Understanding Droughts

Understanding Droughts

Drought is an extended period of unusually dry weather when there is not enough rain.


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Drought is an extended period of unusually dry weather when there is not enough rain. The lack of precipitation can cause a variety of problems for local communities, including damage to crops and a shortage of drinking water. These effects can lead to devastating economic and social disasters, such as famineforced migration away from drought-stricken areas, and conflict over remaining resources.

Because the full effects of a drought can develop slowly over time, impacts can be underestimated. However, drought can have drastic and long-term effects on vegetation, animals, and people. Since 1900, more than 11 million people have died and more than 2 billion people have been affected by drought. Drought is also one of the costliest weather-related disasters. Since 2014 California has lost at least 2 billion-dollars a year, due to drought.

Defining Drought

Drought is a complicated phenomenon, and can be hard to define. One difficulty is that drought means different things in different regions. A drought is defined depending on the average amount of precipitation that an area is accustomed to receiving.

For example, in Atlanta, Georgia, the average rainfall is about 127 centimeters (50 inches) a year. If significantly less rain falls, there may be water shortages and a drought may be declared. However, some arid regions, such as the deserts of the American Southwest, may receive less than about 25 centimeters (10 inches) of rainfall in a non-drought year. A drought in Atlanta could be a very wet period in Phoenix, Arizona!

Determining the start of a drought can be tricky. Unlike many natural hazards that bring about sudden and dramatic results—such as earthquakestornadoes, and hurricanes—the onset of a drought can be gradual and subtle. It can take weeks, months, or even years for the full effects of long-term inadequate rainfall to become apparent.

The end of a drought can also be difficult to determine. While a single rainstorm will provide short-term relief from a drought, it might take weeks or months before levels of precipitation return to normal. The start and end of a drought are often only clear in hindsight.

Causes of Drought

Most droughts occur when regular weather patterns are interrupted, causing disruption to the water cycle. Changes in atmospheric circulation patterns can cause storm tracks to be stalled for months or years. This disruption can dramatically impact amounts of precipitation that a region normally receives. Changes in wind patterns can also be disruptive to how moisture is absorbed in various regions.

Scientists have found a link between certain climate patterns and drought. El Niño is a weather event where the surface water in the Pacific Ocean along the central South American coast rises in temperature. These warmer waters alter storm patterns and are associated with droughts in Indonesia, Australia, and northeastern South America. El Nino events keep climate scientists guessing, by occurring every two to seven years.

La Niña is the counterpart to El Niño, when the surface water in the Pacific Ocean along the coast of South America decreases in temperature. The cooler waters affect storm patterns by contributing to drier-than-normal conditions in parts of North and South America. El Niño and La Niña both usually last about a year. The effects of La Niña on weather patterns are often more complex than El Niño. Two of the most devastating droughts in the history of the United States—the 1930s Dust Bowl and the 1988 drought in the Midwest—are associated with the effects of La Niña.

There is still a lot of debate about the connection between drought and global warming, the current period of climate change. A 2013 NASA study predicts warmer worldwide temperatures will mean increased rainfall in some parts of the world and decreased rainfall in others, leading to both more flooding and more droughts worldwide. Other scientists question the prediction that there will be more droughts and believe global warming will create a wetter climate around the world.

Impacts of Drought

Trees and other plants have adapted to withstand the effects of drought through various survival methods. Some plants (such as grasses) will slow their growth or turn brown to conserve water. Trees can drop their leaves earlier in the season to prevent losing water through the leaf surface. However, if drought conditions persist, much vegetation will die.

Certain plants have adapted so they can withstand long periods without water. Yuccas, for instance, have deep root systems that can seek out water with incredible efficiency. Cacti have spiny, hairy spines, spikes, or leaves that limit how much water they lose to evaporation. Mosses can withstand complete dehydration. Juniper trees can self-prune by steering water only toward the branches required for survival. Other plants only grow when there is enough water to support them. In periods of drought, their seeds can survive under the soil for years until conditions are favorable again.

However, many organisms cannot adapt to drought conditions, and the environmental effects of extended, unusual periods of low precipitation can be severe. Negative impacts include damage to habitats, loss of biodiversity, soil erosion, and an increased risk from wildfires. During the U.S. drought of 1988, rainfall in many states was 50 to 85 percent below normal. Summer thunderstorms produced lightning without rain and ignited fires in dry trees. In Yellowstone National Park 36 percent of the park was destroyed by fire.

Drought can also create significant economic and social problems. The lack of rain can result in crop loss, a decrease in land prices, and unemployment due to declines in production. As water levels in rivers and lakes fall, water-supply problems can develop. These can bring about other social problems. Many of these problems are health-related, such as lack of water, poor nutrition, and famine. Other problems include conflicts over water usage and food, and forced migration away from drought-stricken areas.

While drought is a naturally occurring part of the weather cycle and cannot be prevented, human activity can influence the effects that drought has on a region. Many modern agricultural practices may make land more vulnerable to drought. While new irrigation techniques have increased the amount of land that can be used for farming, they have also increased farmers’ dependence on water.

Traditional agricultural techniques allow land to “rest” by rotating crops each season and alternating areas where livestock graze. Now, with many areas in the world struggling with overpopulation and a shortage of farmland, there is often not enough arable land to support sustainable practices. Over-farming and overgrazing can lead to soil being compacted and unable to hold water. As the soil becomes drier, it is vulnerable to erosion. This process can lead to fertile land becoming desert-like, a process known as desertification. The desertification of the Sahel in North Africa is partly blamed on a prolonged drought whose effects were intensified by farming practices that result in overgrazing.

Increased drought conditions in Kenya have been attributed to deforestation and other human activities. Trees help bring precipitation into the ground and prevent soil erosion. But in 2009, it was reported that one-quarter of a protected forest reserve had been cleared for farming and logging, leading to drought conditions affecting 10 million people around the country.

Historical Droughts

Scientists often study historical droughts to put modern-day droughts in perspective. Since our data from thermometers and rain gauges only goes back about 100 to 150 years, scientists must research paleoclimatology, the study of the atmosphere of prehistoric Earth. Scientists gather paleoclimatic data from tree ringssediments found in lakes and oceans, ice cores, and archaeological features and artifacts. This allows scientists to extend their understanding of weather patterns for millions of years in the past.

Analyzing paleoclimatic data shows that severe and extended droughts are an inevitable part of natural climate cycles. North America has experienced a number of long-lasting droughts with significant effects. It is thought that droughts brought about the decline of the Ancestral Puebloans in the Southwest during the 13th century, and the central and Lower Mississippian societies in the 14th to 16th centuries.

In South America, massive migration out of the once-fertile Atacama Desert 9,500 years ago can be explained by the onset of extreme drought.

In Africa, the Sahel region experienced a dry period from 1400 to 1750 that radically altered the landscape. The water level in Lake Bosumtwi, Ghana, for instance, fell so low that an entire forest grew on the lake’s edges. Today, visitors can still see the tops of trees growing out of the lake—where the water is now more than 15 meters (50 feet) deep.

What scientists have learned by looking at Earth’s drought history is that periods of severe drought are a regular part of nature’s cycle. As devastating as droughts in the last century have been, they are considered relatively minor compared to the severity of earlier droughts that have lasted more than a century.

Major Droughts in the Past Century

The Dust Bowl of the 1930s is probably the most well-known drought experienced by the U.S. By 1934, 80 percent of the U.S. was struggling with moderate-to-severe drought conditions. The drought lasted nearly a decade and had devastating effects on crop production in the Great Plains. The combination of lack of rain, high temperatures, and strong winds affected at least 50 million acres of land. Massive clouds of dust and sand formed as unusually strong winds lifted the dry soil into the air. These clouds could block out the sun for days, giving the period the name “dust bowl.” In 1934, one dust cloud infamously traveled 2,414 kilometers (1,500 miles), from the Great Plains to the eastern U.S.

Mass migration was an indirect effect of the Dust Bowl. Farmers and their families were forced to migrate to other areas in search of work, and by 1940, 2.5 million people had fled the Great Plains. Of those, 200,000 moved to California. The influx of migrants into existing economies already strained by the Great Depression led to a rise in conflict, unemployment, and poverty.

In the 1950s, severe drought returned to the Great Plains and southwestern United States, affecting half of the continental U.S. Low rainfall and high temperatures caused the production of crops in some areas to drop nearly 50 percent. Hay became too expensive for ranchers, and they had to feed their cattle prickly pear cactus and molasses to keep them alive. By the end of the five-year drought in 1957, 244 of Texas’ 254 counties had been declared federal drought disaster areas.

In the late 1980s, the U.S. experienced one of the costliest drought in its history. The three-year spell of high temperatures and low rainfall ruined roughly $15 billion of crops in the Corn Belt. The total of all the losses in energy, water, ecosystems, and agriculture is estimated at $39 billion. Federal assistance programs were able to help many farmers, but a longer-lasting drought would make it more difficult for the government to provide nationwide aid.

Droughts continue to affect the U.S. Texas has been suffering from drought since 2010, with 2011 ranking as the driest year in the state’s history. A September 2012 National Geographic magazine article called Texas “The New Dust Bowl.” By 2013, 99 percent of the state was dealing with drought.

Australia is also a frequent victim of drought. The last decade has been especially severe, earning it the name The Big Dry or The Millennium Drought. Much of the country was placed under water restrictions, wildfires spread in the dry weather, and the water level in some dams fell to 25 percent. In 2007, 65 percent of viable land in Australia was declared to be in a drought. The drought was officially declared over in April 2012.

Droughts that occur in the developing world can cause even greater devastation. The Sahel region in Africa, which includes eight countries, endured a series of droughts in the 1970s and 1980s where annual rainfall dropped by about 40 percent. In the early 1970s, more than 100,000 people died and millions of people were forced to migrate. Conditions continue to be critical in the area due to drought, overpopulation, failing crops, and high food prices. Drought emergencies for the region have been declared four times since 2000.

The Horn of Africa, which includes the countries of Ethiopia, Somalia, Eritrea, and Djibouti, is particularly vulnerable to droughts. Because almost 80 percent of the population is rural and depends on agriculture for food and income, famine often accompanies drought.

Struggles for the region’s limited, remaining resources can lead to conflict and war. In 1984 and 1985, the Horn of Africa suffered one of the worst droughts of the 20th century. The U.N. estimates that in Ethiopia alone, 1 million people died, 1.5 million livestock died, and 8.7 million people were affected by the drought—including being hospitalized, forced to migrate, or forced to change professions. In Sudan, 1 million people died, at least 7 million livestock died, and 7.8 million people were affected.

The cycle of drought-famine-conflict has persisted in the region, with drought conditions returning every few years since 2000. In 2006, drought affected 11 million people across the Horn of Africa, and the resulting crisis killed between 50,000 and 100,000 people and affected more than 13 million.

Forecasting and Measuring Drought

Even though scientists are unable to predict how long a drought will last or how severe it will be, early warning systems and monitoring tools can minimize some of drought’s damaging impacts. There are a number of tools used to monitor drought across the U.S. Due to the limitations of each system, data from different sources are often compiled to create a more comprehensive forecast.

The Palmer Drought Severity Index (PDSI), developed in 1965 by the National Weather Service, is the most commonly used drought monitor. It is a complex measurement system and an effective way to forecast long-term drought. Its limitations are that it does not provide early warnings for drought and is not as accurate for use in mountainous areas because it does not account for snow (only rain) as precipitation. The PDSI is often used by the U.S. Department of Agriculture to determine when to begin providing drought relief.

Information from the Standardized Precipitation Index (SPI) is often used to supplement the PDSI data. The SPI, developed in 1993, is less complex than the PDSI and only measures precipitation—not evaporation or water runoff. Many scientists prefer using the SPI because the time period being analyzed can easily be customized. The SPI can also identify droughts many months earlier than the PDSI. The National Drought Mitigation Center uses the SPI to monitor drought conditions around the U.S.

The U.S. Drought Monitor, started in 1999, is a joint effort between three U.S. government agencies—the Department of Agriculture, the Department of Commerce, and the National Oceanic and Atmospheric Administration (NOAA). The Monitor synthesizes data from academic and federal scientists into a weekly map indicating levels of dryness around the country. It is designed to be a blend of science and art that can be used as a general summary of drought conditions around the country. It is not meant to be used as a drought predictor or for detailed information about specific areas.

The Famine Early Warning System Network (FEWS NET) monitors satellite data of crops and rainfall across Africa and some parts of Central America, the Middle East, and Central Asia. Analysis of the data allows for early intervention to try to prevent drought-induced famine.

Preparing for Drought

People and governments need to adopt new practices and policies to prepare as much as possible for inevitable future droughts. Emergency spending once a crisis has begun is less effective than money spent in preparation. The Federal Emergency Management Agency (FEMA) estimates that every $1 spent in planning for a natural hazard will save $4 in the long term.

Many areas are extremely vulnerable to drought as people continue to be dependent on a steady supply of water. The U.S. Department of Agriculture recommends a series of conservation practices to help farmers prepare for drought. Some preventative measures include installing an efficient irrigation system that reduces the amount of water lost to evaporation, storing water in ditches along fields, regularly monitoring soil moisture, planting crops that are more drought-resistant, and rotating crops to allow water in the soil to increase.

In urban areas, many cities are promoting water conservation by addressing water usage habits. Some enforce water restrictions, such as limiting days when lawns and plants can be watered, and offering free high-efficiency toilets and kitchen faucets.

Some drought-ravaged cities are taking even more extreme measures to prepare for future droughts. In Australia, the city of Perth is planning for a massive wastewater-recycling program that will eventually provide up to a quarter of the city’s water demands by 2060. Perth has been dealing with a decline in rainfall since the mid 1970s. The city, which is on the edge of a huge desert, is also struggling with its history of over-consumption of water. Water-hungry traditions such as planting large, lush lawns and parks will need to be addressed through conservation measures.

Fast Fact

Drought in the USAIn August 2012, drought conditions extended over 70 percent of the United States. Counties in 33 states were designated “disaster counties” by the government. In the beginning of 2013, drought still affected more than 60 percent of the country.

Fast Fact

Dust BowlJohn Steinbeck’s 1939 novel The Grapes of Wrath describes the Dust Bowl drought of the 1930s: “Every moving thing lifted the dust into the air: a walking man lifted a thin layer as high as his waist, and a wagon lifted the dust as high as the fence tops, and an automobile boiled a cloud behind it. The dust was long in setting back again.”

Fast Fact

Yunnan Drought

The ongoing drought in Yunnan Province, China, has forced some families to transport water from more than 10 kilometers (6 miles) away.

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Alison Ince Rodgers
Jeannie Evers, Emdash Editing, Emdash Editing
National Geographic Society
Last Updated

April 3, 2024

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