A drought is a period of unusually dry weather, free of precipitation. It lasts long enough to cause problems, which are usually environmental and economic. For example, a drought can damage crops and cause wildfires.
Of all natural disasters, drought is the most subtle and difficult to define. Farmers cannot tell there is going to be a drought until it is too late. Unlike flash floods or tornadoes, drought is slow to develop. Unlike earthquakes, with destruction to the exterior environment, drought does its damage underground long before dust storms rage across the plains, such as the Dust Bowl, which affected a large portion of the United States in the 1930s. The 1950s saw an additional drought in the Plains and Southwest in the 1950s, and a drought struck the Western and Northwestern states late in the 1980s. Another drought hit the United States in 2012, with 39% of the country experiencing severe to extreme drought, and about 55% in the moderate to extreme categories.
Technically, drought is measured by the decrease in the amount of subsoil moisture that causes crop death or decreased yield (agricultural drought), or by a drop in the water level in surface reservoirs and belowground aquifers, which causes wells to go dry (hydrological drought). Agricultural plus hydrological drought can lead to sociological drought. In this condition, drought affects food and water supplies to the extent that people have to rely on relief donations or are forced to migrate to another area. Meteorological drought occurs when there is a period in which precipitation is significantly lower than average.
Droughts are not recent developments; analysis of rock cores, glacial ice cores, and tree rings reveals prehistorical and historical droughts, some of which lasted for several decades. Tree rings in California, for example, record a 40-year drought 300 years ago.
Droughts occur worldwide repeatedly and cannot be predicted. Some scientists assert that there is a drought somewhere on Earth at any time. Evidence of droughts hundreds of years ago confirms that nature plays a role in these natural disasters. However, people also have a hand in causing drought. For example, when trees are cut down for fuel over widespread areas, the soil cannot hold water effectively and the ground dries out. Even the Dust Bowl, which was blamed on extremely dry weather conditions, was compounded by poor land management. Farming practices left topsoil loose and easily blown away by the wind. Constructing a dam on a large river can provide electricity to a community but cause drought downstream. The effects of climate change are more complicated, but global warming affects climate and will continue to contribute to droughts, even though it cannot be blamed as the single cause of them.
One of the biggest public health problems from drought can be lack of proper sanitation and hygiene because of low water availability. If people are not able to maintain hygiene, the risk of infectious disease increases. Runoff from wildfires can kill fish and other aquatic life in lakes and clog water treatment systems, affecting water quality in nearby communities. In communities that depend on groundwater for drinking and irrigation supplies, drought can make the supply too shallow. In areas that depend on water for energy, drought can affect other important living conditions such as air conditioning or electricity generation, which can affect the health of people in communities. Dust from dry soil or particulates from wildfire smoke can enter the air and cause breathing difficulties.
During severe drought, sociologists find that people must adapt, migrate, or die. Death, however, is usually caused by other factors such as war or poverty, as in the Sahel, where relief food supplies have been hijacked and sold at high prices or where people in remote villages must walk to the distribution centers.
Some migrations have been permanent, as in the migration to California during the Midwest Dust Bowl in the 1930s. Others are temporary, as in the Sahel region, where people migrate in search of food and water, crossing country lines.
The economic toll droughts exert on communities and countries can be even higher than other natural disasters that cause extensive damage to structures. When people have to relocate because of drought, their migration strains the food supplies, economies, and other resources in new communities. In the United States, lack of rain has caused profit losses in Texas, one of the most agriculturally productive states. The total amount of loss from drought in 2006 was estimated at $4.1 billion. In early 2009, 97% of the state of Texas experienced drought.
The direct cause of drought is a continued decrease in optimal rainfall. The reasons clouds do not form over an area or the winds carry rain-bearing clouds elsewhere are complex. Climate change alters the location of increased and reduced rainfall such that some places that always have been well watered experience drought. This variability in the amount of precipitation can influence agricultural production, water resources, and the overall ecosystems of affected areas.
The climate in Australia and other regions have been affected by human-induced aerosols in Asia. Research showed that the temperature and pressure gradients over the Indian Ocean were being altered, and one explanation was the use of aerosols in Asia. Drought occurred in some parts of Australia that rarely encountered lack of rain, and elsewhere uncharacteristic flooding occurred. Researchers suggested that climate change may be shifting patterns southward.
Some scientists argue that El Niño-La Niña events in the western Pacific Ocean are main drivers in causing droughts around the world. El Niño, an eastward flow of warm surface waters, creates a high pressure zone over the equator that results in a change in the high and low pressure zones in other parts of the world. This change in pressure affects the flow of the jet stream and results in a disturbed rainfall pattern, causing, for example, excessive rain in California and drought in southwestern Africa, among other places. La Niña, which usually follows El Niño, is an upwelling of cold, deep waters in the western Pacific Ocean. It causes disturbed pressure zones that result in droughts in the Midwest, among other places.
In 2012 drought prediction remained a new science. Although research has shown that El Niño-La Niña events cause droughts in specific areas, scientists could still not predict when El Niño would occur. Weather satellites can measure subsoil moisture, a good indicator of upcoming drought, but other factors also contribute to drought.
Lack of rain—for example, in the Sahel—is exacerbated by manmade environmental problems, such as deforestation for fuel and not allowing the soil to lay fallow, which conserves soil moisture. Overgrazing by animals such as cattle, goats, and sheep also denudes topsoil, which dries and blows away, a condition known as desertification. Drought then becomes a cycle that feeds on itself: lack of trees reduces the amount of water vapor given off into the atmosphere, and lack of topsoil reduces water retention. The result is that local rainfall is reduced, and the rain that does fall runs off and is not absorbed.
Of all the water on Earth, less than 3% is freshwater. A lot of water is lost in evaporation, especially in arid climates, not only during rainfall but when it is stored in surface reservoirs. Rainwater or snowmelt that seeps into below-ground permeable rock channels, or aquifers, is pumped into wells in many communities. High-tech pumps have contributed to an increased drain on aquifers; if an aquifer is pumped too quickly, it collapses, and the ground above sinks. Some communities recharge their aquifers by pumping water into them when they are low.
The only new water introduced into the hydrologic cycle is purified ocean water. Desalinization plants are expensive to build and maintain and often require burning fossil fuels or wood to run. Future plans include perfecting the use of solar energy and wind energy.
The United Nations has called for an integrated strategy to combat the effects of climate change on drought and global food security. The Centers for Disease Control and Prevention has prepared a guide for public health professionals on how to protect public health during drought conditions. The CDC worked jointly with several agencies and associations to consolidate information into one publication to address public health effects of drought and provide guidance on how to prepare for drought, along with how to respond to late-stage, severe drought conditions.
As of 2010, farm irrigation used most of the world's freshwater supply, but as city populations grow, they become the biggest consumers, and urban conservation measures become imperative; using reduced or low-flow shower heads and toilets are required in new construction in some states. Some communities recycle wastewater for small farms and domestic garden use. Droughts that allow industrial pollutants to increase in concentration and cause the water supply to become toxic are being reduced and resolved under federal law.
Distributing water from more to less abundant supplies by laying pipes and installing pumps within a state or a country requires money and management. If water is fed across state or international boundaries, legal and political negotiations are necessary.
Most people adapt in drought by making the most of their resources, such as building reservoirs or desalination plants or laying pipes connecting to more abundant water supplies. Farmers often invest in high-tech irrigation techniques or alter their crops to grow low-water plants, such as garbanzo beans.
See also Climate change ; Earthquakes ; Sanitation ; Water quality ; Wildfires .
Boken, V. K., A. P. Cracknell, and R. L. Heathcote. Mon-itoring and Predicting Agricultural Drought: A Global Study. New York: Oxford University Press, 2005.
Guile, Melanie. World Issues Come to Australia: Drought and El Nińo. Port Melbourne, Victoria, Australia: Heinemann Library, 2008.
Mortimore, Michael. Adapting to Drought: Farmers, Famines, and Desertification in West Africa. Cambridge: Cambridge University Press, 2009.
Centers for Disease Control and Prevention. “When Every Drop Counts: Protecting Public Health During Drought Conditions.” http://www.cdc.gov/nceh/ehs/Docs/When_Every_Drop_Counts.pdf (accessed October 4, 2012).
Goetz, Alexander F. H. “Landsat Applications: Assessing Drought Impact.” National Aeronautic and Space Administration. August 19, 2010. http://landsat.gsfc.nasa.gov/about/Application6.5.html (accessed December 1, 2012).
National Oceanic and Atmospheric Administration, National Climatic Data Center. “20th Century Drought.” http://www.ncdc.noaa.gov/paleo/drought/drght_history.html (accessed October 4, 2012).
Washington Post Staff. “What We Know About Climate Change and Drought.” http://www.washingtonpost.com/blogs/ezra-klein/wp/2012/07/24/what-we-knowabout-climate-change-and-drought/ (accessed October 5, 2012).
Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 272-0167, http://water.epa.gov .
Revised by Teresa G. Odle