D rought can affect any region of the country, and can have severe and long-lasting impacts on agriculture, energy production, environment, health and safety, fishing, recreation, and many other facets of the economic and social well-being of communities. The likelihood of extended periods of severe drought 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. Although a discussion of these issues is beyond the scope of this report, understanding what drought is and its causes, how it has affected North America in the past, and how drought may affect the United States in the future all bear on actions Congress may take to prepare for and mitigate the effects of drought. This report discusses why drought occurs in the United States; how drought is defined (e.g., why drought in one region of the country is different from drought in another region); and how droughts are classified and reported (e.g., what is meant by moderate, severe, extreme, and exceptional drought). The report briefly describes periods of drought in the country's past that equaled or exceeded drought conditions experienced during the 20 th century, and the effects of those droughts on major western rivers such as the Colorado. Lastly, the report discusses the possible influence of human-induced climate change on drought, and current limits on our understanding for forecasting droughts. The Appendix contains case histories of the 2012-2016 California drought, and previous major droughts in Texas and across the Midwest. 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. 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—many relocating to California's Central Valley—and revealing shortcomings in agricultural and land-use practices. Today, the National Drought Mitigation Center (NDMC) monitors and reports on drought conditions across the nation. Drought conditions are broadly grouped into five categories: D0 (abnormally dry), D1 (moderate), D2 (severe), D3 (extreme), and D4 (exceptional). Some part of the country is almost always experiencing drought at some level. Since 2000, some portion of the land area of the United States has experienced drought of at least moderate intensity (D1) each year ( Figure 1 ). The land area affected by drought can vary widely by year and also within a particular year. For example, in May 2017, only 3.8% of the total U.S. land area was affected by drought of at least moderate intensity (D1). In contrast, in September 2012, 55% of the nation faced drought of at least moderate intensity, and 35% of the country was under severe drought (D2) conditions at that time. Based on weekly estimates of drought conditions since 2000, on average about 26% of the land area across the United States experiences at least moderate intensity in any given year (see the horizontal blue bar in the Moderate [D1] column of Figure 1 ). There is particular concern about locations experiencing the most intense drought conditions: extreme and exceptional drought. Nearly every year, extreme drought (D3) affects some portion of the country. Since 2000, extreme drought or drier conditions have affected approximately 6.5% of the nation on average ( Figure 1 ). Since 2000, exceptional drought (D4) conditions have affected approximately 1.4% of the nation on average. Of particular note were the conditions between June 2011 and October 2011: exceptional drought (D4) occurred over the largest land area—greater than 9%—during those months over the period starting January 2000 until present, with the affected areas concentrated in Texas. The following year, during August 2012, extreme and exceptional drought extended over 20% of the country and was concentrated in the central United States. 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. Conceptually, it may be easier to understand drought through its impacts. For example, when evaluating the impact to agriculture, drought could be defined as a protracted period of deficient precipitation resulting in extensive damage to crops, resulting in loss of yield. 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 likely would be considered more severe—in terms of its impacts—than similar conditions when cropland lies dormant. For policy purposes, drought often becomes an issue when it results in a water supply deficiency. During these deficiencies, less than the average amount of water is available for irrigation, municipal and industrial (M&I) supply, energy production, preservation of endangered species, and other needs. These impacts can occur through multiple mechanisms, such as decreased precipitation and soil moisture affecting dryland farming; low reservoir levels reducing 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 M&I supplies, among others; and 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). Drought also can relate and contribute to other phenomena, such as wildfires and heat waves. To assess and classify the intensity and type of drought, certain measures, or drought indicators, are typically used. Drought intensity, in turn, can be a trigger for local, state, and federal responses to drought. The classification of drought intensity, such as that shown in Figure 3 , may depend on a single indicator or several indicators, often combined with expert opinion from the academic, public, and private sectors. For example, the U.S. Drought Monitor ( Figure 2 ) uses five key indicators, together with expert opinion, with indicators to account for conditions in the West where snowpack is relatively important and with other indicators 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 regions depicted as red in Figure 2 faced extreme drought conditions for the week of October 17, 2017, but they may have contained local areas and individual communities that experienced less (or more) severe drought. Figure 3 illustrates how drought can stretch across nearly the entire nation, or only affect a comparatively small region. On July 17, 2012, abnormally dry or drought conditions covered roughly 72% of the United States, with 35% of the nation experiencing severe drought or worse. In contrast, on May 2, 2017, about 17% of the country faced abnormally dry or drought conditions, and only 4% was classified as experiencing severe drought or worse. Drought and "normal" conditions can vary considerably from region to region. For example, the U.S. Drought Monitor shows that Augusta, GA, faced severe drought in July 2012 (right side of Figure 3 ). Similarly, the city of Colorado Springs, CO, also was in severe drought during the same time period. However, Colorado Springs receives on average a total of 7.63 inches of precipitation over the six-month period of January through June. In contrast, Augusta receives on average of 22.2 inches of precipitation over the same period. During the first six months of 2012, Augusta received 13.28 inches of precipitation, only 60% of its average amount. However, that same amount—13.28 inches—would constitute almost 174% of the amount Colorado Springs typically receives over the same period. Both cities faced severe drought in July 2012, but what is "normal" for Augusta is different from what is "normal" for Colorado Springs. 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 Augusta and Colorado Springs. In addition to the color-coded D0-D4 designations, U.S. Drought Monitor maps often include an "S" and "L" designation to provide additional information about the nature of drought ( Figure 2 ). The "S" designation is intended to indicate existence of short-term effects: a combination of different drought indices that approximates responses to precipitation over days up to a few months. These effects would include impact to agriculture, topsoil moisture, unregulated streamflows, and aspects of wildfire danger.