Drought disasters

Factors involved: Animals, geography, human activity, plants, temperature, weather conditions, wind

Regions affected: Cities, coasts, deserts, forests, islands, lakes, mountains, plains, rivers, towns, valleys

Definition

A drought is an extended period of below-normal precipitation. It is a dry period that is sufficiently long and severe that crops fail and normal water demand cannot be met.

Science

Drought can be defined as a shortage of precipitation that results in below-normal levels of stream flow, groundwater, lakes, and soil moisture. It differs from other geophysical events such as volcanic eruptions, floods, and earthquakes because droughts are actually nonevents—that is, they result from the absence of events (precipitation) that should normally occur. Drought also differs from other geophysical events because it has no recognizable beginning (as opposed to an earthquake) and takes time to develop. Drought may be recognized only when plants start to wilt, wells and streams run dry, and reservoir shorelines recede.

Most droughts occur when slow-moving subsiding air masses dominate a region. Commonly, air circulates in continental interiors, where there is little moisture available for evaporation, thereby providing little potential for precipitation. In order for precipitation to occur, the water vapor in the air must be lifted so that it has a chance to cool, condense into dust particles, and eventually, if conditions are favorable, precipitate. Clearly, there is little opportunity for these conditions to occur when the air is dry and descending.

Another climatological characteristic associated with droughts is that, following their establishment within a particular area, they tend to persist and even increase in areal extent. Air circulation is influenced by the drying-out of soil moisture and its unavailability for precipitation further downwind. Concurrently, the state of the atmosphere that produces the unusual circulation associated with droughts can induce surface temperature variations that in turn foster further development of the unusual circulation pattern. As a result, the process feeds on itself, making the drought last longer and intensify until major atmospheric circulation patterns change.

Drought-identification research has changed over the years from a time when the size of the precipitation deficit was the only factor considered, to the present, when sophisticated techniques are applied to the quantitative assessment of the deviation of the total environmental moisture status. These techniques facilitate better understanding of the severity, duration, and areal extent of droughts.

Early in the twentieth century, drought was identified by the US Weather Bureau—now the National Weather Service—as any period of three weeks or more when the precipitation was 30 percent or more below normal. (Note that normal is defined as the average for a thirty-year period, such as 1951–80 or 1961–90.) The initial selection of three weeks for defining a drought was based entirely on precipitation. Subsequent research has shown that the moisture status of a region is affected by other factors besides precipitation.

Further developments in drought identification during the mid-twentieth century involved examination of the moisture demands that are related to evapotranspiration, the return of moisture to the atmosphere by the combined effects of evaporation and plant transpiration. Some drought-identification studies have examined the adequacy of soil moisture in the root zone for plant growth. The objective of this research is to determine drought probability based on the number of days when the moisture storage in the soil is zero. The US Forest Service used evapotranspiration in developing a drought index for use by fire-control managers. The index was used to provide an indicator of flammability that could lead to forest fires. It had limited applicability to nonforestry users as it was not effective for showing drought as a measure of total environmental moisture stress.

W. C. Palmer developed a drought-identification index in 1965 that became widely adopted. The Palmer Drought Index (PDI) defines drought as the period of time, generally measured in months or years, when the actual moisture supply at a specified location is always below the climatically anticipated or appropriate supply of moisture. Evapotranspiration, soil moisture loss, surface runoff, and precipitation are the required environmental parameters. The PDI values range from +4.0, for an extremely wet moisture status, to −4.0, for extreme drought. Normal conditions have values close to zero. Although the PDI has been used for decades and recognized as an acceptable procedure for including evapotranspiration and soil moisture in drought identification, it has been criticized. For example, the method determines a dimensionless parameter ranging from +4.0 to −4.0 that cannot be compared to variables such as precipitation, which are measured in units (inches) that are immediately recognizable. In addition, the index is not very sensitive to short drought periods, which can negatively affect crops.

In order to overcome these problems with the PDI, other researchers have used water-budget analysis to identify changes in environmental moisture status. The procedure is similar to the Palmer method, as it includes precipitation, evapotranspiration, and soil moisture. However, the values for moisture status deviation are dimensional and expressed in the same units as precipitation—inches. Drought classification using this method yields values ranging from approximately +1.0 inch, for an above-normal moisture status, to −4.0 inches, for extreme drought. As in the PDI, the index is close to zero for normal conditions.

Geography

Many regions of the world have regularly occurring periods of dryness. Three different forms of dryness have a temporal dimension; they are known as perennial, seasonal, and intermittent. Perennially dry areas include the major deserts of the world, such as the Sahara, Arabian, Kalahari, and Australian Deserts. Precipitation in these large deserts is not only very low (less than 10 inches per year) but also very erratic. Seasonal dryness is associated with those parts of the world where most of the precipitation for the year occurs during a few months, leaving the rest of the year rainless. Intermittent dryness pertains to those areas of the world where the total precipitation is reduced in humid regions or where the rainy season in wet-dry climates either does not occur or is shortened.

The major problem for humans is a lack of precipitation where it is normally expected. For example, the absence of precipitation for a week where daily precipitation is the norm is considered a drought. In contrast, it would take two or more years without any rain in parts of Libya in North Africa for a drought to occur. In those parts of the world that have one rainy season, a 50 percent decrease in precipitation would be considered a drought. In other regions that normally have two rainy seasons, the failure of one could lead to drought conditions. Thus, the very word “drought” itself is a relative term, since it has different meanings in different climatic regions. The deficiency of precipitation in one location is therefore not a good indicator of drought, as each place has its own criteria for identifying drought.

Prevention and Preparations

Droughts cannot be prevented, but their effects may be ameliorated. There are two main options for managing droughts: increasing the supply of water and decreasing the demand.

There are several supply enhancement measures that can be instituted. For example, reservoir release requirements can be relaxed. This occurred on the Delaware River during the severe drought of the early 1960s, when the required flow of 3,000 cubic feet per second at Trenton could not be met without jeopardizing the water-supply needs of New York City. Accordingly, the reservoir releases in the upper Delaware were temporarily relaxed. Many states require low flow or conservation flows to be maintained in the channel below a reservoir for waste assimilation and aquatic health. If a drought is severe enough, the conservation flows can be temporarily reduced or even eliminated. Other measures include the temporary diversion of water from one source, such as a recreational lake, to a water-supply reservoir. Interconnections with other water-supply purveyors may be encouraged or mandated. New sources of water could also be obtained from buried valleys that contain stratified glacial deposits with large amounts of groundwater.

Demand reduction measures include appeals for voluntary water conservation. If these do not work, then mandatory water-use restrictions can be imposed. Bans on outside uses of water, such as lawn watering and car washing, are common.

Rescue and Relief Efforts

Drought in the developed world affects crops and livestock but generally does not pose a threat to life, as it does in the developing world. Industrialized societies have existing transportation networks that enable supplies and foodstuffs to be shipped to affected regions. If there are crop and livestock losses, governments can provide disaster relief in the form of low- or no-interest loans to affected farmers, as happened in the eastern United States in the summer of 1999.

The situation in the developing world is much grimmer. Governments often lack the money and resources to distribute supplies to rural populations. Food supplies coming from overseas donors may not reach the intended victims because of inadequate transportation infrastructures. Some relief efforts that could be successful include drilling of deeper wells so as to tap undeveloped water sources. This takes much time, but the extra water may inadvertently encourage more people to stay in an area that may not be sustainable.

Impact

Droughts have had enormous impacts on human societies since ancient times. Crop and livestock losses have caused famine and death. Drought has caused ancient civilizations to collapse and forced many people to migrate. Water is so critical to all forms of life that a pronounced shortage can decimate whole populations.

The effects of drought are profound, even during modern times. For example, the dry conditions in the Great Plains in the 1930s in conjunction with intensive farming resulted in the Dust Bowl, which at one time covered more than 77,000 square miles, an area the size of Nebraska. An estimated 10 billion tons of topsoil was blown away, some of it landing on eastern cities. The Sahel region south of the Sahara in Africa had a severe drought from 1968 to 1974, which decimated local populations. Famine and disease killed several hundred thousand people (100,000 in 1973 alone) and 5 million cattle that were the sole means of support for the nomadic populations. The Sahel drought occurred during the worldwide droughts of 1973–74, which caused crop shortages to Amazonia, Australia, China, India, and the Soviet Union, and forced an increase in food prices around the world.

Historical Overview

Drought is the absence of precipitation. It is a problem particularly where precipitation is marginal, usually because of topographic factors. For example, precipitation is typically less than 20 inches a year over much of the Great Plains of the United States; the area farther west, until the Rocky Mountains are reached, normally has less than 10 inches per year. In this case, precipitation is low because the Rocky Mountains exist between the Great Plains and the Pacific Ocean: Oceans are the source of moisture that becomes precipitation—either rain or snow. The mountains force most rain clouds to drop their moisture before the clouds have passed over the mountains. This is why rainfall is high in the Pacific Northwest and low in the region to the east of the Rocky Mountains.

Precipitation is often marginal in areas where rainfall is seasonal. This condition prevails in much of Africa and in Asia, where precipitation occurs in the form of seasonal monsoons. For central Asia, precipitation that should come in the form of monsoons is interrupted by the Himalayas, a mountain range that lies between central Asia and the Indian Ocean, the source of moisture in that region.

Precipitation is also affected by long-term climatic trends. In general, when the climate is warmer, it tends also to be drier; when the climate is colder, it tends to be wetter. Some climatologists have argued that the recurring droughts in northern and eastern Africa have been a reflection of the warming trend in the climate identified as global warming and climate change. By the 2020s, scientists had found through much research that this extensive period of consistently, quickly increasing global warming could, in large part, be directly linked to human activity, particularly as the world's population had prodigiously expanded and technological advancements had continued to lead to greater amounts of pollution (especially from the burning of fossil fuels and the subsequent emission of carbon dioxide), deforestation, and intensive agriculture. Mean temperatures in the 1990s were higher than any recorded after the end of the Ice Age. These climatological trends are believed to be responsible for a prolonged drought in the American Southwest that undermined the Anasazi Indian culture of that region beginning in the thirteenth century. It is also possible that a comparable drought in central Asia led to the wave of Mongol invasions of Europe in the thirteenth century and the Turkish invasions of the fourteenth century.

While droughts occur with fairly regular frequency in areas of marginal precipitation, they represent an important historical event when they last more than one year. This was the case of the droughts believed to be responsible for the elimination of the early English colonies in Virginia in the sixteenth century. Droughts played a somewhat similar role in the late nineteenth century in the Great Plains of the United States, where farming settlement had been heavily promoted by the government through low-cost sales of public land. The process of moving the nomadic and seminomadic American Indian peoples to reservations had been predicated on the assumption that they would be replaced by permanent White settlers. However, after droughts hit the newly established farms between 1887 and 1896, many of the settlers abandoned their residences.

The twentieth century saw repeated recurring droughts in the sub-Saharan portion of Africa. One that occurred between 1910 and 1915 led many of the pastoral tribes inhabiting the area to move onto marginal land at higher elevations, land less able to support the tribes as their numbers grew. This same area was subjected to recurring droughts in the second half of the twentieth century, which spread to eastern Africa. Because this area has many subsistence farmers, who are unable to survive a lost harvest, the drought problem led to much unrest, with large numbers of people migrating in search of food. The conditions in the Sudan, Somalia, and Ethiopia resulted in repeated calls for emergency food supplies.

In South America, drought is not uncommon along the Pacific coastline, particularly in Chile and Peru. Because the winds tend to blow from east to west, little moisture is moved over the Pacific coastline of South America, and the Andes Mountains prevent moisture that arises from the Atlantic Ocean from reaching the lands to the west of the mountains. Droughts also affect parts of Brazil that are well inland from the Atlantic. Droughts in this area have been increasing since the seventeenth century; at least eight occurred in the twentieth century.

Many parts of Australia also suffer from recurrent droughts. The continent is located outside the main global circulation patterns that bring clouds and rain to inland areas, with the result that only the fringes of the continent are used for intensive cultivation. Most of Australia is suited only to grazing herds that can utilize the sparse vegetation and then move on. In 2002, a severe, long-term drought began in Australia and lasted through about 2010. By 2006, urban areas had begun to feel its effects. Major cities passed heavy restrictions on water usage and debated gray-water recycling programs. Alternate sources of water were sought. Brisbane hoped to set up larger dams and a pipeline, and Perth constructed a desalination plant. Another of Australia's most severe droughts began in 2017, as many regions experienced record dryness over subsequent consecutive years, including during the typically cooler season, with the Murray-Darling Basin being prominently impacted.

Probably the most famous drought in American history was that which hit the southern part of the Great Plains region in the early 1930s. Lands that even under the best of conditions receive only marginal precipitation had been “broken to the plough” in the first two decades of the twentieth century. When precipitation failed to materialize in the early 1930s, many subsistence farmers were driven from the land in what came to be known as the Dust Bowl, as winds blew the unprotected soil off the land.

Another drought affected the Great Plains, even the northern Great Plains, in the late 1980s. Many farmers who had borrowed money to extend their farms were unable to pay back the loans and lost their farms when their crops failed. Another drought hit the southern Great Plains in 1998, destroying a large portion of the cotton crop in Texas. In 1999, the drought conditions moved to the southeastern United States, devastating crops in that region. Coupled with high temperatures, this drought captured public attention.

The 2012–2015 North American drought affected areas in Mexico and eastern Canada, as well as much of the United States. During this time, California suffered the second most severe drought in the state’s history. In Texas, the drought began in 2011 and lasted into 2015. By the summer of 2021, it was reported that according to the PDI, 100 percent of the Western United States was experiencing a drought, while according to the US Drought Monitor's measures, approximately 90 percent was at levels that included extreme and exceptional. The PDI analysis represented the most severe, widespread extent of drought in the area in more than a century. Once again, states such as California, which was additionally experiencing a large number of widespread and devastating wildfires, faced the severest levels of the drought. Because of the lengthy period of sustained, intense drought fueled by climate change in the Western United States, it was labeled as a "megadrought" by scientists as early as 2020, a year that had seen further record-high temperatures, and concerns had risen over decreases in integral water sources such as the Colorado River.

During 2022, simultaneous droughts were recorded in regions around the globe. In August of that year, drought was recorded in 125 countries across six continents, according to data from the European Drought Observatory. Furthermore, such droughts, concurrently affecting multiple regions at once, were predicted to increase over the next several decades. Climate scientists reported in 2022 that concurrent droughts were expected to increase by 40 percent by 2050, causing increased food insecurity and other agricultural and socioeconomic challenges worldwide.

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