The United States' contiguous western and especially southwestern region has experienced widespread drought since about year 2000. Below normal precipitation leads to drought, and is caused by an above average persistence of high pressure over the affected area. Changes in the track of extratropical cyclones, which can occur during climate cycles such as the El Niño-Southern Oscillation, or ENSO, as well as the North Atlantic Oscillation, Pacific Decadal Oscillation, and Atlantic multidecadal oscillation, modulate which areas are more prone to drought. Increased drought frequency and severity is also expected to be one of the effects of global warming.
Drought having an acute economic impact in the history of the United States occurred during the 1930s and 1940s, periods of time known as 'Dust Bowl' years where relief and health agencies became overburdened and many local community banks had to close. The event also led to significant changes in farming practices emphasizing soil conservation and water resource management, inspired further growth in U.S. water storage and delivery infrastructure as part of the New Deal, and influenced other social changes.
Furthermore, global La Niña meteorological events are generally associated with drier and hotter conditions and further exacerbation of droughts in California and the Southwestern and to some extent Southeastern United States. Meteorological scientists have observed that La Niñas have become more frequent over time.
Metrics
The U.S. Drought Monitor provides a national database to track the duration and severity of droughts in the United States. It is hosted by the University of Nebraska–Lincoln with assistance from the United States Department of Agriculture and the National Oceanic and Atmospheric Administration. Their standardized measurements track droughts on a severity scale from "Abnormally Dry" (D0) to "Exceptional" (D4).
Causes
Mechanisms of producing precipitation include convective, stratiform, and orographic rainfall. Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation, while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration. Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice.
If these factors do not support precipitation volumes sufficient to reach the surface over a sufficient period of time, the result is a drought. Drought can be triggered by a high level of reflected sunlight and above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses, and ridges of high pressure areas aloft can prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Once a region is within drought, feedback mechanisms such as local arid air, hot conditions which can promote warm core ridging, and minimal evapotranspiration can worsen drought conditions. Winters during El Niño are warmer and drier than average in the Northwest, northern Midwest, and northern Mideast United States, so those regions experience reduced snowfalls.
Activities resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture and increased social unrest throughout the world, especially in developing nations. Overall, global warming will result in increased world rainfall. Along with drought in some areas, flooding and erosion will increase in others. Paradoxically, some proposed solutions to global warming that focus on more active techniques, solar radiation management through the use of a space sunshade for one, may also carry with them increased chances of drought.
Government responses
Certain regions within the United States are more susceptible to droughts than others. Droughts can be more damaging than tornadoes, tropical cyclones, winter storms and flooding combined. Unlike a hurricane, tornado or flooding, the onset of droughts happen gradually over a long period of time.
In dry areas, removing grass cover and going with a more natural vegetation for the area can reduce the impact of drought, since a significant amount of fresh water is used to keep lawns green. In the Nevada "cash for grass" program, the people are paid to remove grass and put in desert landscaping, which calls for the planting of vegetation which is local in origin and more resistant to drought.
When California suffered a severe drought from 1985 to 1991, a California company, Sun Belt Water Inc. was established for the purpose importing water from Canada in marine transport vessels formerly used for oil transport and converted to water carriers. The idea was commercially viable and Sun Belt Water Inc., was selected by the Goleta Water District to enter a long-term contract. When the government of British Columbia reversed its existing bulk water export policy, the change in government policy led to a claim by Sun Belt Water Inc. against Canada under the provisions of Chapter 11 of the North American Free Trade Agreement (NAFTA).
The National Integrated Drought Information System (NIDIS) Act was signed into law in 2006 (Public Law ). The Western Governors' Association described the need for NIDIS in a 2004 report, Creating a Drought Early Warning System for the 21st Century: The National Integrated Drought Information System. The NIDIS Act calls for an interagency, multi-partner approach to drought monitoring, forecasting, and early warning, led by the National Oceanic and Atmospheric Administration (NOAA). NIDIS is being developed to consolidate data on drought's physical, hydrological and socio-economic impacts on an ongoing basis, to develop drought decision support and simulation tools for critical, drought-sensitive areas, and to enable proactive planning by those affected by drought. NIDIS (www.drought.gov) draws on the personnel, experience, and networks of the National Drought Mitigation Center, the NOAA Regional Climate Centers, and the Regional Integrated Sciences and Assessments (RISAs), among others. Federal agencies and departments partnering in NIDIS include the U.S. Army Corps of Engineers, the Bureau of Reclamation, the U.S. Geological Survey, NASA, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the National Science Foundation, and the Natural Resources Conservation Service.
President Roosevelt on April 27, 1935, signed documents creating the Soil Conservation Service (SCS)—now the Natural Resources Conservation Service (NRCS). Models of the law were sent to each state where they were enacted. These were the first enduring practical programs to curtail future susceptibility to drought, creating agencies that first began to stress soil conservation measures to protect farm lands today. It was not until the 1950s that there was an importance placed on water conservation was put into the existing laws (NRCS 2014).
Events
Pre-1800 The earliest and longest drought discussed in the literature is the "Altithermal Long Drought", which some scholars now believe was in reality two severe shorter droughts (ca. 7000-6500 BP and 6000-5500 BP), separated by an interval. Other early notable droughts in North America include the Fairbank Drought of 500 BC and the Whitewater Drought of 330 C.E.
There were megadroughts in what is now the central and western United States, between 900 and 1300. A megadrought struck what is now the American Southwest 1276–1299 C.E., which severely affected the Pueblo cities, and tree rings also document drought in the lower and central Mississippi River basin between the 14th and 16th century. The droughts of that period may have contributed to the decline and fall of the Mississippian cultures. Data from tree rings indicate that the megadroughts which occurred throughout the twelfth and thirteenth centuries, exceeded anything which occurred within the twentieth century in both spatial extent and duration, including the Dust Bowl of the 1930s and drought in the 1950s, but was co-equal to the drought there in the early 21st century.
The 18th century seems to have been a relatively wet century in North America, but there were apparently droughts in Iowa in 1721, 1736, and from 1771 to 1773.Cleaveland, M.K., Duvick, D.N., 1992. Iowa climate reconstructed from tree rings, 1640–1982. Water Resources Research 28, 2607–2615.
19th Century There were at least three major droughts in 19th-century North America: one from the mid-1850s to the mid-1860s, one in the 1870s, and one in the 1890s. There was also a drought around 1820; the periods from 1816 to 1844 and from 1849 to 1880 were rather dry, and the 19th century overall was a dry century for the Great Plains. While there was little rain-gauge data from the mid-19th century in the middle of the US, there were plenty of trees, and tree-ring data showed evidence of a major drought from around 1856 to around 1865. Native Americans were hard hit, as the bison they depended upon on the Plains moved to river valleys in search of water, and those valleys were full of natives and settlers alike. The river valleys were also home to domestic livestock, which competed against the bison for food. The result was starvation for many of the bison.
The 1870–1877 drought brought with it a major swarm of Rocky Mountain Locusts, as droughts benefit locusts, making plants more nutritious and edible to locusts and reducing diseases that harm locusts. Locusts also grow more quickly during a drought and gather in small spots of lush vegetation, enabling them to swarm, facts which contributed to the ruin of much of the farmland in the American West. The evidence for this drought is also primarily in tree-ring, rather than rain gauge, data.
The 1890s drought, between 1890 and 1896, was the first to be widely and adequately recorded by rain gauges, with much of the American West having been settled. Railroads promised land to people willing to settle it, and the period between 1877 and 1890 was wetter than usual, leading to unrealistic expectations of land productivity. The amount of land required to support a family in more arid regions was already larger than the amount that could realistically be irrigated by a family, but this fact was made more obvious by the drought, leading to emigration from recently settled lands. The Federal government started to assist with irrigation with the 1902 Reclamation Act.
1930s
The Dust Bowl or the Dirty Thirties was a period of severe dust storms causing major ecological and agricultural damage to American and Canadian prairie lands from 1930 to 1936 (in some areas until 1940). The phenomenon was caused by severe drought coupled with decades of extensive farming without crop rotation, fallow fields, cover crops or other techniques to prevent erosion. Deep plowing of the virgin topsoil of the Great Plains had displaced the natural grasses that normally kept the soil in place and trapped moisture even during periods of drought and high winds.
During the drought of the 1930s, without natural anchors to keep the soil in place, it dried, turned to dust, and blew away eastward and southward in large dark clouds. At times the clouds blackened the sky reaching all the way to East Coast cities such as New York and Washington, D.C. Much of the soil ended up deposited in the Atlantic Ocean, carried by prevailing winds which were in part created by the dry and bare soil conditions itself. These immense dust storms—given names such as "Black Blizzards" and "Black Rollers"—often reduced visibility to a few feet (around a meter). The Dust Bowl affected , centered on the panhandles of Texas and Oklahoma, and adjacent parts of New Mexico, Colorado, and Kansas.