Drought in the United States: Causes and Issues for Congress
Released: 2013-04-22T00:00:00

Introduction

This report discusses how drought is defined (e.g., why drought in one region of the country is different from drought in another region), and why drought occurs in the United States. How droughts are classified, and what is meant by moderate, severe, and extreme drought classifications, are also discussed. The report briefly describes periods of drought in the country's past that equaled or exceeded drought conditions experienced during the 20 th century. This is followed by a discussion of the future prospects for a climate in the western United States that might be drier than the average 20 th -century climate. The report concludes with a primer on policy challenges for Congress, such as the existing federal/nonfederal split in drought response and management and the patchwork of drought programs subject to oversight by multiple congressional committees.

Following are brief answers to frequently asked questions related to drought.

What is drought? Drought is commonly defined as a lack of precipitation over an extended period, usually a season or more, relative to some long-term average condition. History suggests that severe and extended droughts are inevitable and part of natural climate cycles. While forecast technology and science have improved, regional predictions remain limited to a few months in advance.

What causes drought? 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 severe western drought in the first decade of the 21 st century, severe droughts of the late 19 th century, and precolonial North American "megadroughts." The 2011 severe drought in Texas is thought to be linked to La Niña conditions (cooler-than-average SSTs) in the Pacific Ocean.

What is the future of drought in the United States ? The prospect of extended droughts and more arid baseline conditions in parts of the United States could suggest new challenges to federal programs and water projects, which were conceived or constructed largely on the basis of 20 th -century climate conditions. Some studies suggest a transitioning of the American West to a more arid climate, possibly resulting from the buildup of greenhouse gases in the atmosphere, raising concerns that the region may become more prone to extreme drought than it was in the 20 th century. Some models of future climate conditions also predict greater fluctuations in wet and dry years; however, the net effect of such fluctuations is difficult to predict.

What is federal drought policy? Although drought impacts can be significant, no comprehensive national drought policy exists. Developing a national policy would be challenging because of split federal and nonfederal responsibilities; the existing patchwork of federal programs; and differences in regional conditions, risks, and available responses. In 2000, the National Drought Policy Commission provided recommendations to Congress to improve drought policy. Congress has acted on some of the recommendations (e.g., authorizing the National Integrated Drought Information System), but not others (e.g., creation of a National Drought Council and a fund to support drought planning). Given current conditions, Congress may review the functioning and adequacy of existing federal responses and programs (e.g., access to and level of assistance provided, incentives for mitigation of drought risk, and preparedness of federal facilities).

Drought in the United States—Overview

The likelihood of extended periods of severe drought, similar to conditions experienced centuries ago, and its effects on 21 st -century society in the United States raise several issues for Congress. These issues include how to respond to recurrent drought incidents, how to prepare for future drought, and how to coordinate federal agency actions. For example, drought often results in agricultural losses, which can have local, regional, and national effects. It also can affect other industries and services, including power and energy resource production, navigation, recreation, municipal water supplies, and natural resources such as fisheries, aquatic species, and water quality. How to address these impacts is an often recurring issue for Congress. Addressing drought on an emergency basis is costly to individuals, communities, and businesses. Additionally, millions and sometimes billions of dollars in federal assistance can be expended in response to drought's social consequences. Thus, another recurrent policy issue is how to prepare and mitigate future drought impacts and how to do so efficiently across the many federal agencies with various and sometimes overlapping drought responsibilities.

Drought has afflicted portions of North America for thousands of years. Severe, long-lasting droughts may have been a factor in the disintegration of Pueblo society in the Southwest during the 13 th century, and in the demise of central and lower Mississippi Valley societies in the 14 th through 16 th centuries. In the 20 th century, droughts in the 1930s (Dust Bowl era) and 1950s were particularly severe and widespread. In 1934, 65% of the contiguous United States was affected by severe to extreme drought, resulting in widespread economic disruption and displacement of populations from the U.S. heartland.

Drought conditions are broadly grouped into five categories: (1) abnormally dry, (2) moderate, (3) severe, (4) extreme, and (5) exceptional. Some part of the country is almost always experiencing drought at some level. Since 2000, no less than 6.6% of the land area of the United States has experienced drought of at least moderate intensity each year. The land area affected by drought of at least moderate intensity varies by year and also within a particular year. For example, since 2000, the total U.S. land area affected by drought of at least moderate intensity has varied from as little as 6.6% (July 6, 2010) to as much as 55% (September 25, 2012). Based on weekly estimates of the areal extent of drought conditions since 2000, the average amount of land area across the United States affected by at least moderate-intensity drought has been 26%.

While the previous percentages refer to the extent of drought nationally, there is particular concern about those locations experiencing the most intense drought conditions. Nearly every year, extreme drought affects some portion of the country. Since 2000, extreme drought or drier conditions have affected approximately 6.4% of the nation on average. During August 2012, extreme drought extended over 20% of the country. Since 2000, exceptional drought conditions have affected approximately 1.4% of the nation on average. Of particular note were the conditions between June and October 2011; exceptional drought occurred over the largest land area—greater than 9%—during those months, with the affected areas concentrated in Texas. This year, 2013, is likely to be another exceptional year in terms of the breadth of drought conditions throughout the country, particularly in the Great Plains and eastern portions of the Midwest. The severe to exceptional drought conditions throughout the central and western parts of the United States appear to be persisting during spring 2013. (See Figure 1 .)

Drought in North America: Summer 2012 into 2013

In mid-August 2012, the extent of the drought conditions was significant: over 70% of the land area of the United States (including Alaska and Hawaii) was affected by abnormally dry and drought conditions. The percentage of land area affected by abnormally dry or drought conditions stayed at or above 65% through February 2013, and remains above 60% as of mid-April, 2013. The intensity of the drought varied across the country, but the regions of extreme and exceptional drought were clustered across the Midwest, Great Plains, Southwest, and in the Southeast, particularly Georgia in 2012. The widespread nature of drought and excessive heat conditions in the Midwest and Great Plains in the summer of 2012 contributed to sharply lower yields for major crops, including corn and soybeans. For example, the average U.S. corn yield was 123.4 bushels per acre, down more than 40 bushels from expectations before the onset of drought. For all crops, crop insurance indemnities for 2012 losses totaled $16 billion. In 2013, continuation of drought in the Great Plains and western parts of the Midwest is affecting prospects for wheat and other crops as well as pasture and forage conditions for livestock producers.

Figure 2 shows that Texas and portions of Florida and Georgia experienced exceptional drought conditions (the worst category of drought) in early 2012, while the upper Midwest, including most of the Mississippi Valley experienced normal conditions. A year later, in early 2013, the drought had eased somewhat in portions of Georgia and Florida, but intensified throughout the center of the country from Texas to the Canadian border. Nearly 12% of the contiguous United States was in exceptional drought conditions from late June 2011 through October 2011, compared to approximately 6% of the country the following year. However, exceptional drought conditions persisted over nearly 6% of the contiguous United States from mid-August 2012 through mid-February 2013. Although less severe for portions of the country, such as Texas and Florida, the 2012-2013 drought affected broader swaths of the agricultural heartland compared to 2011 ( Figure 2 ). The 2012-2013 experience illustrates that the extent, timing, and particular features of areas affected by drought—dryland versus irrigated farm regions, or regions that are still recovering from previous droughts—are important in addition to the relative severity of drought conditions.

Origin of the 2012-2013 Drought

Figure 2 shows a snapshot of drought conditions for March in North America for 2010-2013. In 2010 most of North America and the United States were experiencing near-normal conditions. The extent and severity of the 2012-2013 drought raised questions regarding its origin, and whether the drought was consistent with the range of natural variability in the U.S. Midwest and Plains, or whether it was linked to longer-term changes in the Earth's climate system, such as human-induced global warming.

Although the images presented in Figure 2 may seem to indicate a steady progression of drought in the middle portion of the country from near-normal conditions in 2010 to widespread and intense drought in 2012-2013, a March 2013 analysis concludes that the 2012 intense drought was a discrete extreme event. The report states that "the event did not appear to be just a progression or a continuation of the prior year's record drought event that developed in situ over the central U.S." Instead, the report asserted that the drought developed suddenly, with near normal precipitation during winter and spring 2012 over the Great Plains. The drought resulted from an extreme lack of precipitation during the summer months: 2012 was the driest summer in the historical record for the region, experiencing even less rainfall than the years 1934 and 1936, when the central Great Plains were about 0.5 o C warmer than 2012. Essentially the rains abruptly stopped in May over the central Great Plains, and did not return for the summer.

The report further stated that the 2012 summer drought was a "climate surprise," because summertime Great Plains rainfall has been trending upward since the early 20 th century, and the last major drought occurred 25 years ago in 1988. Further, the report concluded that neither sea surface temperatures, which have been rising generally due to global warming, nor changes in greenhouse gases in the atmosphere, were responsible for producing the anomalously dry conditions over the central Great Plains in 2012. (A brief discussion of climate change and drought is below.)

What Is Drought?

Drought has a number of definitions; the simplest 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: evapotranspiration). An imbalance could result from a decrease in precipitation, 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 an area's climate (e.g., deserts are regions of relatively permanent aridity).

Higher demand for water for human activities and vegetation in areas of limited water supply increases the severity of drought. For example, drought during the growing season would likely be considered more severe—in terms of its impacts—than similar conditions when cropland lies fallow. For policy purposes, drought often 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. These impacts can occur through multiple mechanisms: decreased rainfall and soil moisture affecting dryland farming; low reservoir levels decreasing allocations for multiple purposes (including irrigation, navigation, energy production, recreation, fish and wildlife needs, and other water supplies); low stream flows limiting withdrawals for multiple purposes, including municipal and industrial supplies, among others; decreased exchange of water in lakes resulting in water quality problems limiting recreation (e.g., blue-green algae restrictions in multiple lakes in Oklahoma and Texas during 2011 and 2012 drought conditions). At the national level, drought is monitored and reported by the National Drought Mitigation Center in an index known as the U.S. Drought Monitor, which synthesizes various drought indices and impacts, and represents a consensus view of ongoing drought conditions between academic and federal scientists. Drought can also relate and contribute to other phenomena, such as fires and heat waves.

Drought Is Relative

Drought and "normal" conditions can vary considerably from region to region. For example, the U.S. Drought Monitor shows that the southern tip of Texas, including Corpus Christi, faced extreme to exceptional drought in early April 2013 ( Figure 1 ). Similarly, the city of Albuquerque, New Mexico, was in extreme drought during the same time period. However, Corpus Christi receives on average a total of 5.19 inches of precipitation over the three-month period January through March. In contrast, Albuquerque receives on average 1.54 inches of precipitation over the same period. Both cities faced extreme drought in early April 2013, but what was normal for Corpus Christi was very different for what was normal for Albuquerque.

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, as described above for Corpus Christi and Albuquerque.

Drought Is Multifaceted

In the past, U.S. Drought Monitor maps used an "A" to indicate that the primary physical effects are agricultural (crops, pastures, and grasslands) and an "H" to indicate that the primary impacts of drought are hydrological (to water supplies such as rivers, groundwater, and reservoirs). When both effects are apparent, the letters are combined, appearing as "AH." In the newer versions of the maps, such as the one shown in Figure 1 , the "A" and "H" are replaced with an "S" and "L." These are experimental designations, according to the National Drought Mitigation Center, which produces the U.S. Drought Monitor maps. The "S" designation is intended to indicate a combination of drought indices that reflect impacts that respond to precipitation over several days up to a few months (short-term effects). These would include impact to agriculture, topsoil moisture, unregulated streamflows, and aspects of wildfire danger. The "L" designation approximates responses to precipitation over several months up to a few years (long-term effects). These would include reservoir levels, groundwater, and lake levels. Figure 1 shows that for early April the nation, predominantly the western half, was experiencing a combination of all three types, S, L, and SL.

The U.S. Drought Monitor maps also indicate the intensity of a drought, ranging from abnormally dry (shown as D0 on the maps) to exceptional drought (shown as D4). How these conditions are assessed and how drought is classified is discussed below.

Drought Classification

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 can lead to the flow of billions of dollars in relief to drought-stricken regions. The classification of drought intensity, such as that shown in Figure 1 for April 9, 2013, 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, 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. The U.S. Drought Monitor intensity scheme—D0 to D4—is used to depict broad-scale conditions but not necessarily drought circumstances at the local scale. For example, the large regions depicted as red in Figure 1 faced extreme to exceptional drought conditions for the week of April 9, 2013, but they may contain local areas and individual communities that experienced less (or more) severe drought.

Recent Examples: Texas, California, and Colorado River Basin

Drought in Texas, 2011-2013

In early April 2011, over 80% of Texas was experiencing severe to extreme drought, and nearly 10% of the state was in exceptional drought, the most severe level of drought intensity published by the National Drought Mitigation Center. The 2011 drought in Texas represented a dramatic shift compared to the same time period in 2010, when approximately 4% of the total land area in Texas was experiencing drought conditions, with no exceptional drought conditions anywhere in the state. (See Figure 3 , comparing 2010, 2011, 2012, and 2013.)

Drought conditions worsened in Texas through the beginning of October 2011, when 88% of the state experienced exceptional drought conditions (and only 3% of the state was not classified as extreme or exceptional drought). Drought conditions generally improved throughout the rest of 2011, but large portions of the state were still affected by extreme or exceptional drought until late winter and early spring of 2012, when the eastern portion of the state recovered to normal or abnormally dry conditions (the least severe category) because of above-normal rainfall from December 2011 through February 2012.

In April 2012 the drought in Texas had eased somewhat; slightly more than 50% of the state was experiencing severe drought or worse, down from 80% a year before. Conditions in April 2013 have worsened compared to a year earlier, however; approximately 70% of the state was experiencing at least severe drought in April 2013, and extreme and exceptional drought were 30% and 12%, respectively. (See Figure 3 .)

According to Texas state climatologist John Nielsen-Gammon, 2011 may have been the worst one-year drought on record for Texas. Compounding the effects of abnormally low precipitation, the June-August average temperature in Texas was approximately 2.5 degrees Fahrenheit greater than any previous Texas summer since 1895 and 5 degrees Fahrenheit (F) greater than the long-term average. The 2011 U.S. Drought Monitor showed that Texas had been experiencing both hydrological and agricultural drought, indicating that the drought had caused deficiencies in water supplies as well as deficiencies of water to crops, plants, and grasses.

The most severe Texas drought overall occurred from 1950 to 1957, and had substantial impacts on water supplies across the state because it lasted over many years. Because of the longevity and severity of the 1950s drought, municipal water supplies in Texas today are designed to withstand a drought of similar magnitude, according to the state climatologist. Long-term precipitation patterns in Texas are influenced by a configuration of sea surface temperatures known as the Pacific Decadal Oscillation (PDO). Similar conditions also prevailed from the 1940s through the 1960s, encompassing the Texas drought of record (1950-1957).

The 2007-2009 California Drought and Outlook for 2013

The 2007-2009 California drought was complicated by decades of tension over water supply deliveries for irrigation and M&I uses, and the preservation of water flows to protect threatened and endangered species. Dry conditions that began in 2007 continued through the 2009 water year (October 2008 through September 2009) and into the fall of 2009. According to the California Department of Water Resources, the 2007-2009 drought was the 12 th -driest three-year period in California history since measurements began. Although hydrological conditions were classified as below normal in 2010 and "wet" (well above average) in 2011, the 2012 water year was classified as "below normal" for the Sacramento River basin and "dry" for the San Joaquin River basin. Above-average reservoir storage at the end of 2011 mitigated reductions to water users. Although the drought was declared over in spring 2011, by August 2012, the U.S. drought monitor again showed increasing severity of drought in the eastern portion of the state. Water deliveries to state and federal water project contractors were restricted again in 2012.

California's dry conditions from 2007 through 2009 exacerbated an already tight water supply, where federal and state water deliveries had been reduced in response to a court order to prevent extinction of the Delta smelt. Similar factors are still in play today. Water deliveries from state and federal water projects for 2013 are restricted due to legal actions to protect threatened and endangered species, water quality requirements, and hydrological factors. Some federal agricultural water supply contractors are projected to receive just 20% of contract water supplies from the Bureau of Reclamation's Central Valley Project as of April 5, 2013, while some municipal and industrial contractors are scheduled to receive 70% of contract supplies. The April 9, 2013, drought monitor shows abnormally dry to severe drought conditions in almost every corner of the state. (For more information on project water deliveries, see CRS Report R40979, California Drought: Hydrological and Regulatory Water Supply Issues , by [author name scrubbed], [author name scrubbed], and Cynthia Brougher.)

Conditions in the Colorado River Basin

Spanning parts of Arizona, California, Colorado, New Mexico, Nevada, Utah, and Wyoming, the Colorado River basin is a critical water supply for the West and portions of northwestern Mexico. Based on inflows observed over the last century, the river is over-allocated, and some contend that supply and demand imbalances are likely to increase in the future. The March 2013 forecast is for flows below average for 2013, as has been the case for 10 of the last 13 years. The 2013 forecast for Lake Powell, the basin's largest reservoir, was for half of its normal inflow as the result of below average snowpack; that is, the inflow is anticipated at 5.3 million acre-feet. As called for in the water compact and interim guidelines that prescribe water sharing in the basin, the Bureau of Reclamation will most likely release from Lake Powell 8.2 million acre-feet from the reservoir for downstream purposes, thus resulting in a net drop in the lake level by the end of the water year. The dry conditions in 2013 are following the dry conditions of 2012. If forecasted conditions transpire, the basin will have storage levels in September 2013 near the historic lows of 2005. Reclamation predicts that Lake Powell storage levels will be 44% of capacity—10.78 million acre-feet—at that time.

Drought in part of the basin, particularly the upper basin, which is the source of most of the river's flow, exacerbates tensions over the sharing of the resource and results in difficult tradeoffs among the multiple uses of water (e.g., municipal, agricultural, hydropower, energy, recreation, and ecosystem and species demands). How water resources are allocated among these uses within a state is largely determined by state water law, compliance with federal and state laws (including interstate compacts, and environmental and resource management laws and regulations), and court decisions. In the case of the Colorado River, apportionment of water supplies among the seven basin states is done in accordance with the Colorado River Compact and a body of law known as the "Law of the River."

Low water availability in the Colorado River basin has effects beyond the basin boundaries. For example, Colorado River water is transported from Colorado's Western Slope to the state's Front Range; this water represents a significant contribution to the water available for agricultural and municipal uses in many eastern Colorado counties. Similarly, much of the lower basin allocation (4.4 million acre-feet) is diverted to southern California under the Colorado River Compact.

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 semi permanent 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 of Asia.

Prolonged droughts occur when these atmospheric conditions persist for months or years over a certain region that typically does not experience such conditions for a prolonged period.

Predicting drought 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 other weather system dynamics. Scientists seek to understand how all these variables interact and to further the ability to predict sustained and severe droughts beyond a season or two in advance, which is the limit of drought forecasting abilities today.

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. A number of studies have made the connection between cooler SSTs in the eastern Pacific and the 1998-2004 western drought, three widespread and persistent droughts of the late 19 th century, and past North American "megadroughts" that occurred between approximately 900 and 1300 A.D. 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.

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 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.

Prehistorical and Historical Droughts in the United States

Some scientists refer to severe drought as "the greatest recurring natural disaster to strike North America." 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 proxies in the absence of direct measurements. What these reconstructions illustrate is that the coterminous United States has 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 document that severe multidecadal drought occurred in the American Southwest during the 13 th century, which anthropologists and archeologists suspect profoundly affected Pueblo society. Tree ring drought reconstructions also document severe drought during the 14 th , 15 th , and 16 th centuries in the central and lower Mississippi Valley, possibly contributing to the disintegration of societies in that region.

More recently, a combination of tree ring reconstructions and other proxy data, historical accounts, and some early instrumental records identify three periods of severe drought in the 19 th century: 1856-1865 (the "Civil War drought"), 1870-1877, and 1890-1896. 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. The 1890-1896 drought coincided with a period in U.S. history of federal encouragement of large-scale efforts to irrigate the relatively arid western states under authority of the Carey Act . Congressional debate also occurred over a much larger federal role in western states irrigation, which led to the Reclamation Act of 1902.

In the 20 th 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. (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 19 th and 20 th century severe droughts occurred during a regime of relatively less arid conditions compared to the average aridity in the American West during the 900 to 1300 A.D. megadroughts. One study indicates that the drought record from 900 to 1300 A.D. shows similar variability—drought periods followed by wetter periods—compared to today, but the average climate conditions were much drier and led to more severe droughts.

Drought and Climate Change

The relationship between climate change and future trends in droughts is complex and its scientific understanding appears to be evolving. In 2007 the Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report, which stated that, globally, very dry areas have more than doubled since the 1970s due to a combination of El Nino-Southern Oscillation (ENSO) events and global surface warming. The IPCC report added that very wet areas declined by about 5% globally. The report asserted that documented trends in severe droughts and heavy rains show that hydrological conditions are becoming more intense in some regions.

In 2012, however, the IPCC issued a new report that noted: "There are still large uncertainties regarding observed global-scale trends in droughts." The new report noted that its earlier assessment, that very dry areas have more than doubled since the 1970s, was based largely on only one study, which relied on a measurement largely related to temperature, not moisture. A different study, which looked at soil moisture simulations, found that trends in drought duration, intensity, and severity predominantly were decreasing , not increasing, but with strong regional variation.

The 2012 IPCC report assigned medium confidence that there has been an overall slight tendency toward less dryness in North America (i.e., a wetting trend with increasing soil moisture and runoff). It noted that the most severe droughts in the 20 th century occurred in the 1930s and 1950s, where the 1930s drought was the most intense and the 1950s drought was the most persistent. In comparison to the severe megadroughts that occurred in North America hundreds and thousands of years ago, as documented using paleoclimate evidence (discussed elsewhere in this report), these recent droughts were not unprecedented, according to the 2012 IPCC report.

The report concluded that despite new studies that have furthered the understanding of mechanisms leading to drought, there is still limited evidence to attribute observed changes. The IPCC assessed that there was medium confidence that anthropogenic influence has contributed to changes in drought patterns in the second half of the 20 th century, but gave low confidence to the attribution of changes in drought patterns at the regional level. The report noted that some regions of the world have experienced trends towards more intense and longer droughts, such as southern Europe and West Africa. But in other regions, such as central North America and northwestern Australia, droughts have become less frequent, less intense, or shorter. How the 2011-2012 drought in the central United States may change that assessment in the forthcoming IPCC report (2014) remains to be seen.

Further adding to the complexity and challenge to the scientific understanding of what causes drought was work presented at the European Geosciences Union meeting in April 2013 that attempted to simulate megadroughts that occurred in the past. The simulations produced a number of megadroughts that lasted for decades; however, they did not match the timing of the past documented megadroughts. The scientists presenting their work at the meeting concluded that the model they used seemed to miss some of the dynamics that drive large droughts.

Responding to and Planning for Drought

Several recent droughts triggered federal responses. When a drought is declared for a locality or region by the U.S. President, U.S. Secretary of Agriculture, or a state governor, 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 drought could initiate a federal response and transfer of federal dollars to the affected area.

Before drought severity reaches a level triggering a federal response, many states take action. The National Drought Mitigation Center posts online copies of drought management, mitigation, or response plans for states and localities, nationwide. The California and Texas governors also have in recent years issued state drought emergency declarations triggering state drought assistance. Some states have also instituted water banks and water transfer mechanisms to deal with water supply shortages (e.g., California, Idaho, and Texas).

Federal Aid

When a state's resources are lacking, a state governor may request drought disaster assistance through the U.S. Secretary of Agriculture, who can declare an agricultural disaster due to drought and make available low-interest loans for qualified farmers and ranchers and other emergency assistance. However, under a new Farm Services Agency (FSA) streamlined process, any portion of a county experiencing severe drought according to the U.S. Drought Monitor for eight consecutive weeks can receive a "nearly" automatic USDA disaster declaration. Further, any county for which a portion is identified in the U.S. Drought Monitor as undergoing severe drought (or worse) may also be declared a disaster area.

As of August 13, 2012, more than 1,400 U.S. counties in 33 states had been designated as drought disaster areas by the Secretary. As of April 17, 2013, more than 1,180 counties had been designated as drought disaster areas for the 2013 crop loss purposes (primary and contiguous county designations). For more on emergency assistance, see the box "USDA Emergency Assistance: Status and Legislation."

In addition, 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 resulting in federal aid to affected parties. However, the last presidential drought or water shortage 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. The infrequency of presidential domestic drought declarations increases the uncertainty about the circumstances under which such a declaration is likely to be made. The de facto policy since the 1980s has been that the U.S. Secretary of Agriculture is the lead in responding and declaring drought and eligibility for drought assistance.

Although agricultural losses typically dominate drought impacts, congressional interest in federal drought assistance is not limited to the USDA. For example, the 2012 drought raised interest in whether and to what extent other federal agencies have and are using authorities to assist with managing drought. For example, in addition to operations of federal water resource facilities (discussed below), the U.S. Army Corps of Engineers and the U.S. Bureau of Reclamation also have limited emergency drought authorities and funding; the drought response and recovery authorities for the Corps, Reclamation, and the Farm Service Agency are provided in Appendix A . While the Corps and Reclamation authorities have experienced some use, the frequency, impact, and coordination of their use with other federal and state drought efforts have not been monitored and assessed. Most of the Corps drought response and recovery authorities are limited to providing limited and temporary water from its reservoirs for a fee. The exception is its authority to assist with well construction and transport of water to drought-distressed farmers, ranchers, and political subdivisions; the construction costs are a nonfederal responsibility, while the transport costs can be federally funded. Congress has provided Reclamation with a broader set of authorities that allow it to assist with drought response and recovery; these authorities, however, are limited to the 17 western states and Hawaii.

Other federal agencies are also reviewing drought response actions and committing resources or temporarily changing policies that may ease drought burdens. For example, the President in early August 2012 convened a White House Rural Council to review and assess federal agency activities and capabilities. Shortly following the gathering, the Administration announced new measures to address drought impact, as well as a listing of ongoing federal agency efforts to address the 2012 drought. Among the new efforts announced were waivers for federal trucking regulations and additional emergency funding for crop and livestock producers.

Federal Facilities and Drought

Operations of federal water resource facilities, particularly reservoirs behind dams, can both assist in meeting water supply needs during droughts and be vulnerable to droughts. Federal dams, particularly in the West, were constructed in part to provide multi-year storage to help with variations in seasonal and annual precipitation. Sustained hydrological drought nonetheless affects operations of federally managed reservoirs, dams, locks, hydroelectric facilities, and other components of the nation's water infrastructure. For example, numerous Corps reservoirs have drought management plans that result in the curtailing of some benefits (e.g., navigation, hydropower) in order to maintain other benefits (e.g., in-stream flows to support water quality, aquatic species, and river withdrawals for electric power cooling and municipal and industrial water supplies). The Corps' operations of its facilities in the Apalachicola-Chattahoochee-Flint River system during 2007-2008 illustrate management tradeoffs during drought (see box below).

Similarly, drought conditions in California from 2007 to 2009, coupled with declining fish species, 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 in the West and deliver a total of nearly 30 million acre-feet of water annually, faces operational challenges because of conflicts among its water users during drought in states it serves.

For example, severe drought conditions in 2001 in the Klamath River basin, on the Oregon-California border, exacerbated competition for scarce water resources 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 in-stream flows for three fish species listed as endangered or threatened under the Endangered Species Act sparked congressional debate that continues today. The Klamath basin again experienced drought conditions in 2010 and again in 2012. Project water flows to Klamath refuges were halted from December 2011 through March 2012. Dry conditions contributed to a cholera outbreak among migrating birds during this time, resulting in the death of thousands of birds that visit the refuges. Early spring precipitation improved hydrological conditions such that Reclamation projected full irrigation deliveries for 2012. However, low lake levels and inflows by April 1, 2013, have resulted in Reclamation postponing spring water deliveries by one to two weeks. Final water deliveries are also affected by flows necessary to protect federally listed Coho salmon and two sucker species. A new biological opinion on project operations through February 2014 is expected in April 2013, which may again affect water deliveries.

The droughts in California, the Southeast, and the Klamath River basin underscore an underlying difficulty of managing federal reservoirs to meet multipurpose water needs. In the future, the United States might face severe and sustained periods of drought not experienced in the 20 th century. If so, disputes over federal infrastructure management like those in California, the ACF basin, and Klamath River basin may increasingly determine short-term actions by Reclamation and the Corps, and result in long-term consequences for congressional oversight and funding.

Drought Forecasts for the United States

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. Some studies have suggested that human influences on climate, caused by emissions of greenhouse gases, may be responsible for a drying trend; however, other studies appear to indicate an opposite trend or possibility (see above section on " Drought and Climate Change "). Whether future greenhouse gas-driven warming can be linked to La Niña-like conditions, or other phenomena related to the El Niño-Southern Oscillation, is unclear.

A likely consequence of higher temperatures in the West would be higher evapotranspiration, reduced precipitation, and decreased spring runoff. These impacts would result from an "acceleration" of the hydrologic cycle, due to increased warming of the atmosphere, which in turn increases the amount of water held in the atmosphere. A possible consequence is more frequent, and perhaps more severe, droughts and floods. However, these changes are unlikely to occur evenly across the United States. Observations of water-related changes over the last century suggest that runoff and streamflow in the Colorado and Columbia River basins has been decreasing, along with the amount of ice in mountain glaciers in the West, and the amount of annual precipitation in the Southwest. Yet the understanding of hydrologic extremes, such as drought, is confounded by other effects such as land cover changes, the operation of dams, irrigation works, extraction of groundwater, and other engineered changes. Forecasting drought conditions at the regional scale, for example for river basins or smaller, is difficult because current climate models are less robust and have higher uncertainty at smaller scales. (For example, see box below on the Colorado River's Lake Mead.)

Even though forecasting drought at the regional scale is difficult, understanding potential changes in long-term trends is important for water managers at all levels—federal, state, local, and tribal. Water project operations and state water allocations are typically based on past long-term hydrological trends; significant deviations from such trends may result in difficult challenges for water managers and water users alike. An example of such a dilemma can be observed in the Colorado River basin.

Conditions in the Colorado River basin over the last decade, including recent low reservoir levels in Lake Mead and Lake Powell, and low flows in the Upper Basin, raise the issue of what is the baseline for average hydrologic cycles now and in the future. The allocation of Colorado River water supplies was agreed upon by lower and upper basin states in the early part of the 20 th century based on hydrologic data from what scientists now know was a relatively wet period in the history of the Colorado River basin. If long-term reduced runoff predictions for the basin are borne out (see box above on Colorado River's Lake Mead and earlier section on " Conditions in the Colorado River Basin "), then water allocation policies for regions like the Colorado River basin may again need to be revisited. In the meantime, Colorado River basin states have negotiated "shortage criteria" and "interim guidelines" for managing Colorado River water supplies during times of shortages. Additionally, Reclamation in December 2012 released a new Colorado River basin supply and demand study, which examined several different future water supply and demand scenarios.

Policy Challenges

Severe drought can cause significant economic harm, affect nearly all areas of the country, and exacerbate water competition. Nonetheless, several key factors make comprehensive drought policy at the national level a challenge, including:

the gradual, or "creeping," nature of drought; split federal and nonfederal drought response and management responsibilities; a patchwork of federal programs and oversight with little coordination; and differences in regional conditions and drought risk in terms of the drought hazard, vulnerability, and potential consequences.

Drought conditions often develop slowly, are not easily identified initially, and are challenging to forecast beyond 30 to 90 days. Consequently, drought declarations are made well after onset—typically once impacts are felt. This situation makes it difficult to mitigate or prevent drought impacts. Further, even though drought generally is continuously occurring somewhere in the United States, the unpredictability of its location, duration, and severity complicates preparation for implementation of responses.

When severe meteorological drought affects a region, the supply of available water often shrinks before use is reduced. Adjusting down the use of water as drought persists and supplies shrink can be difficult. Actually, droughts can increase demand on water supplies (e.g., lower soil moisture results in increased demand for irrigation and landscape watering). The lack of flexibility of existing water access and use arrangements sometimes limits the scope and speed of some drought responses. Federal, state, and local authorities make water resource decisions within the context of multiple and often conflicting laws and objectives, competing legal decisions, and entrenched institutional mechanisms, including century-old water rights and long-standing contractual obligations (i.e., long-term water delivery and power contracts). Typically, how access to and competition for water is managed (e.g., permitting of water withdrawals) and how reductions in water supply are managed (e.g., shared reductions under a riparian system of water rights versus reductions based on the priority in time of a water right) is determined by state law and at times through interstate compacts. Additionally, state and local laws can determine how easily water can be transferred among users. These access, reduction, and transfer arrangements can significantly affect the behavior, incentives, and opportunities available to water users during droughts. Fundamental changes to the access, reduction, and transfer arrangements are largely outside of the realm of federal action, and are largely determined by each state; however, such activities might be directly or indirectly influenced by changes in federal policies and activities.

A mismatch between supply and demand during droughts underscores the responsibility of stakeholders to anticipate the influence of drought and plan and act accordingly. The federal government has several drought monitoring and response programs. While drought planning and mitigation responsibilities lie largely at the state and local level, the federal government also provides some drought planning assistance. Much of this assistance historically has been concentrated on the West. Additionally, the federal government often provides emergency funding for drought relief that is primarily aimed at easing the economic impacts. The National Drought Commission and others have noted, however, that federal relief programs and emergency funding provide little incentive for state and local planning and efforts to reduce social and economic vulnerability to drought. A policy issue particularly relevant to state and local decision makers is the role and types of demand management tools to employ during a drought (e.g., lawn watering restrictions, incentives to curtail irrigation during droughts, scarcity pricing). How a state distributes and administers its waters among competing uses can affect what drought response tools are available to it and to water users.

A further challenge is lack of a cohesive national drought policy at the federal level, and lack of a lead agency coordinating 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 are overseen by different congressional committees. Whether this fragmentation results in duplication, waste, and gaps, or whether it reflects the complexity of preparing and responding to drought and the different responses needed by a wide range of stakeholders (e.g., irrigated agriculture, dry land farming, municipal water utilities) is part of the debate about how to proceed with cost-effective management of the nation's drought risk and who bears the consequences of drought. (See box below for an example of how water access and transfer arrangements played a significant role in shaping Australia's drought resilience and adaptation.)

The 2012 drought, like other recent severe weather events, has contributed to the ongoing public and policy discussion about the influence of human actions on climate and the future of U.S. actions on climate change mitigation. For a discussion of climate change science and policy, see CRS Report RL34266, Climate Change: Science Highlights , by [author name scrubbed]; and CRS Report R41973, Climate Change: Conceptual Approaches and Policy Tools , by [author name scrubbed], respectively.

Legislative Action

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 15 years, 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 the National Drought Policy Commission. In 2000, the commission submitted to Congress a comprehensive report that included policy recommendations. Congress has considered recommendations from the commission's 2000 report; to date, it has enacted one part of the recommendations (the National Integrated Drought Information System, discussed below). Congress also considered, but did not enact, legislation creating a National Drought Council during deliberations on the 2008 farm bill. Recent congressional deliberations focused on the 2012 reauthorization of the farm bill, which was given a one-year extension (through 2013) in January 2013, although other legislation is also pending. The commission findings, the proposed council, and the 2008 and 2013 farm bills are discussed below.

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." 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 it to conduct a study and report to Congress on

what is needed to respond to drought emergencies; what federal laws and programs address drought; what are the 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 state law.

In May 2000, the commission submitted its report. The commission concluded that the United States needed to embrace a national drought policy with preparedness at its core. It recommended that Congress enact a National Drought Preparedness Act, which would establish a federal-nonfederal partnership through a National Drought Council. The council would function to further national drought policy goals. The commission's report provided 29 specific recommendations to achieve the goals of national drought policy. Most of the specific recommendations were targeted at the President and federal agencies, coupled with calls for Congress to fund drought-related activities in support of the recommendations. ( Appendix B of this report lists the five goals and illustrative recommendations from the commission's report.) 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 of "crisis management."

National Drought Preparedness Legislation and the 2008 Farm Bill

National Drought Preparedness Act bills were introduced in 2002 (107 th Congress), 2003 (108 th Congress), and 2005 (109 th Congress), but were not enacted. Similar stand-alone legislation was introduced in the 110 th 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 2008 farm bill would have charged the council with creating a national drought policy action plan, which would have incorporated many of the components recommended in the commission's report; however, it was not included in the conference agreement. Although the Senate version of H.R. 2419 did not contain a similar section, 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 following four years. The assistance was available for disasters occurring on or before September 30, 2011. In January 2013, as part of a one-year extension of the 2008 farm bill, four of the five disaster programs were reauthorized through FY2013, but Congress provided no funding. With farm bill programs expiring in 2013, the 113th Congress is expected to consider the reauthorization of an omnibus farm bill, including agricultural disaster programs (see box on page 18 ).

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 and Atmospheric Administration (NOAA) to improve drought monitoring and forecasting abilities. The NIDIS authorization expired in 2012. A NIDIS reauthorization bill, S. 376 , has been introduced in the 113 th Congress.

Conclusion

Drought is a natural hazard with potentially significant economic, social, and ecological consequences. History suggests that severe and extended droughts are inevitable and part of natural climate cycles. Drought has for centuries shaped the societies of North America and will continue to do so into the future. Current understanding is that the physical conditions causing drought in the United States are linked to sea surface temperatures in the tropical Pacific Ocean. For example, the 2011 severe drought in Texas is thought to be linked to La Niña conditions in the Pacific Ocean. Increasingly, studies are projecting the long-term role that droughts may play in regional climate patterns. Nonetheless, available technology and science remains limited to forecasting specific drought a few months in advance for a region. The prospect of extended droughts and more arid baseline conditions in parts of the United States represents a challenge to existing public policy responses for preparing and responding to drought, and to federal water resource projects in particular, because their construction was based largely on 19 th - and 20 th -century hydrologic conditions.

Over time, Congress has created various drought programs, often in response to specific droughts and authored by different committees. Crafting a broad drought policy that might encompass the jurisdiction of many different congressional committees is often difficult. Additionally, although 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. 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 they underlie many of its recommendations to Congress. The currently fragmented approach can be costly to national taxpayers; however, it is not certain that increased federal investment (especially vis-à-vis the potential for tailored local and state investment) in drought preparation, mitigation, and improved coordination would produce more economically efficient outcomes.

The overall costs to the federal government and the nation as a result of extreme drought, apart from relief to the agricultural sector, are difficult to assess in part because of the broad nature of drought's impacts. Drought can result in water restrictions affecting municipal and industrial users, decreased hydropower generation and power plant cooling efficiency, navigation limitations and disruptions, harm to drought-sensitive species (benefits to other species), and increased fire risk, among other effects.

Congress may opt 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 managing drought, Congress also may consider proposals to manage drought impacts, such as assisting localities, industries, and agriculture with developing or augmenting water supplies. This could take multiple forms: construction or permitting of reservoirs, the reallocation of water supplies at existing facilities, promotion of alternative water sources (e.g., reuse, desalination), or water conservation and efficiency. Congress also may move 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.

Appendix A. Drought-Related Federal Response and Recovery Authorities

Appendix B. Excerpts from the 2000 National Drought Policy Commission Report to Congress

In 2000, the National Drought Policy Commission published its report to Congress, Preparing for Drought in the 21 st Century—A Report of the National Drought Policy Commission. The commission concluded that the United States needed to embrace a national drought policy with preparedness at its core. It recommended that Congress enact a National Drought Preparedness Act, which would establish a federal-nonfederal partnership through a National Drought Council. The council would function to further national drought policy goals. The council's recommended duties included promoting cooperation and effective delivery of drought programs, evaluation of regional needs and opportunities, and assessments of drought-related assistance and initiatives, and post-drought impact assessments.

The commission also recommended five broad national policy goals and a number of recommendations for furthering each of the goals. Below are the five goals and below those are some examples of the types of recommendations accompanying each of the goals; many more recommendations are proposed in the commission's report. To date, Congress has enacted legislation furthering Goal 2 on improved observation. Congress also has taken action on insurance and financial strategies; their effect on improving drought preparedness (i.e., reducing drought vulnerability) versus increasing the relief for the impacts of drought has not been evaluated. This type of assessment potentially would fall under the duties of a National Drought Council as recommended by the commission.

Policy Statement

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.

Goals

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 drought.

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 customer-oriented manner.

Illustrative Recommendations for Goal 1 : Planning and Mitigation

Congress should adequately fund existing drought preparedness programs. President should direct appropriate federal agencies to effectively meet the drought planning needs of those areas not traditionally served (e.g., eastern United States). The President should direct all appropriate federal agencies to study their programs for potential impacts on drought. The president should direct all appropriate federal agencies to develop and implement drought management plans for federal facilities (e.g., military bases, federal office complexes, federal prisons).