Drought in the United States: Causes and Issues for Congress
Drought in the United States:
Causes and Issues for Congress
July 15, 2008
Peter Folger, Betsy A. Cody, and Nicole T. Carter
Resources, Science, and Industry Division
Drought in the United States:
Causes and Issues for Congress
Drought is commonly defined as a deficiency of precipitation over an extended
period of time, usually a season or more, relative to some long-term average
condition. Droughts have afflicted the United States, particularly the American
West, for centuries. Drought affects societies because of the combination of reduced
supply (e.g., less precipitation, reduced reservoir levels, a lower groundwater table)
and competing demand (e.g., for irrigation, municipal and industrial supply, energy
production, species protection). This report focuses on the physical causes of
drought, its history in the United States, and what may be expected in the near future.
Although currently drought can be predicted for a particular region for at best a few
months in advance, past history suggests that severe and extended droughts are
inevitable and part of natural climate cycles, particularly in the West.
Drought concerns are increasing as some studies suggest that the American
West may be in transition to a more arid climate, more prone to extreme drought,
than was the norm during most of the 20th century. While drought is most common
in the West, drought can also provoke water resource conflicts in other parts of the
country. For example, the 2007-2008 drought in the Southeast has heightened a
long-standing dispute over water in the Apalachicola-Chattahoochee-Flint River
basin, even though the three states at odds with each other — Georgia, Alabama, and
Florida — receive more rainfall in dry years than many western states receive in
The physical conditions causing drought in the United States are increasingly
understood to be linked to sea surface temperatures (SSTs) in the tropical Pacific
Ocean. Studies indicate that cooler-than-average SSTs have been connected to the
recent severe western drought, severe droughts of the late 19th century, and
precolonial North American “megadroughts.” Some climate model projections
suggest that warming temperatures resulting from increased greenhouse gases in the
atmosphere could return the western United States to more arid baseline conditions
within decades (i.e., conditions similar to those during earlier times).
The prospect of extended droughts and more arid baseline conditions in parts
of the United States could present new challenges to federal water projects, the
construction of which was based largely on 20th century climate conditions. In
responding to competing demands for water (e.g., deliveries to serve agricultural
demands versus supporting endangered species), federal water delivery systems may
have to be re-tuned to match a drier average climate in the West.
The evolving nature of drought, split federal and non-federal responsibilities,
and a patchwork of federal programs and congressional committee jurisdictions make
development of a comprehensive national drought policy difficult. While Congress
has considered some of the recommendations issued by the National Drought Policy
Commission in 2000, comprehensive drought legislation has not been enacted.
Congress may wish to review responses to the 2007-2008 Southeast drought, as well
as revisit recommendations of the National Drought Policy Commission.
In troduction ......................................................1
What Is Drought?..................................................2
Drought Is Relative............................................3
Drought Is Multifaceted.........................................3
Responding to Drought.............................................7
What Causes Drought in the United States..............................9
Prehistorical and Historical Droughts in the United States.............10
Drought Forecasts for the United States...........................12
The National Drought Policy Act of 1998......................14
National Drought Preparedness Legislation and the 2008 Farm Bill.15
National Integrated Drought Information System................15
List of Figures
Figure 1. Example of a Drought Conditions Map in the United States........4
List of Tables
California Drought Emergency Declaration.............................6
El Niño-Southern Oscillation (ENSO).................................11
Drought in the United States:
Causes and Issues for Congress
Every year drought affects some part of the United States. At any given time,
at least 10% of the country is typically experiencing drought of at least moderate1
intensity. Over the last eight years as much as 45% of the country has been affected
by at least moderate intensity drought at one time.2 Over the same time span,
extreme or exceptional drought affected an average of approximately 7% of the
nation, by area.3 In the 20st century, droughts in the 1930s and 1950s were
particularly severe. At the maximum geographic extent of the 1930s drought in
conditions. How droughts are classified, and what is meant by moderate, severe, and
extreme drought classification is discussed further below.
Drought has afflicted portions of North America for thousands of years,
particularly in the West, although severe drought has also occurred in the more
humid Mississippi Valley and southeastern United States. Severe, long-lasting
droughts may have been an important factor in the disintegration of Pueblo society
in the Southwest during the 13th century, and in the demise of central and lowerthth5
Mississippi Valley societies in the 14 through 16 centuries. Severe drought has
occurred and will likely continue to occur periodically in the United States. The
likelihood of extended periods of severe drought, similar to conditions experienced
1 According to data collected by the National Drought Mitigation Center (NDMC) since
2 NDMC reports that during the week of August 26, 2004, 45.9% of the nation faced at least
moderate intensity drought. Over the time period between January 2000 and March 2008,
at least moderate intensity drought has occurred over roughly 28% of the country, on
3 Although in some years no part of the country was under extreme or exceptional drought
during certain months. For example, from January 2000 through the first two weeks of
April 2000, extreme or exceptional drought did not affect any portion of country, according
to the NDMC.
4 Donald A. Wilhite, et al., Managing Drought: A Roadmap for Change in the United States,
(Boulder, CO: The Geological Society of America, 2007), p. 12; at [http://www.
geosociety.org/ meetings /06drought/roadmap.pdf].
5 Cook, Edward R., Richard Seager, Mark A. Crane, and David W. Stahle, “North American
drought: reconstructions, causes, and consequences,” Earth-Science Reviews, vol. 81 (2007):
pp. 93-134. Hereafter referred to as Cook et al., 2007.
centuries ago, and its effects on 21st century society in the United States raise several
issues for Congress.
The recurrence of drought is of concern to Congress for many reasons. Drought
often results in significant agricultural losses, which can have widespread effects.
It can also impact other industries and services, including power production,
navigation, recreation, and natural resources such as fisheries and water quality.
Addressing these impacts on an emergency basis is costly — often resulting in
hundreds of millions and sometimes billions of dollars in federal assistance.
Additionally, drought affects management of federal reservoirs and in many cases
exacerbates existing tensions among competing uses.
This report discusses how drought is defined (e.g., why drought in one region
of the country is different from drought in a different region), and why drought
occurs in the United States. It briefly discusses periods of drought in the country’s
past that equaled or exceeded drought conditions experienced during the 20th century,
including periods during earlier centuries where the American West was substantially
drier, on average, than it is today. This is followed by a discussion of the prospects
for a future climate in the West that may be drier than the average 20th century
climate — perhaps similar to drier periods centuries ago. The report concludes with
a discussion of policy challenges for Congress, such as the existing federal/non-
federal split in drought response and management, and the patchwork of drought
programs subject to oversight by multiple congressional committees. These policy
challenges may become increasingly more difficult to address in the face of a
changing climate that could exacerbate drought conditions in some regions of the
What Is Drought?
Drought has a number of definitions;6 the simplest conceptual definition may
be a deficiency of precipitation over an extended period of time, usually a season or
more. Drought is usually considered relative to some long-term average condition,
or balance, between precipitation, evaporation, and transpiration by plants
(evaporation and transpiration are typically combined into one term:7
evapotranspiration). An imbalance could result from a decrease in precipitation, or
an increase in evapotranspiration (from drier conditions, higher temperatures, higher
winds), or both. It is important to distinguish between drought, which has a
beginning and an end, and aridity, which is restricted to low rainfall regions and is
a relatively permanent feature of climate (e.g., deserts are regions of relatively
6 NDMC, at [http://www.drought.unl.edu/whatis/what.htm].
7 Evapotranspiration may be defined as the loss of water from a land area through
transpiration from plants and evaporation from the soil and surface water bodies such as
lakes and ponds.
8 NDMC, at [http://www.drought.unl.edu/whatis/concept.htm].
An increased demand for water from human activities and vegetation in areas
of limited water supply increases the severity of drought. For policy purposes,
drought becomes an issue when it results in a water supply deficiency: less water is
available than the average amount for irrigation, municipal and industrial supply
(M&I), energy production, preservation of endangered species, and other needs. At
the national level, drought is monitored and reported in an index known as the U.S.
Drought Monitor, which synthesizes various drought indices and impacts, and
represents a consensus view of on-going drought conditions between academic and
Drought Is Relative
“Normal” conditions can vary considerably from region to region. For example,
in early April 2008, extreme drought simultaneously gripped south Texas and
northeast Alabama, according to the U.S. Drought Monitor.9 (See Figure 1.)
However, extreme drought means something different to the city of Laredo, in south
Texas, than it does for the city of Huntsville, in northeast Alabama. Laredo receives10
an average total of 2.62 inches of rain from January through March of each year.
In contrast, Huntsville receives an average of 17.2 inches over the same time period.11
In 2008, Huntsville received 11.26 inches from January through March, four times
the average precipitation for Laredo, but only 65% of what Huntsville normally
receives. Each city received abnormally low rainfall in the first quarter of 2008, but
what is “normal” for each city is different.
To deal with these differences, meteorologists use the term meteorological
drought — usually defined as the degree of dryness relative to some average amount
of dryness and relative to the duration of the dry period. Meteorological drought is
region-specific because atmospheric conditions creating precipitation deficiencies
vary from region to region. Both Huntsville, AL and Laredo, TX experienced
meteorological drought in the first quarter of 2008, even though each city received
different amounts of rainfall.
Drought Is Multifaceted
The Drought Monitor uses an “A” to indicate that the primary physical effects
are agricultural (crops, pastures, and grasslands) and an “H” to indicate that the
primary impacts are hydrological (to water supplies such as rivers, groundwater, and
reservoirs).12 When both effects are apparent, the letters are combined, appearing as
“AH” on Drought Monitor maps. (See Figure 1.) To continue the comparison
9 See U.S. Drought Monitor at [http://www.drought.unl.edu/dm/monitor.html].
10 The National Oceanic and Atmospheric Administration, Southern Regional Climate
Center, Baton Rouge, LA, at [http://www.srcc.lsu.edu/southernClimate/atlas/images/
11 Data for Huntsville International Airport. The National Oceanic and Atmospheric
Administration, National Weather Service Forecast Office, Huntsville, AL, at [http://www.
12 U.S. Drought Monitor, at [http://www.drought.unl.edu/dm/classify.htm].
between Huntsville, AL and Laredo, TX, the U.S. Drought Monitor extreme drought
classification — indicating major crop/pasture losses, and widespread water
shortages/restrictions — for both cities is further classified by an “A” for south Texas
and an “H” for northeast Alabama. The Drought Monitor maps thus indicate the
severity of a drought, ranging from abnormally dry (shown as D0 on the maps) to
exceptional drought (shown as D4), as well as the primary physical effects that are
important to the region affected (A or H). How these conditions are assessed and
how drought is classified are discussed below.
Figure 1. Example of a Drought Conditions Map
in the United States
Source: U.S. Drought Monitor, at [http://www.drought.unl.edu/dm/monitor.html], on April
To assess and classify the intensity and type of drought, certain measures, or
drought indices, are typically used. Drought intensity, in turn, is the trigger for local,
state, and federal responses that could lead to the flow of billions of dollars in relief13
to drought-stricken regions. The classification of drought intensity, such as that
shown in Figure 1, may depend on a single indicator or several indicators, often
combined with expert opinion from the academic, public, and private sectors. The
U.S. Drought Monitor uses five key indicators,14 together with expert opinion, with
indices to account for conditions in the West where snowpack is relatively important,
and with other indices used mainly during the growing season.15 The Drought
Monitor intensity scheme — D0 to D4 — is used to depict broad-scale conditions but
13 For example, the Palmer Drought Index has been widely used by the U.S. Department of
Agriculture to determine when to grant emergency drought assistance. See NDMC at
14 The five key indicators include the Palmer Drought Index, the Climate Prediction Center
soil moisture model, U.S. Geological Survey weekly streamflow data, the Standardized
Precipitation Index, and short- and long-term drought indicator blends. For a discussion of
drought indices, see the NDMC, at [http://www.drought.unl.edu/whatis/indices.htm].
15 U.S. Drought Monitor, at [http://www.drought.unl.edu/dm/classify.htm].
not necessarily drought circumstances at the local scale. For example, the large
regions depicted as red in Figure 1 faced severe drought conditions in early 2008, but
they may contain local areas and individual communities that experienced less (or
more) severe drought.
The “A” and “H” terms shown on Figure 1 give additional information on the
nature of the drought in the affected region. Agricultural drought (“A”) can be
defined as when there is insufficient moisture to meet the needs of a particular crop
at a particular time.16 For example, deficient topsoil moisture during planting might
hinder germination and affect the final yield, even if moisture is replenished later in
the growing season. However, if enough moisture is available for early growth
requirements, although below normal levels, the final yield may not be affected if
subsoil moisture is replenished over the length of the growing season.17
Hydrological drought (“H”) can be defined as deficiencies in water supplies, as
measured by stream flows, lake or reservoir levels, or elevation of the ground water
surface. Hydrological drought usually lags behind agricultural drought because it
takes longer for deficiencies in precipitation to affect the broader hydrologic system.
Lack of rainfall during a critical part of the growing season may have an immediate
impact on farmers — an agricultural drought — but the deficiency may not affect
reservoir or river levels for many months.18 Because a hydrological drought affects
the broader hydrologic system, such as one or several river basins, a severe
hydrological drought could exacerbate competition among water uses: irrigation,
navigation, recreation, M&I, energy production, preservation of endangered species,
An example of hydrological drought is the drought in the southeastern United
States (which includes northeast Alabama and the city of Huntsville, in the example
discussed above, as well as Atlanta and northeast Georgia). A persistent severe
drought in the region, beginning with below-average rainfall in the spring of 2006,19
exacerbated an ongoing interstate conflict among Alabama, Florida, and Georgia over
water allocation in the Apalachicola-Chattahoochee-Flint (ACF) river system. The
decision to draw down Lake Lanier, the uppermost federal reservoir in the ACF
basin, in the fall of 2007 to support minimum flows in the lower basin Apalachicola
River triggered concerns from Atlanta’s municipal and industrial water users over
loss of their principal water supply.20 Conversely, efforts to halt the drawdown drew
criticism from downstream interests concerned about species protection and energy
16 NASA Earth Observatory, at [http://earthobservatory.nasa.gov/Library/DroughtFacts/].
17 NDMC, at [http://www.drought.unl.edu/whatis/concept.htm].
19 NOAA, The National Weather Service, Southeast River Forecast Center, When Did the
Drought Begin, a Focus on the North Georgia and Atlanta Areas (Nov. 16, 2007), at
[ ht t p: / / www.sr h.noaa.gov/ al r / dr ought / J our nal 111607.pdf ] .
20 For more information on the ACF drought conflict, see CRS Report RL34326,
Apalachicola-Chattahoochee-Flint (ACF) Drought: Federal Water Management Issues,
coordinated by Nicole T. Carter; and CRS Report RL34440, Apalachicola-Chattahoochee-
Flint (ACF) Drought: Species and Ecosystem Management, by M. Lynne Corn, Kristina
Alexander, and Eugene H. Buck.
Another example is the California drought, classified as both “A” and “H”. The
California situation is complicated by decades of tension between water supply
deliveries for irrigation and M&I uses, and preserving water flows to protect
threatened and endangered species. The June 4, 2008, drought declaration by
Governor Arnold Schwarzenegger responded to dry conditions and lower than
normal snowpack (see box). These conditions exacerbated an already tight water
supply where federal and state water deliveries had been recently reduced, in
response to a court order, to prevent extinction of the Delta Smelt.21 The governor’s
decision underscores why drought is complex and not simply a result of dry
conditions. The governor’s drought declaration reflected long-simmering tensions
between supply and demand in California, made worse by drought conditions. The
droughts in California and the Southeast underscore an underlying difficulty of
managing federal reservoirs to meet multipurpose water needs.
California Drought Emergency Declaration
California Governor Schwarzenegger declared a statewide drought on June 4, 2008, the
first such declaration in California since 1991. The governor’s declaration and
emergency order included the following observations:
!two consecutive years of below average rainfall;
!snowpack at 67% of normal water content;
!runoff forecast at 55% of normal statewide, and only 41% for the
Sacramento and San Joaquin Rivers;
!water storage in many state reservoirs far below normal;
!Colorado River Basin reservoir storage at 50% of total capacity; and
!restrictions on deliveries from the Sacramento-San Joaquin Delta due
to federal court actions to protect fish species.
The governor also recognized that dry conditions have created a situation of extreme fire
danger in the state, and noted that wildfires last year resulting from dry conditions led to
emergency declarations for 10 southern California counties and millions of dollars in
damages. Governor Schwarzenegger ordered the California Department of Water
Resources (DWR) to
!expedite grant programs for local water districts and agencies for water
!facilitate water transfers to respond to water shortages;
!conduct an aggressive water conservation program;
!convene a committee to expedite drought-related climate research;
!provide technical assistance for improving irrigation efficiency;
!review Urban Water Management Plans for water shortage contingency
!implement State Water Project operations and water exchanges for the
San Joaquin Valley agriculture.
The governor also ordered the DWR to work with the U.S. Department of Agriculture and
the U.S. Bureau of Reclamation to identify federal funding for local water agencies and
farmers to install the best available irrigation management and conservation systems. The
federal role in managing water supplies and providing relief to farmers is likely to grow
in California if the drought continues or worsens.
Source: Governor of the State of California, Office of the Governor, press release (June
21 Natural Resources Defense Council v. Kempthorne, No. 1:05-cv-1207 OWW GSA (E.D.
Cal., Dec. 14, 2007).
Responding to Drought
When a drought is declared by the U.S. President or by a state governor for a
locality or region of the United States, it sets in motion a series of alerts,
recommendations, activities and possible restrictions at the local, regional, or state
level depending on the drought length and severity. Ultimately, a multiyear severe
drought could initiate a federal response and transfer of federal dollars to the affected
area. Before drought severity reaches a level requiring a federal response, however,
many states take action, often according to their version of a drought management
plan.22 For example, the governor of Alabama issued a drought declaration on March
in accordance with the draft Alabama Drought Management Plan.23 The emergency
drought level for Alabama is its most extreme category of drought. According to
Alabama’s Plan, declaring drought does not “... automatically invoke a required
response from the various categories of water users”;24 however, upon confirmation
of a drought emergency, the governor’s office may issue “...public statements that a
drought emergency exists, disaster declarations, and the appropriate implementation
of water conservation and drought emergency ordinances.”25
If the effects of a drought overwhelm state or local resources, the President, at
the request of the state governor, is authorized under the Stafford Act (42 U.S.C.
5121 et seq.) to issue major disaster or emergency declarations that result in the
distribution of federal aid to affected parties.26 For example, on October 20, 2007,
the governor of Georgia requested a presidential drought disaster declaration because
of prolonged exceptional drought conditions existing in the northern third of the27
state. No such presidential declaration has occurred in response to the request.
More frequently, a state governor requests drought disaster assistance through the
U.S. Secretary of Agriculture, who can declare an agricultural disaster as a result of
drought and make available low-interest loans and other emergency assistance
22 As of 2006, 37 states had some form of drought management, mitigation, or response plan
according to the NDMC. Their tally, however, may not reflect plans that are in draft form,
such as Alabama’s Drought Management Plan. For more information, see
[http://www.drought.unl.edu/pl an/stateplans.htm] .
23 For more information, see [http://www.adeca.alabama.gov/Office%20of%20Water%20
24 Alabama Drought Management Plan, p. 7.
25 Alabama Drought Management Plan, p. 8.
26 For more information about the Stafford Act, see CRS Report RL33053, Federal Stafford
Act Disaster Assistance: Presidential Declarations, Eligible Activities, and Funding, by
27 The last presidential drought disaster declaration in the continental United States was for
New Jersey in 1980. More recent drought declarations have been issued for U.S. territories
in the Pacific. See [http://www.fema.gov/news/disasters.fema].
through various U.S. Department of Agriculture programs.28 The U.S. Army Corps
of Engineers (Corps) and the U.S. Bureau of Reclamation (Reclamation) also have
limited drought emergency authorities and funding (e.g., the Reclamation States
Emergency Drought Act, as amended [43 U.S.C. 2211 et seq.]).
Under current U.S. farm policy, financial losses caused by drought and other
natural disasters are mitigated primarily via the federal crop insurance program
(administered by the U.S. Department of Agriculture’s Risk Management Agency).
Since the severe drought of 1988, Congress has also regularly made supplemental
financial assistance available to farmers and ranchers, primarily in the form of crop
disaster payments and emergency livestock assistance. Since 2000, the federal
contribution to the crop insurance program has averaged about $3.3 billion per year,
mostly in the form of a premium subsidy and reimbursements to private insurance
companies.29 Another $1.1 billion in ad-hoc crop disaster payments has been made
available on average each year since 2000.
Even absent federal drought disaster declarations, sustained hydrological
drought can affect operations of federally managed reservoirs, dams, locks,
hydroelectric facilities and other components of the nation’s water infrastructure. As
discussed above, the current Southeast drought directly affects how the Corps
manages its facilities in ACF basin. Similarly, current drought conditions in
California coupled with declining fish species have resulted in operational changes
to Reclamation facilities, including significantly reduced water deliveries to Central
Valley Project contractors, as well as to California’s State Water Project (SWP)
contractors. Reclamation, whose facilities currently serve over 31 million people and
deliver a total of nearly 30 million acre-feet30 of water annually, faces operational
challenges because of conflicts among its water users during drought in states it
28 The Secretary of Agriculture declared a disaster for every county in Georgia in 2007
because of the ongoing drought and severe April 2007 freeze. For more information on this
program, see CRS Report RS21212, Agriculture Disaster Assistance, by Ralph Chite. See
also: CRS Report RL34207, Crop Insurance and Disaster Assistance in the 2008 Farm Bill,
by Ralph Chite.
29 The causes of crop loss can vary dramatically from year to year, although drought is one
of the most common, if not the most common, cause of crop loss. See CRS Report
RS21212, Agriculture Disaster Assistance, and CRS Report RL31095, Emergency Funding
for Agriculture: A Brief History of Supplemental Appropriations, FY1989-FY2008, by Ralph
M. Chite for more information.
30 One acre-foot is enough water to cover one acre of land one foot deep. An acre-foot is
equivalent to 325,851 gallons. For more information about federal water supply programs,
see CRS Report RL30478, Federally Supported Water Supply and Wastewater Treatment
Program, by Betsy A. Cody and others.
31 Reclamation is a central player in water resource management in the West, and a
devastating drought at the end of the 19th century was probably one of the many factors that
led to the 1902 Reclamation Act that launched the federal reclamation effort and
Severe drought conditions in 2001 exacerbated competition for scarce water
resources in the Klamath River Basin, on the Oregon-California border, among
farmers, Indian tribes, commercial and sport fishermen, other recreationists, federal
wildlife refuge managers, environmental groups, and state, local, and tribal
governments. Reclamation’s decision in April 2001 to withhold water from farmers
for instream flows for three fish species listed as endangered or threatened under the
Endangered Species Act sparked congressional debate that continues today.
In the future, the United States may face severe and sustained periods of drought
not experienced in the 20th century. If so, disputes over water management like those
in the ACF and Klamath River Basins may increasingly dominate short-term actions
by Reclamation and the Corps, and result in long-term consequences for
congressional oversight and funding.
What Causes Drought in the United States
The immediate cause of drought is:
the predominant sinking motion of air (subsidence) that results in compressional
warming or high pressure, which inhibits cloud formation and results in lower
relative humidity and less precipitation. Regions under the influence of
semipermanent high pressure during all or a major portion of the year are usually
deserts, such as the Sahara and Kalahari deserts of Africa and the Gobi Desert
Desert regions that experience semipermanent high pressure are arid regions of the
globe, reflecting persistent dry climate conditions, as distinguished from drought,
which is a shorter-term departure from wetter average conditions.
Prolonged droughts occur when atmospheric conditions leading to the
predominant sinking motion of air over a certain geographic area, as a result of large-
scale anomalies in atmospheric circulation patterns, persist for months or years.33
Predicting drought, however, is difficult because the ability to forecast surface
temperature and precipitation depends on a number of key variables, such as air-sea
interactions, topography, soil moisture, land surface processes, and how other aspects
of the dynamics of weather systems influence each other.34 Scientists seek to
understand how all these variables interact and thus improve the ability to predict
sustained and severe droughts more than a season or two in advance.
Reclamation itself. (See Marc Reisner, Cadillac Desert, (New York, New York, Penguinth
Books, 1986), pp. 108-109. Other research suggests that the failures of some late 19
century private irrigation projects, undertaken following passage of the Carey Act [see
footnote 45], may have occurred in part due to drought conditions.)
32 See NDMC, at [http://www.drought.unl.edu/whatis/predict.htm].
In the tropics, a major portion of the atmospheric variability over months or
years seems to be associated with variations in sea surface temperatures (SSTs).
Since the mid- to late-1990s, scientists have increasingly linked drought in the United
States to SSTs in the tropical Pacific Ocean. Cooler-than-average SSTs in the eastern
tropical Pacific region — “la Niña-like” conditions — have been shown to be
correlated with persistently strong drought conditions over parts of the country,
particularly the West.35 A number of recent studies have made the connection
between cooler SSTs in the eastern Pacific and the 1998-2004 western drought,36
three widespread and persistent droughts of the late 19th century,37 and past North
American “megadroughts” that occurred between approximately 900 and 1300 A.D.38
The precolonial megadroughts apparently lasted longer and were more extreme than
any U.S. droughts since 1850 when instrumental records began. Some modeling
studies suggest that within a few decades the western United States may again face
higher base levels of dryness, or aridity, akin to the 900-1300 A.D. period.39
Although the relationship between cooler-than-normal eastern tropical Pacific
SSTs (La Niña-like conditions) and drought is becoming more firmly established,
meteorological drought is probably never the result of a single cause. Climate is
inherently variable, and accurately predicting drought for one region in the United
States for more than a few months or seasons in advance is not yet possible because
so many factors influence regional drought. What is emerging from the scientific
study of drought is an improved understanding of global linkages — called
teleconnections by scientists — between interacting weather systems, such as the El
Niño-Southern Oscillation, or ENSO. (See box, below, for a description of ENSO.)
For example, some scientists link La Niña conditions between 1998 and 2002 with
the occurrence of near-simultaneous drought in the southern United States, southern
Europe, and Southwest Asia.40
Prehistorical and Historical Droughts in the United States
Some scientists refer to severe drought as “...the greatest recurring natural
disaster to strike North America.”41 That claim stems from a reconstruction of
drought conditions that extends back over 1,000 years, based on observations,
historical and instrumental records where available, and on tree-ring records or other
35 Cook et al., 2007.
36 Hoerling, Martin and Arun Kumar, “The perfect ocean for drought,” Science, vol. 299
(Jan. 31, 2003), pp. 691-694. Hereafter referred to as Hoerling and Kumar, 2003.
37 Herweiger, Celine, Richard Seager, and Edward Cook, “North American droughts of the
mid to late nineteenth century: a history, simulation and implication for Mediaeval drought,”
The Holocene, vol. 15, no. 2 (Jan. 31, 2006), pp. 159-171. Hereafter referred to as
Herweiger et al., 2006.
38 Cook et al., 2007.
39 Richard Seager et al., “Model projections of an imminent transition to a more arid climate
in southwestern North America,” Science, vol. 316 (May 25, 2007): pp. 1181-1184.
40 Hoerling and Kumar, 2003.
41 Cook et al., 2007.
proxies42 in the absence of direct measurements. What these reconstructions
illustrate is that the coterminous United States have experienced periods of severe
and long-lasting drought in the western states and also in the more humid East and
Mississippi Valley. The drought reconstructions from tree rings apparently confirm
that severe multidecadal drought occurred in the American Southwest during the 13th
century, which anthropologists and archeologists suspect profoundly affected Pueblo
society. Tree ring drought reconstructions also document severe drought during the
14th, 15th, and 16th centuries in the central and lower Mississippi Valley possibly
contributing to the disintegration of societies in that region.43
El Niño-Southern Oscillation (ENSO)
Under normal conditions, the trade winds blow towards the west in the tropical Pacific
Ocean, piling up the warm surface waters so that the ocean surface off Indonesia is
one half meter higher than the ocean off Ecuador. Upwelling deep waters off the west
coast of South America are 8 degrees Celsius (14.4 degrees Fahrenheit) cooler than
waters in the western Pacific. During El Niño, the trade winds relax, upwelling off
South America weakens, and sea surface temperatures rise. The El Niño events occur
irregularly at intervals of 2-7 years, and typically last 12-18 months. These events
often occur with changes in the Southern Oscillation, a see-saw of atmospheric
pressure measured at sea level between the western Pacific and Indian Ocean, and the
eastern Pacific. Under normal conditions, atmospheric pressure at sea level is high in
the eastern Pacific, and low in the western Pacific and Indian Oceans. As implied by
its name, the atmospheric pressure oscillates, or see-saws between east and west; and
during El Niño the atmospheric pressure builds up to abnormally high levels in the
western tropical Pacific and Indian Oceans — the El Niño-Southern Oscillation, or
ENSO. During a La Niña , the situation is reversed: abnormally high pressure builds
up over the eastern Pacific, the trade winds are abnormally strong, and cooler-than-
normal sea surface temperatures occur off tropical South America. Scientists use the
terms ENSO or ENSO cycle to include the full range of variability observed,
including both El Niño and La Niña events.
Source: Tropical Ocean Atmosphere Project, Pacific Marine Environmental
Laboratory, at [http://www.pmel.noaa.gov/tao/proj_over/ensodefs.html].
More recently, a combination of tree ring reconstructions and other proxy data,
historical accounts, and some early instrumental records identify three periods ofth
severe drought in the 19 century: 1856-1865 (the “Civil War drought”), 1870-1877,
and 1890-1896.44 The 1856-1865 drought, centered on the Great Plains and
Southwest, was the most severe drought to strike the region over the last two
centuries, according to one study.45 The 1890-1896 drought coincided with a period
in U.S. history of federal encouragement of large-scale efforts to irrigate the
42 Proxies are indirect measurements typically used where direct measurements are
unavailable. Tree rings can be used as a proxy for measuring dryness and drought.
Similarly, ice cores from glaciers and polar caps can be used as proxies for measuring
atmospheric temperatures and carbon dioxide concentrations from thousands of years ago.
43 Cook et al., 2007.
44 Herweiger et al., 2006.
relatively arid western states via the Carey Act,46 and with congressional debate over
a much larger federal role in western states irrigation which led to the Reclamation
Act of 1902.
In the 20th century, the 1930s “Dust Bowl” drought and the 1950s Southwest
drought are commonly cited as the two most severe multiyear droughts in the United
States.47 (The 1987-1989 drought was also widespread and severe, mainly affecting
the Great Plains but also instigating extensive western forest fires, including the
widespread Yellowstone fire of 1988.) According to several studies, however, the
19th and 20th century severe droughts occurred during a regime of relatively less arid
conditions compared to the average aridity in the American West during the early
megadroughts. One study indicates that the early drought record shows similar
variability — drought periods followed by wetter periods — compared to today, but
the average climate conditions were much drier;48 thus droughts were more severe.
Drought Forecasts for the United States
As indicated above, predicting the severity and duration of severe drought over
a specific region of the country is not yet possible more than a few months in advance
because of the many factors that influence drought. Nevertheless, some modeling
studies suggest that a transition to a more arid average climate in the American West,
perhaps similar to conditions in precolonial North America, may be underway.49
Some studies have suggested that human influences on climate, caused by emissions
of greenhouse gases, may be responsible for a drying trend.50 Whether future
greenhouse gas-driven warming can be linked to La Niña -like conditions, discussed
above as a possible mechanism for causing drought conditions in the United States,
46 The Carey Act, signed into law on Aug. 18, 1894 (Chapter 301, Section 4, 28 Stat. 422),
initially made available up to 1 million acres of federal land in each state provided that the
state met several requirements for the eventual development of water resources for
reclamation. Some observers have suggested that the failure of the Carey Act to foster
irrigation projects in all the land made available, compounded in part by the 1890-1896
drought, led to the Reclamation Act of 1902 and the emergence of the Bureau ofth
Reclamation in the 20 century. (See Marc Reisner, Cadillac Desert, (New York, New
York, Penguin Books, 1986)).
47 Fye, F., D.W. Stahle, and E.R. Cook, “Paleoclimate analogs to twentieth century moisture
regimes across the United States,” Bulletin of the American Meteorological Society, vol. 84,
48 For example, one report showed that 41% of the area studied in the American West was
affected by drought during the years 900 to 1300, versus 30% between 1900 and 2003, a
49 Richard Seager, et al., “Model projections of an imminent transition to a more arid climate
in southwestern North America,” Science, vol. 316 (May 25, 2007): pp. 1181-1184.
50 Tim P. Barnett, et al., “Human-induced changes in the hydrology of the western United
States,” Science, vol. 319 (Feb. 22, 2008), pp. 1080-1082.
51 Cook et al., 2007.
A likely consequence of higher temperatures in the West would be higher
evapotranspiration, reduced precipitation, and decreased spring runoff. A recent,
controversial study asserts that water storage in Lake Mead on the Colorado River
has a 50% probability by 2021 to “run dry” and a 10% chance by 2014 to drop below
levels needed to provide hydroelectric power under current climate conditions and
without changes to water allocation in the basin.52 This study raised awareness of the
vulnerability of western water systems, but invoked criticism that global climate
models are insufficient to forecast climate change effects at the regional scale.53
Some western water officials were especially critical of the report’s assertions. One
noted that Reclamation and other agencies recently developed new criteria for the
allocation of Colorado River water in times of shortages (shortage criteria), including
drought, leaving the likelihood Lake Mead would run dry “absurd.”54 The study was
based on predictions of future warming in the West without increased precipitation.
If reduced runoff predictions bear out, then water allocation policies for regions like
the Colorado River basin may need to be revisited.55
Severe drought can exacerbate water competition in nearly all regions of the
United States at some time; in other words, no area of the country is immune to
drought. However, several key factors have made it difficult to address drought
policy in a systematic fashion at the national level. Some key policy challenges
!the “creeping” nature of drought;
!split federal and non-federal drought response and management
!a patchwork of federal programs and program oversight, and little
coordination at the federal level.
Drought conditions often develop slowly and are not easily identified.
Consequently, drought declarations are made well after the onset of drought
conditions — typically once impacts are felt. This situation makes it difficult to
mitigate or prevent drought impacts. Further, even though drought is taken to be
inevitable, the unpredictability of its timing, location, and severity makes it difficult
to address systematically. When severe meteorological drought affects a region, such
52 Tim P. Barnett and David W. Pierce, “When will Lake Mead go dry?” Water Resources
Research, vol. 44 (March 29, 2008), p. W03201, DOI:10.1029/2007WR006704.
53 Felicity Barringer, “Lake Mead could be within a few years of going dry, study finds,”
New York Times (Feb. 13, 2008); at [http://www.nytimes.com/2008/02/13/us/13mead.html].
54 Jenny Dennis, “Stunned Scientists: ‘When Will Lake Mead go Dry?’” Rim Country
Gazette (February 28, 2008), quoting Larry Dozier, Central Arizona Project deputy general
manager, at [http://ahacreativeink.com/newspapers/featurepage02-28-08GAZETTE.pdf].
55 CRS has not determined to what degree scenarios considered in the Barnett/Pierce study
overlapped with those considered in studies supporting the new shortage criteria for
Colorado River water allocations under the Colorado River Compact.
as the Southeast and Southwest droughts, or the 2001 Klamath River Basin drought,
the supply of available water often shrinks before demand is reduced.
The resulting mismatch between supply and demand underscores the
responsibility of local, state, and federal authorities, as well as the private sector, to
anticipate the influence of drought and plan accordingly. The federal government has
several drought monitoring and response programs, the latter of which are primarily
aimed at easing the economic impacts of drought. Drought planning and mitigation
responsibilities lie largely at the non-federal level (i.e., at the state and local levels),
but the federal government also provides some drought planning assistance.
Additionally, the federal government often provides emergency funding for drought
relief. The National Drought Commission (discussed below) and others have noted
federal relief programs and emergency funding provide little incentive for state and
local planning and drought mitigation.
A further challenge is that there is no cohesive national drought policy at the
federal level, nor is there a lead agency that coordinates federal programs. Rather,
several federal programs have been developed over the years, often in response to
specific droughts. Additionally, occasional widespread economic effects have
prompted creation of several federal relief programs, These programs in turn are
overseen by several different congressional committees.
Congress has long recognized the lack of coordinated drought planning and
mitigation activities among federal agencies and the predominance of a crisis
management approach to dealing with drought. Over the last decade, legislative
action has focused on the question of whether there is a need for a national drought
policy. For example, in 1998, Congress passed the National Drought Policy Act
(P.L. 105-199), which created a National Drought Policy Commission. Congress
also considered, but did not enact, legislation creating a National Drought Council
during deliberations on the 2008 farm bill. Congress has considered
recommendations from the commission’s 2000 report; to date, it has enacted one part
of the recommendations. Both the commission findings and the proposed council are
The National Drought Policy Act of 1998. In passing the National
Drought Policy Act of 1998, Congress found that “at the Federal level, even though
historically there have been frequent, significant droughts of national consequences,
drought is addressed mainly through special legislation and ad hoc action rather than
through a systematic and permanent process as occurs with other natural disasters.”56
Further, Congress found an increasing need at the federal level to emphasize
preparedness, mitigation, and risk management. Those findings are consistent with
a recognition of the inevitability, albeit unpredictability, of severe drought occurring.
The act created the National Drought Policy Commission, and required the
commission to conduct a study and submit a report to Congress on:
56 The National Drought Policy Act of 1998, P.L. 105-199 (42 U.S.C. 5121 note).
!what is needed to respond to drought emergencies;
!what federal laws and programs address drought;
!what are pertinent state, tribal, and local laws; and
!how various needs, laws, and programs can be better integrated
while recognizing the primacy of States to control water through
In May 2000 the commission submitted its report,57 which included 29 specific
recommendations to achieve the goals of national drought policy, including the
establishment of a National Drought Council. (The Appendix of this report lists the
five goals in the commission’s report.) Most of the specific recommendations were
targeted at the President and federal agencies, coupled with calls for Congress to fund
drought-related activities supporting the recommendations. An overarching
recommendation was for Congress to pass a National Drought Preparedness Act to
implement the commission’s recommendations. As background for its
recommendations, the commission noted the patchwork nature of drought programs,
and that despite a major federal role in responding to drought, no single federal
agency leads or coordinates drought programs — instead, the federal role is more of58
National Drought Preparedness Legislation and the 2008 Farm Bill.
National Drought Preparedness Act bills were introduced in 2002 (107th Congress),thth
2003 (108 Congress), and 2005 (109 Congress), but were not enacted. Similar
stand-alone legislation has not been introduced in the 110th Congress; however, the
House-passed version of H.R. 2419, the Farm, Nutrition, and Bioenergy Act of 2008
(also known as the 2008 farm bill), contained a section creating a National Drought
Council. This section of the farm bill would have charged the council with creating
a national drought policy action plan that would incorporate many of the components
recommended in the commission’s report; however, it was not included in the
conference agreement. The Senate version of H.R. 2419 contained no similar
section, although the Senate bill authorized permanent disaster payments in hopes of
precluding the need for ad hoc disaster payments. The conference agreement on the
2008 farm bill (P.L. 110-246, enacted June 18, 2008) included a new $3.8 billion
trust fund to cover the cost of making agricultural disaster assistance available on an
ongoing basis over the next four years.
National Integrated Drought Information System. Although Congress
has not enacted comprehensive national drought preparedness legislation, it acted on
the second of five commission goals by passing the National Integrated Drought
Information System (NIDIS) Act of 2006 (P.L. 109-430). That goal called for
enhanced observation networks, monitoring, prediction, and information delivery of
drought information. P.L. 109-430 established NIDIS within the National Oceanic
57 The report is available via the National Drought Mitigation Center, at
[http://www.ndmc.unl.edu/ pubs/pfd21main.html ].
58 See full report, p. 1, at [http://www.ndmc.unl.edu/pubs/pfd21main.html].
and Atmospheric Administration (NOAA) to improve drought monitoring and
While many water allocation and other water management responsibilities
largely lie at the state or local level, localities and individuals often look to the
federal government for relief when disasters occur. Over time, Congress has created
various drought programs, often in response to specific droughts and authored by
different committees. Crafting a systematic or broad drought policy that might
encompass the jurisdiction of many different congressional committees is often
difficult. This is similar to the situation for flood policy, and water policy in general,
at the national level. The National Drought Policy Commission recognized these
patterns and that recognition underlies many of the commission’s60
recommendations. The current fragmented approach can be costly to national
taxpayers; however, it is not certain that increased federal investment in drought
preparation, mitigation, and improved coordination would produce more
economically efficient outcomes.
The overall costs to the federal government as a result of extreme drought, apart
from relief to the agricultural sector, are more difficult to assess. As discussed above,
the operation of the nation’s complex federal water infrastructure is affected by
drought. Thus, Congress may wish to examine how the two major federal water
management agencies, the Corps and Reclamation, plan for and respond to severe
drought and account for its impacts. For example, Congress may wish to explore
how a national drought policy may or may not address the complex factors that have
led to the current stalemate over a tri-state water allocation agreement in the ACF
basin. How a national drought policy would apply to and potentially assist in
alleviating conflicts over water use in other complex river basins managed by
Reclamation and the Corps, such as the Colorado River, Klamath River, Missouri
River, Sacramento and San Joaquin Rivers, is also at issue.
In its report accompanying the NIDIS Act of 2006, the House Committee on
Science wrote: “Experts in drought mitigation argue that substantial losses from
drought are not inevitable. With adequate forecasting and monitoring capabilities,
government and business can adjust their activities and substantially mitigate the61
extent and severity of many impacts of drought.” The National Drought Policy
Commission identified forecasting and monitoring activities as one important aspect
of the nation’s overall drought management policy, as well as numerous other facets
of federal drought management. Congress may choose to revisit the commission’s
recommendations and reevaluate whether current federal practices could be
supplemented with actions to coordinate, prepare for, and respond to the
unpredictable but inevitable occurrence of drought. Given the daunting task of
59 More information about NIDIS is available at [http://www.drought.gov].
60 Infra, note 52.
61 U.S. House, Committee on Science, National Integrated Drought Information System Act
of 2006, H.Rept. 109-503 (June 15, 2006), p. 3.
managing drought Congress may also wish to consider proposals to manage drought
impacts, such as assisting localities with water supply augmentation via water
conservation and reuse projects, and assisting with development of demand
management techniques such as facilitating water transfers, water markets, and
variable water pricing.
The following is an excerpt from the 2000 National Drought Policy Commission
Report: Preparing for Drought in the 21st Century — A Report of the National
Drought Policy Commission.
!Favor preparedness over insurance, insurance over relief, and
incentives over regulation.
!Set research priorities based on the potential of the research results
to reduce drought impacts.
!Coordinate the delivery of federal services through cooperation and
collaboration with nonfederal entities.
Goal 1. Incorporate planning, implementation of plans and proactive mitigation
measures, risk management, resource stewardship, environmental considerations, and
public education as the key elements of effective national drought policy.
Goal 2. Improve collaboration among scientists and managers to enhance the
effectiveness of observation networks, monitoring, prediction, information delivery,
and applied research and to foster public understanding of and preparedness for
Goal 3. Develop and incorporate comprehensive insurance and financial strategies
into drought preparedness plans.
Goal 4. Maintain a safety net of emergency relief that emphasizes sound stewardship
of natural resources and self-help.
Goal 5. Coordinate drought programs and response effectively, efficiently, and in a