Drought in the United States: Causes and Current Understanding

Summary

Drought is a natural hazard, often with significant societal, economic, and environmental consequences. Public policy issues related to drought range from how to identify and measure drought to how best to prepare for, mitigate, and respond to drought impacts, and who should bear associated costs. Severe droughts in 2011 and 2012 in Texas and the midcontinent region, the current drought in California and parts of other western states, and 15 years of dry conditions in the Southwest have fueled congressional interest in drought and its near-term effects on water supplies and agriculture, as well as in long-term issues, such as drought forecasting and possible links between drought and human-induced climate change.

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 drought conditions since 2000, the average amount of land area across the United States affected by at least moderate-intensity drought has been 27%.

What is drought? Drought has a number of definitions; the simplest may be a deficiency of precipitation over an extended time period, usually a season or more, compared with average conditions. Higher demand for water for human activities and vegetation in areas of limited water supply, and warmer conditions, increase the severity of drought. For example, drought during the growing season may be considered more severe—in terms of its impacts—than similar conditions when cropland lies fallow.

Some scientists refer to severe drought as a recurring natural disaster in North America. Reconstructions of drought conditions that extend back over 1,000 years—based on observations, historical and instrumental records, and tree rings—illustrate that portions of the conterminous United States have experienced periods of severe and long-lasting drought termed megadroughts. For example, drought reconstructions from tree rings document that severe multi-decadal drought occurred in the American Southwest during the 13^th century. These megadroughts have affected flows in major western rivers. For example, during the years 1130-1154, estimated Colorado River flows were less than 84% of normal. Recent data suggest that Colorado River flows since 2000 are approaching those previous lows—flows have been below average for 13 of the 15 years between 2000 and 2014.

One question is whether the parts of the United States, in particular the American Southwest, including California, may be experiencing the beginning of a modern-era megadrought. Because such megadroughts occurred previously, there is the possibility of a return to the long-term drought conditions experienced in previous centuries. Further, some postulate that droughts could be exacerbated if the future climate is warmer due to the buildup of greenhouse gases in the atmosphere. Predicting the severity and persistence of severe drought over a specific region of the country, however, is not yet possible more than a few months in advance because of the many factors that influence drought (e.g., precipitation, heat, soil moisture). The prospect of extended droughts and more arid baseline conditions—possibly exacerbated by greenhouse gas-driven warmer temperatures—in parts of the United States may challenge existing public policy responses for preparing for and responding to drought.

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. It also discusses how droughts are classified and what is meant by moderate, severe, and extreme drought classifications. 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 description is followed by a discussion of the nature and extent of recent droughts that affected Texas and the U.S. midcontinent and the current drought in California. Last, the report discusses prospects for a climate in the western United States that might be drier than the average 20^th-century climate and the possible influence of human-induced climate change.

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, among other policy choices. A discussion of these issues is beyond the scope of this report.¹

Drought has affected portions of North America throughout history. 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.² 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,³ resulting in widespread economic disruption and displacement of populations from the U.S. heartland—many relocating to California's Central Valley—and revealing shortcomings in agricultural and land-use practices.

(January 4, 2000, through May 19, 2015)

Paleoclimate reconstructions using precipitation proxies like tree rings have enabled researchers to estimate and plot the history of flows for important western rivers such as the Colorado River in the Rockies, the Sacramento and San Joaquin Rivers draining the Sierra Nevada Mountains in California, and the Klamath River draining southern Oregon and northern California.³⁶ The Colorado River basin has experienced generally lower-than-normal flows for the past 15 years (based on roughly 100 years of observed flow records), affecting lake levels in Lake Mead and Lake Powell. Comparing previous low-flow periods compiled from other sources may indicate what could be in store for the Colorado River, Lake Powell, and Lake Mead. For example, a 2007 study showed that for the years 1130-1154, estimated Colorado River flows were less than 84% of normal (with normal defined as the mean annual flow between 1906 and 2004, about 15 million acre-feet, or MAF, measured at Lee's Ferry, Arizona).³⁷ Prior to the current dry period, the lowest 25-year mean of observed flows occurred between 1953 and 1977, but Colorado River flows were 87% of normal, still higher than the 1130-1154 period.³⁸ In addition, the 25-year period of exceptionally low flow in the mid-1100s occurred within a generally dry 62-year period, 1118-1179, that was characterized by a series of multiyear low-flow pulses and the absence of years with flow much above 15 MAF.

Whether the current 15-year dry spell in the Colorado basin is creating low-flow levels on the Colorado River that are similar to the low-flow levels experienced in the mid-1100s—and how likely it is that the dry period could extend for years or even decades more—remains an outstanding question. Flow data seem to suggest that Colorado River flows since 2000 are approaching those historic low levels. According to Bureau of Reclamation data, during 12 of the 15 years between 2000 and 2014, the Colorado River flows at Lee's Ferry were below the 1906-2014 average, and the average amount of flow calculated for the 15-year period was about 12.3 MAF, or 83% of the 1906-2014 average.³⁹ The most recent flow data for 2013 and 2014 are provisional; however, if the numbers do not change appreciably, then Colorado River flows for the past 15-year period—83% of the 1906-2014 average—would be lower than for the 1130-1154 period. That outcome would suggest that dry conditions since 2000 may have rivaled the extreme dry periods during medieval times.⁴⁰

For the Sacramento and San Joaquin Rivers, both of which are critical to California's water supply and the agricultural production in the Central Valley, paleoclimate reconstructions have shown that low flows in the 1920s and 1930s rank among the most extreme in the context of the last millennium.⁴¹ However, 1580 was the driest single year. In 1580, flows on the Sacramento River were only 45% of the Sacramento River flow in 1924, and flows on the San Joaquin River were only 54% of the San Joaquin River flows for 1924, the second-driest single year of the entire reconstructed period. Thus, it appears that despite the severity of drought in the 1920s and 1930s, and more recent California droughts in the late 1980s to early 1990s and in 1975 to 1977, flows on both rivers were lower due to drought during the 1500s. An outstanding question is whether the current four-year California drought is affecting flows on the Sacramento and San Joaquin Rivers to a similar extent as did the 1920s-1930s drought or even the exceptionally dry year in 1580.

Over the past five years, portions of the country have been gripped with extensive and extreme to exceptional drought. As noted above, drought conditions nearly always occur in some part of the United States; however, the intensity of the Texas drought in 2011, the widespread nature of the midcontinent drought in 2012-2013, and the return of drought to California in 2012-2015 have focused national and congressional attention on those regions. The national importance of agriculture in those areas, combined with drought conditions (i.e., lack of precipitation, dry soil conditions, and farming and ranching needs), illustrates the nature of drought: namely, it is the combination of lack of precipitation and acute demand for water that poses challenges for water managers. The following section briefly describes these specific regional droughts.

Texas experienced varying levels of drought from 2011 to late spring 2015, when record amounts of rain fell in parts of the state, essentially ending the multiyear drought.⁴² However, the drought in Texas was most extensive and most severe in 2011. For example, mid-February 2011 conditions in Texas were dramatically different compared with mid-February 2010, when only about 7% of the total land area in Texas was abnormally dry and no part of the state was experiencing even moderate drought.⁴³

According to Texas state climatologist John Nielsen-Gammon, 2011 may have been the worst one-year drought on record for Texas.⁴⁶ Compounding the effects of abnormally low precipitation, the June-August average temperature in Texas was approximately 2.5 degrees Fahrenheit higher than during any Texas summer since record keeping began in 1895 and 5 degrees Fahrenheit higher than the long-term average.⁴⁷

Note: As shown in the table to the right of the map of Texas, 100% of the land area of the state was in drought and 97% of the state was in extreme to exceptional drought.

Although seasonal forecasts did not predict the summer 2012 drought in the Great Plains, a 2014 retrospective report stated that some modeling results indicated a regime shift toward warmer and drier western Great Plains and Southwest conditions during the 10 years to 15 years prior to the 2012 drought.⁵⁵ The report noted that the shift likely is due to natural decadal variability, but its existence would have increased the probability of a severe summer Great Plains drought, such as the 2012 event. The report concluded that the 2012 Great Plains drought resulted mostly from natural variations in weather.

As of October 1, 2014—the beginning of the 2015 water year⁵⁶—nearly 60% of California was experiencing exceptional drought, the most severe U.S. federal drought classification. The dry conditions resulted, in part, because the 2014 water year was the third driest on record in terms of precipitation (2013 was the driest calendar year on record, whereas the 1924 water year was the driest on record).⁵⁷ California typically receives more than 80% of its annual precipitation in the November-to-April winter precipitation season, and the season between November 2013 and April 2014 for the state as a whole was the sixth driest since records began in 1895.⁵⁸

Water users that receive supplies from California state and federal water projects are experiencing unprecedented water supply shortages due to the drought. Severe water supply shortages also hampered the state during a recent three-year drought (2007-2009). Other severe droughts in California over the last 100 years include a six-year drought (1987-1992); a two-year drought (1976-1977); and an extended dry period during the 1920s and 1930s that included the single driest water year on record—1924. (The driest winter on record was 1976-1977.) Studies of relict tree stumps, tree rings, and other evidence indicate that parts of California have experienced numerous multiyear droughts, some of which lasted for decades or even centuries during the past 2,000 years.⁶⁰ Whether the current drought is the worst in California's history currently is being debated. The role of human-induced climate change also is in debate, in particular whether the record high temperatures in California in 2014, combined with low precipitation, indicate a fingerprint of human influence on climate.

Although California experienced the driest calendar year on record in 2013, the severity of drought impacts is not solely related to the lack of precipitation and the duration of dryness. For example, drought risks to U.S. citizens generally increase in proportion to the population affected by drought, as well as in proportion to the resources and economic activity dependent upon average water supply and soil conditions. This principle is illustrated by the current situation in California, where more than 22 million people rely on water delivered from the Sacramento and San Joaquin Rivers' Delta confluence with San Francisco Bay (Bay-Delta)—also the home and nursery for several species of federal- and state-listed threatened and endangered species. Farms in the state's Central Valley also rely on water coming from the Bay-Delta, as well as runoff from the Sierra Mountains. Because the state is in its fourth year of drought, and because it is the top U.S. agricultural producer in terms of cash receipts, drought impacts in California could be felt nationwide.⁶¹

The 2007-2009 California drought⁶² was complicated by decades of tension over water supply deliveries for irrigation and municipal and industrial uses and over 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's history since measurements began.⁶³ Hydrological conditions were classified as below normal in 2010, but they were classified as wet (well above average) in 2011 for California, and the drought was declared over in spring 2011.⁶⁴ However, the 2012 water year was classified as below normal for the Sacramento River basin and dry for the San Joaquin River basin,⁶⁵ and by August 2012 the U.S. drought monitor again showed increasing severity of drought in the eastern portion of the state. Although above-average reservoir storage at the end of 2011 somewhat mitigated reductions to water users, water deliveries to state and federal water project contractors were restricted again in 2012, as well as in 2013.⁶⁶

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,⁶⁷ as well as to comply with state water-quality standards in the Bay-Delta. These factors are still in play in 2015 and are challenging water managers charged with protecting certain species from harm while also supplying water for farms, cities, and fish and wildlife.

The relationship between climate change and future trends in droughts is complex, and the scientific understanding of this relationship 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 Niño-Southern Oscillation (ENSO)⁶⁸ events and global surface warming.⁶⁹ The 2007 IPCC report added that very wet areas declined by about 5% globally. The report asserted that documented trends in severe droughts and heavy rains showed that hydrological conditions were becoming more intense in some regions.⁷⁰

In 2014, the IPCC released its most recent climate assessment, which stated that for North America decreases in snowpack already are influencing seasonal stream flows. However, the report had medium-to-high confidence that recent droughts (and floods, and changes in mean streamflow conditions) cannot yet be attributed to climate change.⁷⁷ Further, the report stated that it is not yet possible to attribute changes in drought frequency in North America to anthropogenic climate change.⁷⁸ The report noted that changes in these events, however, may be indicative of future conditions.

Predicting the intensity and duration of severe drought over a specific region 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 under way.⁸³

Likely consequences of higher temperatures in the West include higher evapotranspiration, reduced precipitation, and decreased spring runoff.⁸⁴ 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.⁸⁵ A possible consequence is more frequent, and perhaps more severe, droughts and floods, although these changes are likely to occur unevenly across the United States. 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.⁸⁶

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 typically are based on past long-term hydrological trends; significant deviations from such trends may result in difficult challenges for water managers and water users alike.⁸⁷

Conversely, it is also difficult to predict when droughts will end, and in some cases significant regional droughts can end relatively abruptly with the occurrence of major precipitation events. The May 2015 storms that dropped record amounts of precipitation on Texas and Oklahoma essentially ended the Texas drought, for example. In California, high precipitation events known as atmospheric rivers (ARs) increasingly are considered critical to ending droughts in the state. These weather features, sometimes referred to as drought busters, are extremely important contributors to California precipitation during the winter months. The immediate cause of the current California drought appears to be a region of persistent atmospheric high pressure over the northeast Pacific Ocean offshore Oregon and Washington. The resilient ridge, as it is sometimes referred to, has changed the atmospheric circulation patterns so the ARs are blocked from reaching the northern California coastline. How the location, timing, and intensity of major precipitation events, such as ARs, may affect droughts in future climate scenarios is not well understood.

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. Nonetheless, available technology and scientific understanding remain limited to forecasting any particular 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 and within regions such as the Southwest—possibly exacerbated by human-induced increases in atmospheric temperatures—represents a challenge to existing public policy responses for preparing and responding to drought.

Footnotes

For a discussion of various policy issues regarding drought, see, for example, CRS Report IF10196, Drought Policy, Response, and Preparedness, by [author name scrubbed] and [author name scrubbed]; CRS Report IF10133, California Drought: Water Supply and Conveyance Issues, by [author name scrubbed]; CRS Report R43408, Emergency Water Assistance During Drought: Federal Non-Agricultural Programs, by [author name scrubbed], [author name scrubbed], and [author name scrubbed], among others.

Edward R. Cook, et al., "North American drought: reconstructions, causes, and consequences," Earth-Science Reviews, vol. 81 (2007): pp. 93-134. Hereinafter, referred to as Cook et al., 2007.

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.

These are the categories used by the National Drought Mitigation Center (NDMC). The NDMC helps prepare the U.S. Drought Monitor and maintains its website.

NDMC data collected since 2000. U.S. Drought Monitor at the NDMC in Lincoln, NE. See http://droughtmonitor.unl.edu/MapsAndData/DataTables.aspx.

In some years or months, however, no part of the country was under extreme or exceptional drought. For example, from January 2000 through early April 2000, extreme or exceptional drought did not affect any portion of the country.

NDMC, "What Is Drought?" at http://www.drought.unl.edu/DroughtBasics/WhatisDrought.aspx.

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, ponds, and man-made reservoirs.

Permanently arid conditions reflect the climate of the region, which is the composite of the day-to-day weather over a longer period of time. Climatologists traditionally interpret climate as the 30-year average. See NDMC, "What is Climatology?" at http://www.drought.unl.edu/DroughtBasics/WhatisClimatology.aspx.

For more on fire, see CRS Report R43077, Wildfire Management: Federal Funding and Related Statistics, by [author name scrubbed] and [author name scrubbed].

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, http://drought.unl.edu/Planning/Monitoring/ComparisonofIndicesIntro/PDSI.aspx.

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 NDMC, http://droughtmonitor.unl.edu/AboutUs/ClassificationScheme.aspx.

Ibid.

National Weather Service Forecast Office, Observed Weather Reports, Reno, NV, at http://www.nws.noaa.gov/climate/index.php?wfo=rev.

National Weather Service Forecast Office, Observed Weather Reports, Sacramento, CA, at http://w2.weather.gov/climate/index.php?wfo=sto.

Also, while drought may affect one region at a given time, other regions may experience too much water, and possibly flooding, at the same time. Water-related disasters, drought and flood, may occur simultaneously in different regions of the country.

See NDMC, "Predicting Drought: High Pressure," at http://drought.unl.edu/DroughtBasics/PredictingDrought.aspx.

Ibid.

Ibid.

Cook et al., 2007.

Martin Hoerling and Arun Kumar, "The perfect ocean for drought," Science, vol. 299 (January 31, 2003), pp. 691-694. Hereinafter referred to as Hoerling and Kumar, 2003.

Celine Herweiger, 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 (January 31, 2006), pp. 159-171. Hereinafter referred to as Herweiger et al., 2006.

Cook et al., 2007.

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.

Richard Seager, Columbia University, quoted in Justin Gillis, "Science Linking Drought to Global Warming Remains Matter of Dispute," New York Times, February 16, 2014, p. A11.

Hoerling and Kumar, 2003.

Cook et al., 2007.

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.

Cook et al., 2007.

Herweiger et al., 2006.

Ibid.

The Carey Act, signed into law on August 18, 1894 (Chapter 301, §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 of Reclamation in the 20^th century. See Marc Reisner, Cadillac Desert (New York, New York, Penguin Books, 1986).

Under the Reclamation Act of 1902, the federal government constructed hundreds of dams, reservoirs, and related facilities to provide water to local farmers to "reclaim" the arid West through irrigation of arid lands.

Falko K. Fye, David W. Stahle, and Edward R. Cook, "Paleoclimate Analogs to Twentieth Century Moisture Regimes Across the United States," Bulletin of the American Meteorological Society, vol. 84 (2003), pp. 901-909.

For example, one report showed that 42% 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 29% reduction in the average area affected by drought between the two periods. See Cook et al., 2007.

See David M. Meko, et al., "Medieval Drought in the Upper Colorado River Basin," Geophysical Research Letters, vol. 34, no. 10 (May 24, 2007); and David M. Meko, Connie A. Woodhouse, and Ramzi Touchan, Klamath/San Joaquin/Sacramento Hydroclimate Reconstructions from Tree Rings, Draft Final Report to California Department of Water Resources, February 7, 2014, at http://www.water.ca.gov/waterconditions/docs/tree_ring_report_for_web.pdf.

Meko et al., 2007.

Ibid.

U.S. Bureau of Reclamation, Lower Colorado Region, "Colorado River Basin Natural Flow and Salt Data-Current Natural Flow Data 1906-2012," at http://www.usbr.gov/lc/region/g4000/NaturalFlow/current.html. Data from the spreadsheet provided by Reclamation on that website were extracted from the AnnualWYTotal Natural Flow worksheet, column U, per recommendation of Dr. David Meko, University of Arizona, Laboratory of Tree-Ring Research, personal communication, June 10, 2015. Flow data for water years 2013 and 2014 were provided by Dr. James Prairie, Hydrologic Engineer, U.S. Bureau of Reclamation, personal communication, June 11, 2015. Data for water years 2013 and 2014 are provisional flow data, because of an at least 1.5-year lag between the end of current year data and the calculation of natural flows available in the upper Colorado River basin for that year. Documentation for the natural flow calculation methods is available at http://www.usbr.gov/lc/region/g4000/NaturalFlow/NaturalFlowAndSaltComptMethodsNov05.pdf.

The 1130-1154 low-flow period lasted 25 years, so those flows and the current dry period and accompanying low flows for the 2000-2014 15-year period in the upper Colorado River are not directly comparable. For comparison, Colorado River flows at Lee's Ferry for the 25-year period between 1990 and 2014 were 90% of the 1906-2014 average, higher than the average flows during the 25-year period from 1130 to 1154, which averaged 84% of the 1906-2014 average.

Meko et al., 2014.

As of June 2, 2015, only about 10% of Texas was experiencing abnormally dry or moderate drought conditions. One month earlier, 40% of the state was experiencing abnormally dry or drought conditions. Oklahoma also faced drought conditions earlier in 2015; as of May 5, 2015, nearly 60% of the state was in at least moderate drought conditions. As with Texas, late spring heavy rains have nearly eliminated the drought in Oklahoma.

U.S. Drought Monitor, February 16, 2010. For comparison, the U.S. Drought Monitor on February 15, 2011, reported that over 87% of the state was experiencing abnormally dry or drought conditions, and only 12.5% of the state was not abnormally dry or in drought, http://droughtmonitor.unl.edu/MapsAndData/MapArchive.aspx.

See the U.S. Drought Monitor, Texas, on October 4, 2011, at http://droughtmonitor.unl.edu/MapsAndData/DataTables.aspxl.

John W. Nielsen-Gammon, The 2011 Texas Drought: A Briefing Packet for the Texas Legislature, October 31, 2011, p. 29, http://climatexas.tamu.edu/files/2011_drought.pdf. Possibly 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—longer than year-to-year changes—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).

For the contiguous United States, nearly 80% of the land area was affected by abnormally dry or drought conditions. U.S. Drought Monitor, "U.S. Drought Monitor Map Archive," accessed August 14, 2012, at http://droughtmonitor.unl.edu/MapsAndData/MapArchive.aspx.

U.S. Drought Monitor, Tabular Data Archive, http://droughtmonitor.unl.edu/MapsAndData/DataTables.aspx.

Martin Hoerling et al., An Interpretation of the Origins of the 2012 Central Great Plains Drought, National Oceanic and Atmospheric Administration, Assessment Report: NOAA Drought Task Force Narrative Team, March 20, 2013, at ftp://ftp.oar.noaa.gov/CPO/pdf/mapp/reports/2012-Drought-Interpretation-final.web-041113.pdf.

Ibid., p.1.

Ibid., p. 4.

Ibid., p. 10.

Ibid., p. 22.

M. Hoerling et al., "Causes and Predictability of the 2012 Great Plans Drought," Bulletin of the American Meteorological Society, vol. 95, no. 2 (February 2014), pp. 269-282.

For planning and water allocation purposes, water managers typically rely on "water-year" data. Each water year begins October 1 and ends September 30, thereby including the late-fall and winter months preceding winter/spring water allocation decisions. The media and others often use a calendar-year baseline, thus creating some confusion regarding "driest" years on record. For the purpose of this report, CRS relies on water-year data and water-year types defined by the Bureau of Reclamation and the California Department of Water Resources, not calendar-year data.

California Department of Water Resources, "Water Year 2014 Ends as 3^rd Driest in Precipitation," press release, September 30, 2014, at http://www.water.ca.gov/waterconditions/.

Richard Seager et al., Causes and Predictability of the 2011-2014 California Drought, National Oceanic and Atmospheric Administration, Assessment Report, December 2014, p. 3, at http://cpo.noaa.gov/sites/cpo/MAPP/Task%20Forces/DTF/californiadrought/california_drought_report.pdf. Hereinafter referred to as NOAA Assessment Report 2014.

U.S. Drought Monitor, June 2, 2015.

Rachel Berkowitz, "A Dry State in the West," Physics Today, July 2014, at http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.4004. See also Scott Stine, "Extreme and Persistent Drought in California and Patagonia During Mediaeval Time," Nature, vol. 369 (June 16, 1994), pp. 546-549.

California is the country's largest agricultural producer in terms of cash farm receipts—accounting for 14.7% ($21.2 billion) of the U.S. total agricultural exports in 2013 (the last year for which data are available http://www.cdfa.ca.gov/statistics/). California is also the largest producer of many crops.

For more information about the hydrology and policy issues involved in the 2007-2009 California drought, see CRS Report R40979, California Drought: Hydrological and Regulatory Water Supply Issues, by [author name scrubbed], [author name scrubbed], and [author name scrubbed].

California Department of Water Resources, California's Drought of 2007-2009—An Overview, September 2010, at http://www.water.ca.gov/waterconditions/drought/docs/DroughtReport2010.pdf.

Office of Governor Edmund G. Brown, Jr., "A Proclamation by the Governor of the State of California—Drought," at http://gov.ca.gov/news.php?id=16997.

See California Data Exchange Center, Department of Water Resources, "Chronological Reconstructed Sacramento and San Joaquin Valley Water Year Hydrologic Classification Indices," at http://cdec.water.ca.gov/cgi-progs/iodir/WSIHIST.

The Delta smelt is a species of fish listed as threatened under the federal Endangered Species Act and as endangered under the California Endangered Species Act. Natural Resources Defense Council v. Kempthorne, No. 1:05-cv-1207 OWW GSA (E.D. Cal., December 14, 2007).

A discussion of ENSO is provided in the text box entitled "El Niño-Southern Oscillation (ENSO)," above.

S. D. Solomon et al., "Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change," 2007, Cambridge University Press, Cambridge, United Kingdom and New York, NY.

Ibid., Summary for Policy Makers.

C. B. Field et al., IPCC, "Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation," 2012, Cambridge University Press, Cambridge, United Kingdom and New York, NY, p. 170.

The 2007 IPCC fourth assessment noted that "difficulties in the measurement of precipitation remain an area of concern in quantifying the extent to which global- and regional-scale precipitation has changed." S. D. Solomon et al., "Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change," 2007, Changes in Surface Climate: Precipitation, Drought and Surface Hydrology, chapter 3, section 3.3.1.

C.B. Field et al., IPCC, 2012, p. 170.

According to the report, confidence in the validity of a finding is based on the type, amount, quality, and consistency of evidence and on the degree of agreement. Confidence is expressed qualitatively: low, medium, high.

C.B. Field et al., IPCC, 2012, p. 172.

The IPCC report was referring primarily to hydrological drought and not to water demand, water infrastructure, and other factors that enhance the effects of hydrological drought.

P. Romero-Lankao et al., North America, Intergovernmental Panel on Climate Change, Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel On Climate Change, Cambridge, United Kingdom, and New York, New York, 2014, pp. 1443-1444, at http://www.ipcc.ch/pdf/assessment-report/ar5/wg2/WGIIAR5-Chap26_FINAL.pdf.

Ibid., p. 1456.

Daniel Griffin and Kevin J. Anchukaitis, "How Unusual is the 2012-2014 California Drought?," Geophysical Research Letters, published online December 3, 2014, DOI: 10.1002/2014GL062433.

For a description of the PDSI, see the National Drought Mitigation Center, at http://drought.unl.edu/Planning/Monitoring/ComparisonofIndicesIntro/PDSI.aspx.

Richard Seager et al., Causes and Predictability of the 2011-2014 California Drought, National Oceanic and Atmospheric Administration, Assessment Report by the NOAA Drought Task Force, December 2014, http://cpo.noaa.gov/sites/cpo/MAPP/Task%20Forces/DTF/californiadrought/california_drought_report.pdf.

California receives most of its precipitation during the winter and early spring months, November through April.

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.

Research results are emerging, however, that suggest that local and regional patterns of precipitation may be variable and that parts of a region or a state could receive higher precipitation than the current average, even if the overall trend over the broader area is towards less precipitation. See K. T. Redmond, "Climate Change in the Western United States: Projections and Observations," Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract U11D-02, 2009.

National Research Council, Committee on Hydrologic Science, Global Change and Extreme Hydrology: Testing Conventional Wisdom, Washington, D.C., 2011, p. 3.

Ibid., p. 9.

P.C.D. Milly et al., "Stationarity Is Dead: Whither Water Management?," Science, vol. 319 (February 4, 2008), p. 574.