day high flows was plotted, and a 5-year moving 

average of the differences was calculated (fig. 17A).

Table 8. Selected peak-flow statistics for the Waccamaw and Lumber Rivers and Drowning Creek, 1940-94

Recurrence 

interval, 

in years

2

5

10

25

50

100

Flow, in cubic feet per second

Waccamaw . . _. River Lumber R,ver

3,850

5,920

7,400

9,370

10,900

12,500

4,720

6,980

8,540

10,600

12,100

13,700

Drowning 

Creek

1,290

2,390

3,480

5,430

7,420

9,980

Flow, in cubic feet per second 

per square mile

Waccamaw , . _. River Lumber River

5.66

8.71

10.9

13.8

16.0

18.4

3.84

5.68

6.95

8.63

9.85

11.2

Drowning 

Creek

7.05

13.1

19.0

29.7

40.5

54.5

High Flows 23
16

14

12

10

0

uj 16

^

i M 

i

cc

111 

CL 

Q

LOWESS-SMOOTHED 1-DAY HIGH FLOW, 

WACCAMAW RIVER 

1 LOWESS-SMOOTHED 1-DAY HIGH FLOW,

LUMBER RIVER

 WACCAMAW RIVER 1-DAY HIGH FLOW 

> LUMBER RIVER 1-DAY HIGH FLOW

12

10

8

111

05 8

CC

111

CL

fc 6

111

U_

O .

5 4 

o 

- 2

I

u. o 

16

14

12

10

1 LOWESS-SMOOTHED 7-DAY HIGH FLOW,

WACCAMAW RIVER 

1 LOWESS-SMOOTHED 7-DAY HIGH FLOW,

LUMBER RIVER

' WACCAMAW RIVER 7 DAY HIGH FLOW 

> LUMBER RIVER 7-DAY HIGH FLOW

' LOWESS-SMOOTHED 30-DAY HIGH FLOW,

WACCAMAW RIVER 

1 LOWESS-SMOOTHED 30-DAY HIGH FLOW,

LUMBER RIVER

' WACCAMAW RIVER 30-DAY HIGH FLOW 

> LUMBER RIVER 30-DAY HIGH FLOW

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Figure 16. LOWESS-smoothed plots of annual 1 -day, 7-day, and 30-day 

high flows for the Waccamaw and Lumber Rivers, 1940-94.

24 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
11

10

LU

CE 

<

§

s

0. 

O

CE 

LU 

O_

t

UJ -4

5 11

3 

O 

z 10

2 8

LU

o

LLJ 

CE 

LLJ

A A

A A

A A _________

ZERO DIFFERENCE

5-YEAR MOVING AVERAGE OF DIFFERENCE

DIFFERENCE IN ANNUAL 1-DAY HIGH FLOW,

WACCAMAW RIVER AND LUMBER RIVER

B

A A

-2

ZERO DIFFERENCE

5-YEAR MOVING AVERAGE OF DIFFERENCE

DIFFERENCE IN ANNUAL 7-DAY HIGH FLOW, 

WACCAMAW RIVER AND LUMBER RIVER

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 

Figure 17. Difference between annual (A) 1-day and (B) 7-day high flows for the Waccamaw and Lumber Rivers.

High Flows 25
The moving average shows a fairly clear increasing 

trend in the difference between Waccamaw River and 

Lumber River 1-day high flows, consistent with the 

trend in the difference between the LOWESSsmoothed curves (fig. 16A). A similar pattern is more 

evident in the 7-day high flows (fig. 17B). The analysis 

does not indicate whether the trend is the result of an 

increase in Waccamaw River high flows relative to 

Lumber River high flows, a decrease in Lumber River 

high flows relative to the Waccamaw River, or both. 

Previously discussed analyses showed that the 

difference between Waccamaw River and Lumber 

River monthly mean flows per square mile decreased 

during the period of record, with most of the change 

occurring after 1966 (fig. 10A). Analyses of low flows 

indicated that n-day low flows for the Waccamaw 

River exhibited a slightly increasing trend and that 

Lumber River n-day low flows exhibited a statistically 

significant decreasing trend during the period of 

record. Heath (1975) concluded that channelization 

and artificial drainage leads to an increase in the 

magnitude of the highest flows because of increased 

drainage efficiency in the basin following channelization or ditching. These factors suggest that the 

increasing difference between Waccamaw and Lumber 

River n-day high flows may be the result of a general 

increase in Waccamaw River high flows relative to 

Lumber River high flows.

Base-Flow Separation

Streamflow can be divided into runoff and base 

flow. The distinction is primarily based on the time of 

arrival of water in the stream rather than the path 

followed to reach the stream. Runoff generally 

consists of overland flow and a substantial portion of 

interflow (water which moves laterally through the 

upper soil layer to the stream). Base flow is considered 

to be primarily ground water contributions to 

streamflow. On average, about two-thirds of the total 

streamflow volume in the Coastal Plain is base flow 

(Wilder and others, 1978).

Base Flows

The base-flow index (bfi) is defined as the ratio 

of the base-flow volume to the total streamflow 

volume. The annual bfi for the Lumber River exceeded 

the corresponding value for the Waccamaw River

every year during 1940-94, with the exception of 

1945, 1958, and 1977 (fig. 18). On average, streamflow in the Waccamaw River at site 1 (fig. 1) consisted 

of 53.3 percent base flow, with the annual percentage 

ranging from 28 percent (1942 and 1981) to 72 percent (1953 and 1958). In contrast, base flow in the 

Lumber River at site 3 (fig. 1) accounted for 70.6 

percent of the streamflow during 1940-94, with annual 

bfi's ranging from 0.55 (1985) to 0.84 (1940 and 

1956). As previously shown, n-day low flows for the 

Lumber River were significantly greater than those for 

the Waccamaw River (fig. 15; table 6), which is 

consistent with relatively low base flow in the 

Waccamaw.

The Waccamaw River annual mean bfi (0.53) is 

significantly less than the value of 0.67 reported by 

Wilder and others (1978) as the average for the North 

Carolina Coastal Plain, whereas the Lumber River bfi 

(0.706) is in good agreement with the Coastal Plain 

average. The Waccamaw River bfi values also exhibit 

greater annual variability than the Lumber River 

values (fig. 18). The lowest Waccamaw River bfi 

values were associated with dry years (1942, when 

annual streamflow was 51 percent of the long-term 

average; 1981, when annual flow was 67 percent of 

average; and 1985, when annual flow was 57 percent 

of average). There is, however, no clear relation 

between the annual bfi and the annual flow for either 

the Waccamaw River or the Lumber River (fig. 19).

Monthly mean base flows for the Waccamaw 

and Lumber Rivers were determined, and the time 

series was smoothed using LOWESS (f= 0.5) to 

identify trends in base flow. The two curves depicting 

monthly mean base flow (fig. 20) exhibit the same 

general trends trends which are consistent with those 

seen in the monthly mean streamflow data (fig. 10A). 

Waccamaw River smoothed monthly mean base flows 

(fig. 20) were about 10 times greater than corresponding smoothed 7- and 30-day low flows (fig. 15B and 

C). This indicates that there is generally a large 

difference between the lowest 30-day mean flow 

during a year and the average base-flow condition 

during the same year in the Waccamaw Basin. In 

contrast, Lumber River base flows (fig. 20) were only 

about 2 to 3 times greater than corresponding 7- and 

30-day low flows (fig. 15B and C).

Differences between the Lumber and Waccamaw River smoothed base flows increased in the midto late-1950's during and immediately following the

26 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
WACCAMAW RIVER 

LUMBER RIVER

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 

Figure 18. Ratio of total annual base flow to total annual flow for the Waccamaw and Lumber Rivers, 1940-94.

1995

1950-57 drought (fig. 20). As previously shown, this 

drought was more severe in the Waccamaw Basin than 

in the Lumber Basin. Differences between Lumber 

and Waccamaw River smoothed monthly base flows 

declined in the 1970's, when streamflows were 

generally greater than average (fig. 20). Again, there 

was a pattern of increasing differences between 

Waccamaw and Lumber base flows during the 1980's 

and early 1990's, when flows were generally less than 

average.

As with the total flow (table 5), the Wilcoxon 

rank sum test was used to statistically compare 

distributions of monthly mean base flow for selected 

10-year periods in the Waccamaw River and the 

Lumber River. The distribution of Waccamaw River 

base flows for 1985-94 was statistically different from 

the distributions for the previous four 10-year periods 

(table 9). However, the same was true for the Lumber 

River. The distribution of monthly mean base flows in

the Waccamaw River for the other 10-year periods was 

not statistically different from one another, but some 

differences were noted in the Lumber River.

Runoff

Runoff per unit drainage area is greater in the 

Waccamaw Basin than in the Lumber Basin (fig. 20). 

LOWESS-smoothed monthly runoff curves for the 

two basins exhibit the same general trends (fig. 20), 

with about the same range in values. Trends in runoff 

are similar to trends in total flow (fig. 10A).

The difference between smoothed Waccamaw 

River and Lumber River runoff is approximately 

inversely related to the corresponding differences in 

base flow. When the difference between Waccamaw 

and Lumber River base flow decreases (1940's to 

mid-1950's), the difference between smoothed 

monthly runoff increases. Since 1980, the Waccamaw 

River runoff per unit drainage area appears to have 

increased relative to Lumber River runoff (fig. 20).

Base-flow Separation 27
0.9

§0.8

z

< 0.7

_i

O

O 

t 0.6

co

0.5

 D 

Z

<

0.4

LL

o

< 0.3 

o:

0.2

A WACCAMAW RIVER (Site 1) 

 LUMBER RIVER (Site 3) 

(Sites located in figure 1)

 

A m A 

A** -

A A

A

.«. 

A 

A A

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

RATIO OF ANNUAL FLOW TO LONG-TERM ANNUAL MEAN FLOW

Figure 19. Relation of base-flow index to ratio of total annual base flow to annual mean flow for the Waccamaw and 

Lumber Rivers.

2.2

The ratios of monthly base flow to monthly 

runoff for the Waccamaw River ranged from 0.01 to 

41.2, and from 0.15 to 82.3 for the Lumber River. The 

monthly ratios of base flow to runoff for the Waccamaw and Lumber Rivers were low in the late 1940's 

and increased to their highest values in the 1950's 

during the drought period, when runoff was low 

relative to total flow (fig. 21). During the drought in 

the early 1980's (fig. 9), the ratio for the Lumber River 

again increased as expected (fig. 21). However, the 

ratio of monthly base flow to runoff during this period 

decreased to extremely low values for the Waccamaw 

River indicating an increased proportion of runoff in 

the total flow, which also is suggested by the 

LOWESS-smoothed curves of monthly mean runoff 

(fig. 20). As shown earlier (fig. 9), the drought was less 

severe in the Waccamaw than in the Lumber Basin, but 

flows in the Waccamaw River were still less than 

average during this period. This increase in runoff 

relative to base flow is not consistent with patterns

exhibited during previous periods having lower than 

average flow.

The distribution of monthly mean runoff in the 

Waccamaw River for 1985-94 was statistically different from those of the previous two 10-year periods 

(table 9). Likewise, the distribution for 1965-74 was 

statistically different from those of 1945-54 and 

1955-64. Similar patterns were not evident in the 

monthly distributions of runoff for the Lumber River.

Logarithmically transformed monthly mean 

base flows (in cubic feet per second per square mile) 

for the Waccamaw River were regressed against those 

for the Lumber River, and likewise for the monthly 

mean runoff. Lumber River base flow was more highly 

correlated with Waccamaw River base flow (r2 = 0.60) 

than was Lumber River runoff with Waccamaw River 

runoff (r2 = 0.40). This may be explained by (1) the 

difference in the size of the two drainage basins 

28 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
0.8

0.7

LU

cc 0.6

<

O 

CO

cc 

£0.5

Q

O 

O

LU

co 0.4 

cc

£0.3 

O

m

o 

Z 0.2

I

LL

0.1

 LOWESS-SMOOTHED WACCAMAW RIVER MONTHLY MEAN BASE FLOW

 LOWESS-SMOOTHED LUMBER RIVER MONTHLY MEAN BASE FLOW

.... LOWESS-SMOOTHED WACCAMAW RIVER MONTHLY MEAN RUNOFF

 - - - LOWESS-SMOOTHED LUMBER RIVER MONTHLY MEAN RUNOFF

I I I

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 

Figure 20. LOWESS-smoothed monthly mean base flow and runoff for the Waccamaw and Lumber Rivers, 1940-94.

Table 9. Results of rank sum tests comparing monthly distributions of runoff and base flow for the Waccamaw 

and Lumber Rivers during selected 10-year periods

[ , not applicable; bold values are those considered statistically significant, having a p-statistic less than 0.1]

10-year 

period

1945-54

1955-64

1965-74

1975-84

1985-94

Waccamaw River p-statistic for 

10-year period

1945-54 1955-64 1965-74 1975-84

Lumber River p-statistic for 

10-year period

1945-54

Base flow

0.318

.295 0.483

.479 .258 0.218

.066 .016 .007 0.090

0.055

.012

.455

.048

1955-64 1965-74

0.276

.062 0.032

.0005 .0001

1975-84

0.038

Runoff

1945-54

1955-64

1965-74

1975-84

1985-94

0.447

.074 0.044

.258 .198 0.248

.264 .327 .017 0.107

0.136

.230

.090

.325

0.372

.444 0.354

.068 .131 0.041

Base-flow Separation 29
10

cc

\ 7

O

S

ei 6

LL 

UJ

< 5

m

cc

WACCAMAW RIVER 

LUMBER RIVER

1940 1945 1950 1955 1960 -1965 1970 1975 1980 1985 1990 1995 

Figure 21 . Three-year moving average of the ratio of base flow to runoff for the Waccamaw and Lumber Rivers, 1940-94.

there is a greater likelihood that precipitation events 

will cover the entire Waccamaw Basin than the larger 

Lumber River Basin; and (2) the possibility that 

artificial drainage has increased the efficiency of the 

drainage network in the Waccamaw Basin relative to 

that in the Lumber Basin. There was no apparent 

temporal trend in the residuals from the two regression 

relations.

Flow Variability

Flow variability was evaluated by using computations of a 12-month moving range of monthly flow 

and a 10-day moving range of daily flow (Barringer 

and others, 1994). The moving range was computed 

for total flow, runoff, and base flow. There was 

essentially no difference between the flow variability

described using the LOWESS-smoothed 12-month 

moving range and the LOWESS-smoothed 10-day 

moving range.

There was greater variability in Waccamaw 

River total flow (in cubic feet per second per square 

mile) than in Lumber River total flow (fig. 22A). 

Trends in total flow variability were generally the 

same for the Waccamaw and Lumber Rivers; however, 

the difference in the smoothed 12-month moving 

range for the two rivers was slightly greater at the end 

of the period of record than at the beginning 

(fig. 22A). This also was true for the difference between the LOWESS-smoothed 10-day moving ranges 

for the two rivers.

The variability in runoff generally exceeded the 

variability in base flow in the Waccamaw River

30 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
4.0

3.5

3.0

2.5

2.0

1.5

1.0

LU

0.5 

4.0

LU

cr

w 

cr 

£3.0

Q

§2.5

ill

w

cr 

£2.0

o 

m

O 1.0

O 0.5 

^4.0

3.5 

3.0 

2.5 

2.0 

1.5 

1.0

0.5

LOWESS-smoothed 12-month moving 

range of total flow

WACCAMAW RIVER 

LUMBER RIVER

LOWESS-smoothed 12-month moving 

range of runoff

WACCAMAW RIVER 

LUMBER RIVER

LOWESS-smoothed 12-month moving 

range of base flow

WACCAMAW RIVER 

LUMBER RIVER

1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Figure 22. LOWESS-smoothed curves of 1 2-month moving range of (A) total flow, 

(B) runoff, and (C) base flow for the Waccamaw and Lumber Rivers, 1940-94.

Flow Variability 31
(fig. 22B and C). The opposite was true for the 

Lumber River, however, with greater variability in the 

base flow than the runoff. As previously shown, 

streamflow in the Lumber River consists of about 

70 percent base flow and 30 percent runoff, whereas 

base flow constitutes about 53 percent of the total flow 

in the Waccamaw River. Despite the differences in the 

magnitude of base flow and runoff variability, trends in 

base flow and runoff 12-month moving ranges were 

generally the same for the Waccamaw and Lumber 

Rivers except for the base-flow range prior to about 

1960. The increasing variability in Waccamaw River 

base flow prior to 1960 likely reflects recovery from 

the severe effects on base flows of the 1940-43 and 

1950-57 droughts.

The distributions of the 12-month moving range 

values were compared for four 10-year periods: 

1949-58, the driest 10 years on record; 1970-79, the 

wettest 10 years on record; 1975-84; and 1985-94, the 

most recent period and the second driest 10 years on 

record. The 1975-84 period was selected to compare 

the previous 10-year period with the most recent 

10-year period.

The range in total flow variability, as measured 

by the interquartile range of the 12-month moving 

range values (difference between the 75th and 25th 

percentiles), was smallest during 1985-94 (fig. 23A). 

In fact, the interquartile range for 1985-94 (0.79) was 

significantly less than any of the values for the other 

10-year periods (1.53, 1.63, and 1.36, respectively). 

Median 12-month moving ranges of total flow were 

lower during the drier 10-year periods (1949-58 and 

1985-94) than the wetter 10-year periods.

Runoff variability, as measured by the interquartile range of the 12-month moving ranges, was 

smallest during the driest (1949-58) and wettest 

(1970-79) 10-year periods (fig. 23B). The difference 

between the maximum and minimum 12-month 

moving range values was greatest during 1985-94, 

possibly suggesting an increase in variability in the 

runoff component of streamflow. Base-flow variability, however, was much smaller during 1985-94 

than the other three 10-year periods, for which the 

interquartile ranges were approximately equal 

(fig. 23C). This smaller base-flow variability is 

reflected in the variability of total flows for the 1985- 

94 period.

The median value of the 10-day moving range 

of daily flows for the wettest 10 years (1970-79) was 

about double the median value for the driest 10-year

Lij 5

1949-58 1970-79 1975-84 1985-94

O < Z§ 

<0 

o: co

It

!§

1949-58 1970-79 1975-84 1985-94

Ocr

o: LU

o "- 

80

1949-58 1970-79 1975-84 

EXPLANATION 

 OUTLIERS

95 PERCENTILE

75 PERCENTILE

MEDIAN

25 PERCENTILE

5 PERCENTILE

1985-94

Figure 23. Boxplots of Waccamaw River distributions of 

monthly 12-month moving ranges of (A) total flow, (B) runoff, 

and (C) base flow for the periods 1949-58,1970-79, 

1975-84, and 1985-94.

32 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
10,000

o

0 

LU

co

1,000

o

CD 

Z> 

(J

? 100

o

Q 

LL 

O

LU 

O

cc

o z

o

Q

6

10

0.1

1949-58 

1970-79 

1975-84 

1985-94

5 15 25 35 45 55 65 75 85 95 96 97 98 99 

PERCENTAGE OF TIME 10-DAY MOVING RANGE OF DAILY FLOW WAS EQUALED OR EXCEEDED

Figure 24. Percentage of time 10-day moving range of daily flow values were equaled or exceeded for the Waccamaw River, 

1940-94.

period (1949-58) (fig. 24). The distributions of now 

variability (as indicated by 10-day moving ranges of 

daily flows) for the four different 10-year periods were 

about the same for the larger variabilities (greater than 

about the 50-percent exceedance level (fig. 24)), 

although the magnitudes of the variabilities differed. 

The distributions of flow variability differed for exceedance levels less than 50 percent (shapes of curves 

are different). In particular, at an exceedance level of 

less than 5 percent, the flow variability during 1985-94 

increased relative to variability during the other three 

10-year periods (fig. 24). However, the change in 

variability at this exceedance level was small less 

than 5 ft^/s.

CONCLUSIONS

Streamflow characteristics of the Waccamaw 

River at Freeland, N.C., were described for the period

1940-94. Flows in the Waccamaw River were 

compared to those in the Lumber River at Boardman 

and Drowning Creek near Hoffman for the same 

period. Precipitation at three locations was characterized for the study period and used to interpret flow 

data. Monthly flow statistics, flow durations, n-day 

low and high flows, and base-flow conditions were 

evaluated.

Long-term mean and median precipitation 

amounts at the three rainfall sites were generally the 

same for the period 1940-94. The distribution of 

annual rainfall at Elizabethtown was not statistically 

different from the distributions at Lumberton and 

Laurinburg. This suggests that any long-term differences in flows between the Waccamaw and Lumber 

Basins are not the result of differences in precipitation. 

Periods of four or more consecutive years of less than 

average precipitation at Elizabethtown occurred in

Conclusions 33
1940-43, 1951-54, 1965-68, and 1978-81, with the 

greatest multi-year deficit in 1951-54. Periods of four 

or more years of greater than average precipitation at 

Elizabethtown occurred during 1955-60 and 1991-94. 

Precipitation at Elizabethtown may have been underreported in the early part of the study period, particularly in 1947.

Thirty percent of rainfall, on average, becomes 

streamflow in the Waccamaw and Lumber Basins. 

Fifty-seven percent of the total annual flow in the 

Waccamaw River typically occurs during January to 

April, with the greatest flows occurring during March. 

Conversely, the four months with the greatest precipitation at Elizabethtown are June to September, when 

44 percent of the total annual precipitation occurs. 

Despite the higher rainfall amounts, streamflow is 

lower during June to September than during January to 

April because evapotranspiration is greater during the 

summer months.

The period 1940-43 was one of less than average rainfall and streamflow, and was the second or 

third most severe drought on record in the Waccamaw 

and Lumber Basins. The 1950-57 drought was the 

most persistent on record in North Carolina. The 

passage of Hurricane Diane in August 1955 briefly 

interrupted the drought, particularly in the Waccamaw 

Basin. Streamflows were generally greater than 

average from 1958 until 1980, except for the period 

1966-69. During 1958-83, when annual rainfall was 

4 percent greater than average, 33 percent (or 

10 percent more than average) of the rainfall became 

streamflow. The period 1980-94 was characterized by 

three droughts (1980-81, 1985-88, and 1990-93) with 

lower than average streamflow. Between 1984 and 

1994 when rainfall was 3 percent less than average, 

26 percent of annual rainfall in the Waccamaw Basin 

became streamflow. Although precipitation was 

greater than average during 1991-94, flows were 

generally less than average, possibly because of 

higher-than-average summer temperatures. The 

distribution of monthly mean flows for the period 

1985-94 was statistically different from the 

distributions for the periods 1955-64, 1965-74, and 

1975-84, but was not different from the distribution for 

1945-54.

During 1940-63, flows in the Waccamaw and 

Lumber Basins were essentially identical relative to 

average conditions. Following 1963, droughts in the 

Waccamaw Basin seem to have been less severe than 

in the Lumber Basin. The 5-year moving average of

monthly flows and LOWESS-smoothed curves of 

monthly flows indicate some change in the relation 

between Waccamaw River flows and Lumber River 

flows. Prior to the 1980's, flows per unit drainage area 

in the Waccamaw Basin were generally less than those 

in the Lumber Basin, but after 1980 the opposite was 

true. The LOWESS-smoothed curves indicate an 

increase in Waccamaw flows relative to Lumber flows 

during the same period.

The lowest 10-year average flow in the Waccamaw River was during 1949-58 (0.749 (ft3/s)/mi2), 

and the second lowest, which did not include any of 

the 1949-58 period, was during 1985-94 (0.866 

(ft /s)/mi2). The highest 10-year average flow was 

during 1970-79 (1.235 (ft3/s)/mi2). At exceedance 

levels less than about 60 percent, daily mean flows 

during 1985-94 were not greatly different from those 

during 1949-58. However, at exceedance levels of 

90 percent and greater, daily mean flows during 

1985-94 approached those of 1970-79, and were at 

least three times greater than those during 1949-58.

Annual n-day low flows (in cubic feet per 

second per square mile) in the Lumber River were 

typically several times greater than those in the 

Waccamaw River. However, although the series of 

annual n-day low flows in the Lumber Basin exhibited 

a decreasing trend from the 1970's until 1994, n-day 

low flows in the Waccamaw appeared to be increasing 

somewhat significantly. This is consistent with the 

observation that at the higher exceedance levels during 

1985-94, daily mean flows were several times greater 

than those during 1949-58, despite the fact that flows 

during both periods were much less than average. This 

apparent increase in low flows could be the result of 

channelization or artificial drainage.

Computed flood flows for the Waccamaw River 

having recurrence intervals of 25, 50, and 100 years 

were only about 10 percent less than those computed 

for the Lumber River, which drains an area almost 

twice the size of the Waccamaw. Annual n-day high 

flows in cubic feet per second per square mile were, 

with a few exceptions, higher for the Waccamaw than 

corresponding values for the Lumber during 1940-94. 

There was an increasing trend in the difference 

between Waccamaw and Lumber annual «-day high 

flows, primarily as a result of increases in Waccamaw 

n-day high flows.

On average, streamflow in the Waccamaw River 

consisted of 53.3 percent base flow, but base flow 

accounted for about 70.6 percent of the total flow in

34 Streamflow Characteristics of the Waccamaw River at Freeland, North Carolina, 1940-94
the Lumber River, which is more typical of other 

Coastal Plain streams. Hence, it is not unexpected that 

n-day low flows in cubic feet per second per square 

mile in the Lumber River are greater than those in the 

Waccamaw River. More annual variability also 

occurred in the Waccamaw base-flow index (0.28 to 

0.72) than in the Lumber River base-flow index (0.55 

to 0.84). Runoff per unit drainage area is greater in the 

Waccamaw Basin than in the Lumber Basin. There is 

some indication that the ratio of base flow to runoff in 

the Waccamaw River changed relative to that in the 

Lumber River beginning in the late 1970's.

Greater variability occurred in Waccamaw River 

total flow than in Lumber River total flow for the 

entire period. Trends in streamflow variability were 

generally the same for the Waccamaw and Lumber 

Rivers, but the difference in the smoothed 12-month 

moving range for the two rivers was greater at the end 

of the period of record than at the beginning. The 

variability in runoff generally exceeded the variability 

in base flow in the Waccamaw River, but the opposite 

was true for the Lumber River. The range in total flow 

variability was least during 1985-94, and was significantly less than any of the values for the other 10-year 

periods. Runoff variability was less during the driest 

(1949-58) and wettest (1970-79) 10-year periods. Base 

flow variability, however, was much smaller during 

1985-94 than during the other three 10-year periods, 

for which the interquartile ranges were approximately 

equal. This smaller base-flow variability is reflected in 

the variability of total flows for the 1985-94 period.

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References 35