Variability of Hydrological Droughts in the Conterminous 

United States, 1951 through 2014

Scientific Investigations Report 2017–5099

U.S. Department of the Interior

U.S. Geological Survey

Variability of Hydrological Droughts in the 

Conterminous United States, 1951 through 

2014

By Samuel H. Austin, David M. Wolock, and David L. Nelms

Scientific Investigations Report 2017–5099

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Deputy Director 

exercising the authority of the Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Austin, S.H., Wolock, D.M., and Nelms, D.L., 2018, Variability of hydrological droughts in the conterminous United 

States, 1951 through 2014: U.S. Geological Survey Scientific Investigations Report 2017–5099, 16 p., https://doi.

org/10.3133/sir20175099.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)
Contents

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

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

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

Previous Studies ...................................................................................................................................2

Methods of Study...........................................................................................................................................2

Variability of Hydrological Droughts, 1951 Through 2014........................................................................3

Spatial Variability ..................................................................................................................................3

Temporal Variability..............................................................................................................................6

Comparison of Findings from This Study with Findings from Previous Investigations ....................11

Limitations.....................................................................................................................................................12

Summary and Conclusions.........................................................................................................................14

Acknowledgments.......................................................................................................................................14

References Cited .........................................................................................................................................14

iii
iv

Figures

1. Map showing locations of streamgages used in the analysis of drought

variability in the conterminous United States, 1951–2014......................................................3

2. Maps showing mean hydrological drought A, frequency, B, duration, and C,

severity in the conterminous United States, 1951–2014.........................................................4

3. Maps showing maximum drought A, duration and B, severity in the

conterminous United States, 1951–2014 ...................................................................................7

4. Graphs showing relation of mean maximum hydrological drought duration and

severity to mean hydrological drought frequency in the conterminous United

States, 1951–2014..........................................................................................................................8

5. Maps showing differences in mean A, frequency, B, duration, and C, severity of

droughts in the conterminous United States, 1983–2014 compared to 1951–82 ................9

6. Map showing change in precipitation in the conterminous United States

between a recent period (1983 through 2014) and an older period (1951

through 1982)...............................................................................................................................13

Tables

1. Hydrological drought summary statistics for the conterminous United States, by

ecoregion, 1951 through 2014. ...................................................................................................5

2. Changes in hydrological drought frequency, duration, and severity in the

conterminous United States for a recent period (1983–2014) compared to an

older period (1951–82), by ecoregion, 609 sites experienced drought in both time

periods. ........................................................................................................................................10
v

Conversion Factors

International System of Units to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

kilometer (km) 0.62137 mile (mi.)

Abbreviations

CONUS Conterminous United States

GCM General Circulation Model

mm millimeter

PDSI Palmer Drought Severity Index

USGS U.S. Geological Survey

Variability of Hydrological Droughts in the Conterminous 

United States, 1951 through 2014

By Samuel H. Austin, David M. Wolock, and David L. Nelms

Abstract

Spatial and temporal variability in the frequency, duration, and severity of hydrological droughts across the conterminous United States (CONUS) was examined using 

monthly mean streamflow measured at 872 sites from 1951 

through 2014. Hydrological drought is identified as starting 

when streamflow falls below the 20th percentile streamflow 

value for 3 consecutive months and ending when streamflow 

remains above the 20th percentile streamflow value for 3 consecutive months. Mean drought frequency for all aggregated 

ecoregions in CONUS is 16 droughts per 100 years. Mean 

drought duration is 5 months, and mean drought severity is 

39 percent on a scale ranging from 0 percent to 100 percent 

(with 100% being the most severe). Hydrological drought 

frequency is highest in the Western Mountains aggregated 

ecoregion and lowest in the Eastern Highlands, Northeast, and 

Southeast Plains aggregated ecoregions. Hydrological drought 

frequencies of 17 or more droughts per 100 years were found 

for the Central Plains, Southeast Coastal Plains, Western 

Mountains, and Western Xeric aggregated ecoregions. Drought 

duration and severity indicate spatial variability among the 

sites, but unlike drought frequency, do not show coherent 

spatial patterns. A comparison of an older period (1951–82) 

with a recent period (1983–2014) indicates few sites have 

statistically significant changes in drought frequency, drought 

duration, or drought severity at a 95-percent confidence level.

Introduction

As the global climate has warmed there has been increasing concern that climate change will have substantial effects 

on the occurrence and severity of hydrological droughts 

(Rind and others, 1990; Hoerling and Eischeid, 2007; Seager and others, 2007; Sheffield and Wood, 2008; Dai, 2011a; 

Dai, 2011b; Dai, 2013), which are defined here as periods of 

below-normal streamflow (Maidment, 1992; Patterson and 

others, 2013). Although a number of studies have examined 

drought occurrence (Andreadis and others, 2005; Sheffield 

and others, 2009; McCabe and Wolock, 2014), relatively few 

investigations have described the spatial and temporal variability of hydrological droughts in the United States (Patterson 

and others, 2013). In one of the most complete analyses of 

droughts in the United States, Andreadis and others (2005) 

examined the duration, frequency, severity, and extent of 

agricultural and hydrological droughts across the United States 

during 1925–2003. In their analyses, Andreadis and others 

(2005) used model-simulated soil-moisture storage and runoff 

to quantify agricultural droughts and hydrological droughts, 

respectively. Andreadis and others (2005) found that the 

droughts in the 1930s and 1950s were the most severe largearea droughts of the 20th century, whereas the early 2000s 

drought in western United States was among the most severe 

for small areas and short durations. Additionally, Andreadis 

and others (2005) found that the most severe agricultural 

droughts were associated with the most severe hydrological 

droughts.

This study builds on the work of Andreadis and others 

(2005) and amplifies the work of Patterson and others (2013). 

In a study of hydrological droughts in southeastern United 

States, Patterson and others (2013) defined a hydrological 

drought as a period when monthly mean streamflow falls 

below the 20th percentile value for a specific site for 3 consecutive months and ends when the streamflow remains above 

the 20th percentile streamflow value for 3 consecutive months. 

Patterson and others (2013) found that 71 percent of droughts 

were shorter than 6 months, whereas only 7 percent were multiyear events. They also report that there was little evidence of 

long-term increases or decreases in drought characteristics in 

the southeastern United States during the 20th century.

This study used measured streamflow data rather than 

model-simulated data, which were used by Andreadis and 

others (2005). This study also evaluated hydrological drought 

characteristics for streams throughout the conterminous United 

States (CONUS) rather than those confined to southeastern 

United States, as identified by Patterson and others (2013).

Purpose and Scope

This report documents characterizations of hydrological droughts empirically determined using the methodology 

articulated by Patterson and others (2013) for streamflow data 

for the entire CONUS. The characterization of the spatial and 

temporal variability of hydrological droughts for the entire 

CONUS from 1951 through 2014 complements the work of 

Andreadis and others (2005) and Patterson and others (2013).

In this report, mean drought frequency, duration, and 

severity are illustrated in figures. Statistics on hydrological 
2 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

droughts are listed in tables and are available in a companion 

data release at https://doi.org/10.5066/F70C4T19.

Previous Studies

Hydrological droughts have been quantified in previous 

studies using various methods (Downer and others, 1967; Llamas and Siddiqui, 1969; Sen, 1976, 1980a,b; Dracup and others, 1980a, b; Frick and others, 1990; Loaiciga and Leipnik, 

1996; Fernández and Salas, 1999a, b; Mishra and Singh, 

2011). Yevjevich (1967) introduced an objective approach to 

the definition and investigation of continental hydrological 

droughts, describing concepts and a theory of runs used in the 

probabilistic characterization of droughts. Significant research 

on the probabilistic characterization of droughts has been 

done since Yevjevich (1967), and the research demonstrated 

that hydrological droughts often exhibit probabilistic characteristics (Sen, 1980a; Loaiciga and Leipnik, 1996; Chung and 

Salas, 2000; Mishra and others, 2009). Karl and others (1987) 

used the Palmer Drought Severity Index (PDSI) to investigate the approximate precipitation required to ameliorate or 

terminate any ongoing drought across the CONUS. Strzepek 

and others (2010) forecasted the effect of climate change on 

the frequency and intensity of droughts in the CONUS over 

the 21st century and computed standardized precipitation 

indices, such as the PDSI using output from General Circulation Models (GCM) for three emissions scenarios. Nalbantis 

and Tsakiris (2009) developed a methodology for forecasting 

hydrological drought severity using a simple index of cumulative streamflow from overlapping monthly periods within each 

hydrological year in a non-stationary Markov chain. Cayan 

and others (2010) forecasted future dryness in the southwestern United States using hydrological models from downscaled 

climate change simulations. Groisman and others (2004) summarize in situ observed trends in contemporary changes of the 

hydrological cycle for the CONUS and suggest that physical 

models of the global climate system are best at identifying the 

causes of observed trends.

Methods of Study

Daily streamflow data were obtained from the U.S. Geological Survey (USGS) National Water Information System 

(http://waterdata.usgs.gov/nwis) for streamgages listed in 

version 2 of the USGS Geospatial Attributes of Gages for 

Evaluating Streamflow (GAGES-II) dataset (Falcone and others, 2011). These sites are considered USGS reference gages, 

representing hydrologic conditions which are least disturbed 

by human influences as compared to other watersheds. Their 

drainage areas range from 0.21 square kilometer (km2

) to 

49,802 km2. No further differentiation, such as the degree of 

flow regulation, was made beyond this identification of reference condition. Sites with records that are at least 80 percent 

complete for calendar years 1951 through 2014 were used for 

the analyses (872 sites, fig. 1). Drainage areas of these sites 

range from 3 km2 to 47,725 km2. The daily streamflow data for 

each of the selected sites were aggregated to provide calendar 

year time series of monthly mean streamflow for each site 

(Austin and others, 2018).

The monthly mean streamflow values for each site were 

converted to monthly percentile values on the basis of the 

period of record for each site. For example, the lowest and 

highest January monthly flow values at a site were assigned 

values of 1 and 100, respectively. Characteristics of hydrological drought then were identified using the methodology 

presented by Patterson and others (2013). Per Patterson and 

others (2013), hydrological drought begins when streamflow 

falls below the 20th percentile streamflow value for 3 consecutive months and ends when streamflow remains above the 

20th percentile streamflow value for 3 consecutive months. 

Other studies have used similar thresholds to define drought 

(Andreadis and others, 2005; Sheffield and others, 2009). 

Three hydrological drought characteristics were computed: 

(1) drought frequency, (2) drought duration, and (3) drought

severity. Drought frequency was computed as the number

of droughts over the period of record divided by the number

of years in the period of record multiplied by 100 to express

frequency as the number of droughts likely to occur on average within a 100-year period. Drought duration (D) is defined

as the number of months counted from the beginning of the

drought period to the end of the drought period. Drought

severity (S) was computed as

S = 100 – (Pave) (1)

where 

Pave is the sum of monthly mean streamflow 

percentiles during a drought divided by 

the duration of the drought ΣP

D





 



 normalized 

to a scale of 0 percent (least severe) to 

100 percent (most severe).

The mean percentile departure value was subtracted from 

100 so that higher values of drought severity indicate lower 

magnitude flow conditions (Patterson and others, 2013). Note 

that the methodology employed in this study to define drought 

periods does not identify short-term (less than 3 months) 

droughts. 
Variability of Hydrological Droughts, 1951 Through 2014 3

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

Aggregated ecoregion

EXPLANATION

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Streamgage location

0 250 500 1,000 KILOMETERS

Ecoregions: From Falcone, J.A., 2011, GAGES-II: geospatial 

attributes of gages for evaluating streamflow, U.S. Geological

Survey digital spatial dataset, accessed December 25, 2013, at

http://water.usgs.gov/lookup/getspatial?gagesII_Sept2011. 

Base from U.S. Geological Survey, 1994, 

The National Map: State Boundaries of the 

Conterminous United States, 1:2,000,000.

Figure 1. Locations of streamgages used in the analysis of drought variability in the conterminous United States, 1951–2014. Colors 

identify ecological regions. (SE, Southeast)

Variability of Hydrological Droughts, 

1951 Through 2014

This study examined the spatial and temporal variability of drought frequency, duration, and severity, providing 

an empirically determined characterization of hydrological 

droughts. Spatial and temporal variability of hydrological 

droughts from 1951 through 2014 were evaluated using measured streamflow data rather than model-simulated data, and 

evaluations of hydrological drought characteristics were made 

for streams throughout the entire CONUS rather than those of 

a particular region. Results are presented for the CONUS and 

associated with 9 of 10 aggregated ecoregions, after Falcone 

and others (2011), to facilitate summary and discussion. No 

information is provided for the South Florida aggregated 

ecoregion (fig. 1).

Spatial Variability

For the CONUS, the mean hydrological drought frequency (the number of droughts per 100 years) is 16, which is 

about one drought every 6 years on average (fig. 2A, table 1). 

Hydrological drought frequency is highest in the Western 

Mountains (19) and lowest in the Eastern Highlands (12), 

Northeast (12), and Southeast Plains (12).

Mean hydrological drought duration for the CONUS is 

5 months (fig. 2B, table 1), and the mean drought severity for 
4 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

Base from U.S. Geological Survey, 1994, 

The National Map: State Boundaries of the 

Conterminous United States, 1:2,000,000.

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Aggregated ecoregion

EXPLANATION

Mean severity

(scaled from 0 to 100)

0 to 33

34 to 36

37 to 40

41 to 45

46 to 94

C

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Aggregated ecoregion

EXPLANATION

Mean duration

(months)

3.0 to 3.8

3.9 to 4.3

4.4 to 4.8

4.9 to 5.6

5.7 to 11.9

B

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Aggregated ecoregion

EXPLANATION

Mean frequency

(droughts per 100 years)

1.6 to 9.7

9.8 to 14.3

14.4 to 18.1

18.2 to 22.1

22.2 to 36.9

A

0 250 500 1,000 KILOMETERS

Figure 2. Mean hydrological drought A, frequency, B, duration, and C, severity in the conterminous United States, 1951–2014. Colored 

dots show magnitude. Colored areas show aggregated ecoregions. (SE, Southeast)
Variability of Hydrological Droughts, 1951 Through 2014 

5

Table 1. Hydrological drought summary statistics for the conterminous United States, by ecoregion, 1951 through 2014. [%, percent; SE, Southeast]

Hydrological droughts summary statistic Units

Ecoregion

Central 

Plains

Eastern 

Highlands

Mixed 

Wood 

Shield

Northeast

SE 

Coastal 

Plains

SE 

Plains

Western 

Mountains

Western 

Plains

Western 

Xeric

All 

regions

Mean hydrological drought frequency number of droughts per 100 years 17 12 16 12 17 12 19 16 18 16Mean hydrological drought duration Months 5 4 4 4 5 4 5 5 5 5Maximum hydrological drought duration Months 27 16 15 15 53 25 29 40 28 9Minimum hydrological drought duration Months 3 3 3 3 3 3 3 3 3 3Mean maximum hydrological drought duration Months 9 6 7 7 9 7 9 10 10 9Mean hydrological drought severity Index ranging from 0% to 100% 39% 41% 38% 40% 39% 40% 38% 41% 40% 39%Maximum hydrological drought severity Index ranging from 0% to 100% 94% 88% 83% 75% 95% 88% 95% 95% 95% 95%Minimum hydrological drought severity Index ranging from 0% to 100% 17% 17% 31% 33% 19% 17% 17% 19% 29% 17%Mean maximum hydrological drought severity Index ranging from 0% to 100% 64% 59% 60% 58% 64% 61% 64% 67% 67% 63%
6 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

the CONUS is 39 percent (fig. 2C, table 1). Unlike drought 

frequency, there are no aggregated ecoregions in which 

drought duration and severity are consistently high or low. 

Areas of coherent high mean drought duration include parts of 

the Rocky Mountains and east-central California, whereas low 

mean drought duration occurred in parts of the Pacific Northwest, the southern California coast, the Mid-Atlantic region, 

and northern New England. Areas of high mean drought severity are found in parts of southern California, eastern Texas, 

and the southern Mississippi basin. Areas of low mean drought 

severity are found in parts of the Rocky Mountains, Michigan, 

and Florida (figs. 2B and 2C, table 1).

Of special concern are long droughts expressed as 

maximum drought duration. The mean value of the maximum 

drought duration for all sites is 9 months, with a maximum 

drought duration of 53 months and a minimum drought 

duration, as a consequence of our methodology, of 3 months 

(fig. 3A, table 1). The longest of all droughts occurred in the 

Southeast Coastal Plain (53 months) followed by a drought 

in the Western Plains (40 months). Mean maximum drought 

durations were longest in the Western Xeric (10 months) and 

the Western Plains (10 months) aggregated ecoregions, and the 

shortest durations were in the Eastern Highlands (6 months).

Drought severities among all of the sites ranged from 

17 percent to 95 percent (fig. 3B, table 1). The highest drought 

severity rating (95%) occurred for the Southeast Coastal Plain, 

Western Mountains, Western Plains, and Western Xeric aggregated ecoregions. The lowest drought severity rating (17%) 

occurred in the Central Plains, Eastern Highlands, Southeast 

Plains, and Western Mountains aggregated ecoregions.

Among aggregated ecoregions, mean maximum hydrological drought duration and drought severity varied with 

mean hydrological drought frequency, with lower values of 

mean maximum drought duration and severity corresponding 

to lower values of mean drought frequency, and higher values 

of mean maximum drought duration and severity corresponding to higher values of mean drought frequency. Most low 

values of mean maximum hydrological drought duration and 

severity were associated with aggregated ecoregions in the 

eastern CONUS, except the Southeast Coastal Plain, and high 

values were associated with aggregated ecoregions in the 

western CONUS (fig. 4).

Temporal Variability

To examine temporal changes, the mean drought statistics (frequency, duration, and severity) for an older period 

(1951–82) were subtracted from the mean statistics for a 

recent period (1983–2014), and the differences in drought 

frequency, duration, and severity were mapped. Positive differences indicate greater frequency, duration, and severity in 

the more recent period (fig. 5). Drought duration and drought 

severity values at each site were grouped by time period, 

and student t-tests were used to compare the grouped means 

at each site, identifying long-term statistically significant 

differences in drought duration and severity at a 95-percent 

confidence level (p < 0.05). Because student t-tests compare 

significant differences in the means of two groups of values, 

having one frequency value available for each site for each 

time period precludes student t-tests of frequency differences 

at each site. Sites were therefore grouped by region, and 

student t-tests were used to identify the long-term statistically 

significant differences in drought frequency for the group of 

sites in each region at a 95-percent confidence level (p < 0.05).

Changes in drought frequency (mean frequency 

1983–2014 minus mean frequency 1951–82) are variable 

across the CONUS. Overall, 51 percent of sites experienced 

a decrease in drought frequency, 40 percent experienced an 

increase in drought frequency, and 9 percent experienced no 

change. Five of 9 aggregated ecoregions (Eastern Highlands, 

Southeast Coastal Plain, Western Mountains, Western Plains, 

and Western Xeric) exhibited changes in drought frequencies 

that are not statistically significant (p < 0.05). Four aggregated 

ecoregions (Central Plains, Mixed Wood Shield, Northeast, 

and Southeast Plains), all associated with the eastern part 

of the CONUS, exhibited statistically significant drought 

frequency differences (fig. 5A-inset, table 2). Drought frequencies decreased in the Central Plains and Northeast but 

increased in the Mixed Wood Shield and Southeast Plains. 

In the Central Plains, 88 of 108 sites have decreased drought 

frequency (81%), followed by the Northeast with 32 of 

44 sites with decreased drought frequency (73%). The Mixed 

Wood Shield had 8 of 12 sites with increased drought frequency (67%), followed by the Southeast Coastal Plain with 

24 of 42 sites with increased drought frequency (57%). Parts 

of some aggregated ecoregions exhibit clusters of sites with 

coherent changes in drought frequency. For example, drought 

frequency has increased in parts of the southeastern United 

States—Texas, California, the Western Mountains, and the 

Mixed Wood Shield (red and orange points fig. 5A).

Changes in drought duration and drought severity also 

have been variable across the CONUS; regions of similar 

changes in these drought statistics are not apparent (figs. 5B, 

5C, table 2). Overall, 49 percent of sites experienced a 

decrease in drought duration, 47 percent experienced an 

increase in drought duration, and 4 percent experienced no 

change in drought duration. However, 56 percent of sites 

experienced a decrease in drought severity, and 44 percent 

experienced an increase in drought severity (table 2). Drought 

duration and drought severity differences were calculated for 

the 609 sites that experienced drought in both time periods 

(1983–2014 and 1951–82); most of the differences were not 

statistically significant (p < 0.05) (figs. 5B-inset, 5C-inset, 

table 2). Parts of northern California, southern Oregon, and 

the area from Texas to Virginia, however, show that most sites 

have increased drought durations (red and orange points fig. 

5B), and increased drought severity predominates in parts, but 

not all, of the Southeast, Western Plains, and Western Mountains (red and orange points 5C). In the Southeast Plains, 46 

of 75 sites have increased drought duration (61%), and 42 of 

75 sites have increased drought severity (56%). In the Western 
Variability of Hydrological Droughts, 1951 Through 2014 7

0 250 500 1,000 KILOMETERS

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Aggregated ecoregion

EXPLANATION

Maximum duration

(months)

3 to 5

6 to 7

8 to 9

10 to 12

13 to 53

Central Plains

Eastern Highlands

Mixed Wood Shield

Northeast

SE Coastal Plains

SE Plains

South Florida

Western Mountains

Western Plains

Western Xeric

Aggregated ecoregion

EXPLANATION

Maximum severity

(scaled from 0 to 100)

0 to 54

55 to 61

62 to 67

68 to 75

76 to 95

A

B

Base from U.S. Geological Survey, 1994, 

The National Map: State Boundaries of the 

Conterminous United States, 1:2,000,000.

Figure 3. Maximum drought A, duration and B, severity in the conterminous United States, 1951–2014. Colored dots show magnitude. 

Colored areas show aggregated ecoregions. (SE, Southeast)
8 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014 Mean maximum hydrologic drought severity index, ranging from 0 to 100

55.0

57.5

60.0

62.5

65.0

67.5

70.0

11 12 13 14 15 16 17 18 19 20

Mean hydrologic drought frequency, in number of droughts per 100 years

Y = 48.48 + 0.9174*X

RMSE: 2.16

R2

: 0.578

Mean maximum hydrologic drought duration, in months

5

6

7

8

9

10

11

12

11 12 13 14 15 16 17 18 19 20

Mean hydrologic drought frequency, in number of droughts per 100 years

Y = 1.511 + 0.438*X

RMSE: 0.90

R2

: 0.641

SE Plains

(range = 22)

with

Northeast

(range = 12)

underneath

Eastern Highlands

(range = 13)

SE Coastal Plains

(range = 50)

with

Central Plains

(range = 24)

underneath

SE Plains

(range = 71)

Eastern Highlands

(range = 71)

Northeast

(range = 42)

SE Coastal Plains

(range = 76)

with

Central Plains

(range = 77)

underneath

Western Plains

(range = 37)

All Regions

(range = 6)

Mixed Wood Shield

(range = 12)

Western Xeric

(range = 25)

Western Mountains

(range = 26)

Mixed Wood Shield

(range = 52)

All Regions

(range = 78)

Western Plains

(range = 76)

Western Xeric

(range = 66)

Western Mountains

(range = 78)

Figure 4. Relation of mean maximum hydrological drought duration and severity to mean hydrological drought frequency in the 

conterminous United States, 1951–2014. Lines show changes in mean maximum hydrological drought duration and severity as functions 

of mean hydrological drought frequency. Shading identified the 95-percent confidence interval of each relation. Circle size indicates the 

relative range of values in each ecoregion and all ecoregions (blue circle). (RMSE, root mean square error; R2, R-squared coefficient of 

determination; SE, Southeast)
Variability of Hydrological Droughts, 1951 Through 2014 9

EXPLANATION

Difference in frequency

(droughts per 100 years)

Minus 39 to minus 13

Minus 12 to minus 3

Minus 2 to 3

4 to 16

17 to 42

EXPLANATION

Difference in duration

(months)

Minus 16.0 to minus 1.50

Minus 1.49 to minus 0.40

Minus 0.39 to 0.33

0.34 to 1.24

1.25 to 6.25

EXPLANATION

Difference in severity

Minus 12.9 to minus 5.7

Minus 5.6 to minus 2.5

Minus 2.4 to 0.2

0.3 to 3.3

3.4 to 9.9

Central Plains

Mixed Wood Shield

Northeast

SE Plains

EXPLANATION

Decrease

Increase

EXPLANATION

Decrease

Increase

EXPLANATION

Sites with statistically significant 

drought severity differences

Ecoregions with statistically significant 

drought frequency differences

Sites with statistically significant 

drought duration differences

0 250 500 1,000 KILOMETERS

C

B

A

Figure 5. Differences in mean A, frequency, B, duration, and C, severity of droughts in the conterminous United States, 1983–2014 

compared to 1951–82. Frequency insert shows ecoregions with statistically significant differences in drought frequency. Points on 

duration and severity insets show sites with statistically significant differences in drought duration and severity at the 95-percent 

confidence level (p < 0.05). Dot colors indicate direction or magnitude of difference. (SE, Southeast)
10 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

Table 2. Changes in hydrological drought frequency, duration, and severity in the conterminous United States for a recent period 

(1983–2014) compared to an older period (1951–82), by ecoregion, 609 sites experienced drought in both time periods. 

[%, percent; SE, Southeast; <, less than]

Drought frequency

Ecoregion

Total 

sites

Increased drought 

frequency

Decreased drought 

frequency

No change in drought 

frequency

Statistically significant 

for sites in the region?

(Number) (Number) (%) (Number) (%) (Number) (%) (Yes or No)

Central Plains 108 11 10% 88 81% 9 8% Yes

Eastern Highlands 41 12 29% 22 54% 7 17% No

Mixed Wood Shield 12 8 67% 4 33% 0 0% Yes

Northeast 44 7 16% 32 73% 5 11% Yes

SE Coastal Plains 42 24 57% 14 33% 4 10% No

SE Plains 75 40 53% 28 37% 7 9% Yes

Western Mountains 169 82 49% 73 43% 14 8% No

Western Plains 72 36 50% 29 40% 7 10% No

Western Xeric 46 21 46% 21 46% 4 9% No

All regions 609 241 40% 311 51% 57 9%

Drought duration

Ecoregion

Total 

sites

Increased drought 

duration

Decreased drought 

duration

No change in drought 

duration

Sites with statistically 

significant differences 

(p < 0.05)

(Number) (Number) (%) (Number) (%) (Number) (%) (Number)

Central Plains 108 38 35% 67 62% 3 3% 6

Eastern Highlands 41 16 39% 22 54% 3 7% 3

Mixed Wood Shield 12 6 50% 6 50% 0 0% 2

Northeast 44 15 34% 27 61% 2 5% 1

SE Coastal Plains 42 23 55% 17 40% 2 5% 0

SE Plains 75 46 61% 25 33% 4 5% 1

Western Mountains 169 81 48% 85 50% 3 2% 5

Western Plains 72 42 58% 27 38% 3 4% 3

Western Xeric 46 19 41% 25 54% 2 4% 3

All regions 609 286 47% 301 49% 22 4% 24

Drought severity

Ecoregion

Total 

sites

Increased drought 

severity

Decreased drought 

severity

No change in drought 

severity

Sites with statistically 

significant differences 

(p < 0.05)

(Number) (Number) (%) (Number) (%) (Number) (%) (Number)

Central Plains 108 38 35% 70 65% 0 0% 9

Eastern Highlands 41 18 44% 23 56% 0 0% 2

Mixed Wood Shield 12 5 42% 7 58% 0 0% 1

Northeast 44 17 39% 27 61% 0 0% 4

SE Coastal Plains 42 24 57% 18 43% 0 0% 3

SE Plains 75 42 56% 33 44% 0 0% 3

Western Mountains 169 75 44% 94 56% 0 0% 14

Western Plains 72 30 42% 42 58% 0 0% 8

Western Xeric 46 19 41% 27 59% 0 0% 5

All regions 609 268 44% 341 56% 0 0% 49
Comparison of Findings from This Study with Findings from Previous Investigations 11

Plains, 42 of 72 sites have increased drought duration (58%), 

and 30 of 72 sites have increased drought severity (42%). In 

the Southeast Coastal Plains, 23 of 42 sites have increased 

drought duration (55%) and 24 of 42 sites have increased 

drought severity (57%). Some aggregated ecoregions exhibit 

decreased drought duration and severity. In the Central Plains, 

67 of 108 sites (62%) experienced decreased drought duration, 

and 70 of 108 sites experienced decreased drought severity 

(65%). In the Northeast, 27 of 44 sites (61%) show decreased 

drought duration and decreased drought severity. In the Western Xeric, 25 of 46 sites (54%) experienced decreased drought 

duration, and 27 of 46 sites experienced decreased drought 

severity (59%). In the Eastern Highlands, 22 of 41 sites (54%) 

experienced decreased drought duration, and 23 of 41 sites 

(56%) experienced decreased drought severity. The Mixed 

Wood Shield experienced no net change in drought duration; 

6 of 12 sites (50%) show increases, and 6 of 12 sites (50%) 

show decreases. The Mixed Wood Shield experienced a small 

decrease in drought severity with 5 of 12 sites (42%) showing an increase and 7 of 12 sites (58%) showing a decrease 

(table 2).

Comparison of Findings from This 

Study with Findings from Previous 

Investigations

The Southeast Coastal Plains aggregated ecoregion 

has a high percentage of sites exhibiting increased drought 

frequency (57%), a mean drought duration of (5 months), the 

longest single hydrological drought event (53 months), and 

a high maximum drought severity of (95%) (tables 1 and 2). 

These results are consistent with other studies, such as Patterson and others (2013), in which a significant increase was 

identified for the joint probability of nearby basins simultaneously experiencing drought conditions from 1970 to 2010, and 

a late 20th century increase was identified for the conditional 

probability of these basins simultaneously being in drought. 

Patterson and others (2013) suggest that combined changes 

in climate, streamflow, and water demand affect southeastern 

droughts and assert that decreasing average streamflow during 

non-drought periods coupled with increasing water demand 

since the 1970s led to pressure on limited water supplies in 

the southeastern United States. These conclusions are reinforced by Andreadis and others (2005), who note an increase 

in global aridity since the 1970s and project increased aridity 

in the 21st century over most of the Americas, and Rind and 

others (1990), who identify an increasing number of droughts 

in the United States using the Goddard Institute for Space 

Studies general circulation model (GISS GCM) and note that 

drought effects became apparent beginning in the 1990s.

In this study the potential for persistent drought in the 

western United States is indicated by high frequencies of 

hydrological drought in the Western Mountains (19 droughts 

on average per 100 years), Western Xeric (18 droughts per 

100 years), and Central Plains (17 droughts per 100 years) 

combined with relatively long mean drought durations 

(5 months) and high maximum drought severity percentages 

(95%, 95%, and 94%, respectively). This is reinforced by 

Andreadis and others (2005) who determined that droughts 

in the western United States in the early 2000s were among 

the most severe for the period of record in their analysis of 

20th century droughts in the CONUS. Furthermore, Andreadis 

and others (2005) determined that the most severe agricultural droughts were among the most severe droughts of any 

kind for the period of record, and in the West most droughts 

were hydrological droughts. Seager and others (2007) identify 

a broad consensus among the results from climate models 

that the southwestern CONUS will dry significantly in the 

21st century and that a transition to a more arid climate is 

already underway. This observation is reinforced by systematic analysis of climate model simulations by Hoerling and 

Eischeid (2007) to determine whether an ongoing drought 

in the southwestern United States, within the time period of 

their analysis, is an early sign of departures from the historical 

norm. Hoerling and Eischeid (2007) conclude that the southwestern United States appears to be entering a new drought 

era, with conditions in this region shifting from droughts that 

were principally precipitation driven events enhanced by 

temperature to a near perpetual state of drought in the coming 

decades as a consequence of steadily increasing temperature.

Groisman and others (2004) found century-long increases 

in temperature and precipitation in the CONUS with the bulk 

of these increases occurring during the past three decades 

(1980–2010). Groisman and others (2004), for example, found 

that increases in precipitation and streamflow are associated with the eastern CONUS, whereas increased dryness 

is associated with the southwestern CONUS. These results 

correspond to some of the changes in regional hydrological 

drought identified in this study. For example, most sites in the 

Northeast aggregated ecoregion (73%) have exhibited decreasing drought frequencies, and 50 percent of sites in the Western 

Plains aggregated ecoregion have exhibited increasing drought 

frequencies (table 2).

Sheffield and others (2004) show that previously reported 

increases in global drought are overestimated because the 

PDSI used to make the predictions contains a simplified 

model of potential evaporation that responds only to changes 

in temperature and thus responds incorrectly to the global 

warming that has occurred over recent decades (1980–2010). 

Using high-resolution land-surface hydrological simulations 

to produce a drought-forecasting index, Sheffield and others (2004) suggest that more realistic calculations, based on 

underlying physical principles that take into account changes 

in available energy, humidity, and wind speed, show little 

change in global drought over the past 60 years (1956–2015). 

Their results coincide with general observations from this 

study for the entire CONUS. This study shows that, on the 

basis of a comparison of an older period (1951–82) with a 

recent period (1983–2014), for the CONUS little change in 
12 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

hydrological drought frequency, duration, or severity has 

occurred. Variable changes in hydrological drought frequency 

were identified across the CONUS with 51 percent of sites 

experiencing decreases, 40 percent experiencing increases, 

and 9 percent experiencing no change (table 2). Changes in 

hydrological drought duration and severity across the CONUS 

are also variable. Overall 49 percent of sites experienced 

a decrease in drought duration, 47 percent experienced an 

increase in drought duration, and 4 percent experienced no 

change in drought duration. However, 56 percent of sites experienced a decrease in drought severity, 44 percent experienced 

an increase in drought severity, and 0 percent experienced no 

change in drought severity. We find that most differences in 

drought duration and severity are not statistically significant 

(p < 0.05) (figs. 5B inset, 5C inset, table 2).

Limitations

Instances of serial dependence, when the value of a 

datum is affected by one or more previous data values, are 

a potential cause of change in measured flow patterns possibly resulting from a long-term trend, seasonal variations, 

dependence on some other serially correlated variable, or 

some combination of these (Helsel and Hirsch, 2002). This 

study found that any limitations associated with serial dependence of streamflows appear to be negligible. Precipitation 

is a primary climate driver of drought. Mapped changes in 

precipitation that correspond with our analysis of differences 

in hydrological drought illustrate the affect of precipitation as 

a driver of drought. Plotted differences in precipitation values 

(Daly and others, 2008) for the recent period (1983 through 

2014) minus values for the older period (1951 through 1982) 

show temporal changes in precipitation consistent with our 

reported temporal changes in hydrological drought frequency 

(figs. 5A and 6). These similarities in hydrological drought and 

precipitation changes signal a foundational relation between 

streamflow and precipitation over several time intervals in 

which any small-scale correlations associated with serial 

dependence of values, localized changes in flow patterns, or 

combinations of these influences are negligible.

Sheffield and others (2012) describe other potential 

physical interactions of temperature and precipitation during 

droughts in which evaporation may decrease because of less 

precipitation, resulting in higher temperatures caused by less 

evaporative cooling and a larger heat flux warming the air. 

Droughts have also been shown to co-vary with Pacific Ocean 

and Atlantic Ocean surface temperatures, and some assessments of drought made using indices such as the PDSI show 

expected increases in drought frequency and severity resulting 

from climate change (McCabe and others, 2004; Seager and 

others 2007; Sheffield and others 2009; Dai, 2013). Although 

combined interactions among precipitation, evaporation, 

transpiration, and temperature can affect hydrological drought 

variability in certain areas of the CONUS, in this study, we 

found that changes in precipitation, as a primary driver, correspond with overall changes in frequency of hydrological 

droughts.
Limitations 13

Precipitation difference, in millimeters

(recent minus older period)

Base from U.S. Geological Survey, 1994, The National Map: 

State Boundaries of the Conterminous United States, 1:2,000,000

0 250 500 1,000 KILOMETERS

Minus 1,885 to minus 100

Minus 99 to minus 20

Minus 19 to 20

21 to 100

101 to 1,197

EXPLANATION

Figure 6. Change in precipitation in the conterminous United States between a recent period (1983 through 2014) and an older period 

(1951 through 1982). Precipitation differences correspond to hydrological drought differences from Falcone and others (2010). Changes 

in precipitation are consistent with the temporal changes in drought frequency shown in figure 5.
14 Variability of Hydrological Droughts in the Conterminous United States, 1951 through 2014

Summary and Conclusions

As global climate continues to warm, concern continues 

to increase that climate change will have substantial effects on 

the occurrence and severity of hydrological droughts. When 

comparing an older period (1951–82) with a recent period 

(1983–2014), we found few sites with statistically significant 

changes in drought frequency, drought duration, or drought 

severity at a 95-percent confidence level. Some aggregated 

ecoregions (the Mixed Wood Shield and the Southeast Plains) 

and subsets of aggregated ecoregions (parts of the southeastern 

United States, parts of Texas and California) have experienced 

increases in drought frequency. Other aggregated ecoregions 

(the Central Plains and the Northeast) have experienced 

overall decreases in drought frequency. Four of 9 aggregated 

ecoregions have statistically significant differences in drought 

frequency. Changes in hydrological drought duration and 

severity during the specified periods were variable throughout 

the conterminous United States (CONUS). Parts of northern 

California, southern Oregon, and the area from Texas to Virginia experienced increases in drought duration. Parts of the 

Southeast, Western Plains, and Western Mountains aggregated 

ecoregions experienced increases in drought severity. At most 

sites, however, the observed differences in drought duration and drought severity were not statistically significant at 

p < 0.05.

Relatively long mean drought durations, high mean 

maximum drought severity percentages, and high frequencies of hydrological drought are evident in certain parts of 

the CONUS, such as the Western Plains, Western Mountains, 

Western Xeric, Central Plains, and Southeast Coastal Plains 

aggregated ecoregions. When evaluated within the context of 

other studies these results indicate a continued potential for 

hydrological droughts in these parts of the CONUS.

Acknowledgments

The authors thank Pixie Hamilton, Eric Evenson, and 

Michael Norris of the U.S. Geological Survey for proposing 

and supporting this project. George E. Harlow, Jr., and Stacey 

Archfield of the U.S. Geological Survey are thanked for helpful advice and suggestions.

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