Droughts occur in virtually all climates. Of all the weather-related phenomena that can cause severe economic impacts in the United States, droughts come in second only to hurricanes, according to the National Climatic Data Center. But unlike hurricanes, which are easily identified and straightforward to classify in terms of wind speeds, droughts are much tougher to define.
Definition of drought
Most people think of a drought as a period of unusually dry weather that persists long enough to cause problems such as crop damage and water supply shortages. But because dry conditions develop for different reasons, there is more than one definition of drought.
"Drought is caused by not only lack of precipitation and high temperatures but by overuse and overpopulation," said David Miskus, a drought expert and meteorologist at the National Oceanic and Atmospheric Administration's Climate Prediction Center.
In the 1980s, two researchers uncovered more than 150 published definitions of drought. In an effort to bring some order to measuring drought, the scientists grouped the definitions into four basic approaches: meteorological, hydrological, agricultural and socioeconomic. The first three categories track drought as a physical phenomenon. The last category deals with drought as a supply and demand problem, through the impacts of water shortfalls.
These definitions usually specify the beginning, end and degree of severity of drought by comparing the current precipitation over a certain time period to a historical average. Scientists include both rain and snow in precipitation measurements, because some regions, such as mountainous West, rely on winter snow for much of their yearly water.
Here are descriptions of the four main categories of drought.
Meteorological drought is specific to different regions. For example, 20 inches (51 centimeters) of rainfall in a year is normal in West Texas, but the same amount would be less than half the yearly average in Virginia.
Agricultural drought accounts for the water needs of crops during different growing stages. For instance, not enough moisture at planting may hinder germination, leading to low plant populations and a reduction in yield.
Hydrological drought refers to persistently low water volumes in streams, rivers and reservoirs. Human activities, such as drawdown of reservoirs, can worsen hydrological droughts. Hydrological drought is often linked with meteorological droughts.
Socioeconomic drought occurs when the demand for water exceeds the supply. Examples of this kind of drought include too much irrigation or when low river flow forces hydroelectric power plant operators to reduce energy production.
In the United States, the Palmer Drought Severity Index (PDSI, weekly index from CPC shown), devised in 1965, was the first comprehensive drought indicator. It is considered most effective for unirrigated cropland. The Palmer Drought Severity Index combines temperature, precipitation, evaporation, transpiration, soil runoff and soil recharge data for a given region to produce a single negative number that indicates drought conditions. This index serves as an estimate of soil moisture deficiency and roughly correlates with drought severity. The PDSI is the most commonly used index for drought monitoring and research. It has been widely used in tree-ring based reconstructions of past droughts in North America and other regions.
In 1999, the U.S. Drought Monitor replaced the PDSI as the nation's drought indicator. "One index can't cover the whole United States," Miskus said. "The Drought Monitor uses a lot of different tools to assess drought." [Infographic: Drought Grips U.S.]
The nationwide Drought Monitor categorizes drought into five levels of severity:
- abnormally dry (category D0, corresponding to a PDSI between -1.0 and .1.9)
- moderate drought (D1, PDSI between -2.0 and -2.9)
- severe drought (D2, PDSI between -3.0 and -3.9)
- extreme drought (D3, PDSI between -4.0 and -4.9)
- exceptional drought (D4, PDSI between -5.0 and -5.9)
History of U.S. droughts
Droughts have been cited as a scourge of humankind since biblical times, but the nation's most devastating drought on record occurred in the 1930s during the so-called "Dust Bowl" years. According to the National Climatic Data Center, the drought affected almost the entire Plains and covered more than 60 percent of the country at its peak in July 1934. It caused the migration of millions of people from the Plains to other parts of the country, especially the West Coast.
Researchers think a high-pressure ridge over the West Coast deflected moisture-bringing storms in 1934, causing the severe Dust Bowl droughts. This ridging pattern has been in place during some of the West's worst droughts, such as the 1976 and 2013 California droughts, two of the worst dry spells in the state's history.
Persistent high pressure in the atmosphere curbs cloud formation and leads to lower relative humidity and less precipitation. In the West, prolonged droughts occur when large-scale high-pressure patterns persist for months or more, blocking storms carrying winter rains and snow.
More recent U.S. droughts, such as those of the 1950s, 1988 and 2000, have also had serious economic and societal impacts. Between 1980 and today, 16 drought events have cost a combined $210 billion in the United States, and thousands of people have died due to the effects of drought.
In June 2012, 55.8 percent of the land in the lower 48 U.S. states were in drought, the highest figure in the history of the U.S. Drought Monitor.
A dry future?
As the human population increases in arid regions as well as wet ones, so will the demand for water and — with water supplies dropping at a faster rate — so will the likelihood of drought. In fact, population booms can trigger droughts almost by themselves.
A severe drought that gripped the Southeast from 2005 to 2007 was largely attributed by climate scientists to a 50 percent rise in the region's population during the preceding 15 years, which placed unprecedented demands on the water supply.
Aside from the human population explosion, global warming is also expected to fuel increased frequency and severity of droughts in many parts of the globe in future. According to projections by the Intergovernmental Panel on Climate Change, droughts will especially increase in subtropical areas, such as the U.S. Southwest, Australia and parts of Africa and Europe, as Earth's warming causes more evaporation and shifts weather patterns, pushing the paths of storms that bring thirst-quenching rains farther north.
Additional reporting by Natalie Wolchover, Staff Writer