Drought

Drought is an unusually long period of below-normal precipitation. It is a relative rather than an absolute condition, but the end result is a water shortage for plant growth, affecting the people who live in that region and beyond. Drought is particularly disastrous for farmers and the practice of agriculture.

Impact of Drought

Droughts have had enormous impacts on human societies since ancient times. The most obvious effect is crop and livestock failure, which have caused famine and death through thousands of years of human history. Drought has resulted in the demise of some ancient civilizations and, in some instances, the forced mass migration of large numbers of people. Water is so critical to all forms of life that a pronounced shortage could, and has, decimated whole populations.

The effects of drought remain profound. The dry conditions in the Great Plains of North America in the early 1930s in conjunction with extensive and improper farming activities resulted in the occurrence of the Dust Bowl, which at one point covered more than 200,000 square kilometers, or an area about the size of Nebraska. During the early 1960s, a severe drought affected the Mid-Atlantic states. Parts of New Jersey experienced sixty consecutive months of below-normal precipitation, so depleting local water supplies that plans were actively considered to bring rail cars of water into Newark and other cities in the northern part of the state, as the reservoirs that usually supplied the region were practically dry. The Sahel region south of the Sahara in West Africa had a severe drought beginning in the late 1960s and largely continuing into the early 2020s, creating an enormous negative impact on the local population, livestock, and vegetation. Hundreds of thousands of people starved, thousands of animals died, and many tribes were forced to migrate south to areas of more reliable precipitation.

Drought Characteristics

Almost all droughts occur when slow-moving air masses that are characterized by subsiding air movements dominate an area. Often, the air comes from continental interiors where the amount of moisture that is available for evaporation into the atmosphere is very limited. When these conditions occur, the potential for precipitation is low for a number of reasons. First, the humidity in the air is already low, as the continental air mass is distant from maritime (moist) influences. Second, air that subsides undergoes adiabatic heating at the rate of 10 degrees Celsius per 1,000 meters. The term “adiabatic” refers to a change of temperature within a gas (such as the atmosphere) that occurs as a result of compression (descending air) or expansion (rising air), without any input or extraction of heat from external sources. For example, assume that air at a temperature of 0 degrees Celsius is passing over the Sierra Nevada in eastern California at an elevation of 3,500 meters. As the air descends and reaches Reno, Nevada, at an elevation of 1,200 meters, the higher atmospheric pressure found at lower elevations results in compression and heating at the dry adiabatic rate of 10 degrees Celsius per 1,000 meters, yielding a temperature in the Reno area of 23 degrees Celsius. Thus, adiabatic heating from subsiding air masses results in a decline in relative humidity and an increase in moisture-holding capacity. In addition, the movement of air under these conditions is usually unfavorable for vertical uplift and the beginning of condensation. The final factor that reduces precipitation potential is the decrease in cloudiness and corresponding increase in sunshine, which in turn leads to an increase in potential evapotranspiration demands, which favor soil moisture loss.

Another characteristic associated with droughts is that once they have become established within a particular location, they appear to persist and even expand into nearby regions, resulting in desertification in extreme cases. This tendency is apparently related to positive feedback mechanisms. For example, the drying out of the soil influences air circulation and the amount of moisture that is then available for precipitation farther downwind. At the same time, the atmospheric interactions that lead to unusual wind systems associated with droughts can induce surface-temperature variations that, in turn, lead to further development of the unusual circulation pattern. Thus, the process builds on itself, causing the drought to both last longer and intensify. The situation persists until a major change occurs in the circulation pattern in the atmosphere.

Many climatologists concur with the concept that precipitation is not the only factor associated with drought. Other factors that demand consideration include moisture supply, the amount of water in storage, and the demand generated by evapotranspiration. Although the scientific literature of climatology is replete with information about the intensity, length, and environmental impacts of drought events, the role of individual climatological factors that can increase or decrease the severity of a drought is not as fully understood.

Drought Identification

Research in drought identification has been changing over the years. Drought was once considered solely in terms of precipitation deficit. Although that lack of precipitation is still a key atmospheric component of drought, sophisticated techniques are now used to assess the deviation from normal levels of the total environmental moisture status. These techniques have enabled investigators to better understand the severity and length of drought events, as well as the extent of the affected area.

Drought has been defined in numerous ways. Some authorities consider it to be merely a period of below-normal precipitation, while others relate it to the likelihood of forest fires. Drought is also said to occur when the yield from a specific agricultural crop or pasture is significantly less than expected. It has also been defined as a period when soil moisture or groundwater decreases to a critical level.

Drought was identified early in the twentieth century by the US Weather Bureau as any period of twenty-one or more days when precipitation was 30 percent or more below normal. Subsequent examination of drought events that were identified by this method revealed that soil moisture reserves were often elevated during these events to the extent that there was sufficient water to support vegetation. It was also determined that the amount of precipitation preceding the drought event was ample or even heavy. Thus, it became apparent that precipitation should not be used as the sole measure to identify drought. Subsequent research has shown that the moisture status of an area is affected by additional factors.

Further developments in drought identification during the middle decades of the twentieth century began to focus on the moisture demands that are associated with the evapotranspiration in an area. Evaporation is primarily the process by which water in liquid form is converted into water vapor at the surface and conveyed into the atmosphere. To a lesser extent, this also includes the conversion of “solid water,” as ice and snow, ultimately to vapor either directly or through an intermediate liquid state. Transpiration refers to the loss of moisture by plants to the atmosphere. Although evaporation and transpiration can be studied and measured separately, it is convenient to consider them in applied climatological studies as the single process of evapotranspiration.

There are two ways to define evapotranspiration. The first is actual evapotranspiration, which is the actual or real rate of water-vapor return to the atmosphere from the earth and vegetation; this process could also be called “water use.” The second is potential evapotranspiration, which is the theoretical rate of water loss to the atmosphere if one assumes continuous plant cover and an unlimited supply of water. This process could also be called “water need,” as it indicates the amount of soil water needed if plant growth is to be maximized. Procedures have been developed that enable one to calculate the potential evapotranspiration for any area as long as one has monthly mean temperature and precipitation values.

Some drought-identification studies have focused on agricultural drought, looking at the adequacy of soil moisture in the root zone for plant growth. This procedure involved the evaluation of precipitation, evapotranspiration, available soil moisture, and the water needs of plants. The goal of this research was to determine drought probability based on the number of days when soil moisture storage is reduced to zero.

Evapotranspiration was also used by the Forest Service of the US Department of Agriculture when it developed a drought index to be used by fire-control managers. The purpose of the index was to provide a measure of flammability that could create forest fires. This index has limited applicability to nonforestry users, as it is not effective for showing drought as an indication of total environmental stress.

Palmer Drought Index

One of the most widely adopted drought-identification techniques was developed by W. C. Palmer in 1965. The method, which became known as the Palmer Drought Index (PDI), defines drought as the period of time—usually measured in months or years—when the actual moisture supply at a given location is consistently less than the climatically anticipated or appropriate supply of moisture. The calculation of this index requires the determination of evapotranspiration, soil moisture loss, soil moisture recharge, surface runoff, and precipitation. The Palmer Drought Index values range from approximately +4.0 for an extremely wet moisture status class to –4.0 for extreme drought. Normal conditions have a value close to 0. Positive values indicate varying stages of abundant moisture, whereas negative values indicate varying stages of drought.

Although the Palmer Drought Index is recognized as an acceptable procedure for incorporating the role of potential evapotranspiration and soil moisture in magnifying or alleviating drought status, there have been some criticisms of its use. For example, the method produces a dimensionless parameter of drought status that cannot be directly compared with other environmental moisture variables, such as precipitation, which are measured in units (centimeters, millimeters) that are immediately recognizable. In addition, the index is not especially sensitive to short drought periods, which can affect agricultural productivity.

In order to address these shortcomings, other researchers use water-budget analysis to identify deviations in environmental moisture status. The procedure is similar to the Palmer method inasmuch as it incorporates the environmental parameters of precipitation, potential evapotranspiration, and soil moisture. However, the moisture status departure values are expressed in the same units as precipitation and are therefore dimensional. Drought classification using this index method ranges from approximately 25 millimeters for an above-normal moisture status class to –100 millimeters for extreme drought. The index would be close to 0 for normal conditions.

Significance

Drought is invariably associated with some form of water shortage, yet many regions of the world have regularly occurring periods of dryness. Three different forms of dryness—perennial, seasonal, and intermittent—have a temporal dimension. Perennially dry areas include the major deserts of the world, such as the Sahara, Arabian, and Kalahari. Precipitation in these areas is not only very low but also very erratic. Seasonal dryness is associated with regions where the bulk of the annual precipitation comes during a few months of the year, leaving the rest of the year without rain or other precipitation. Intermittent dryness is associated with those instances where the overall precipitation is reduced in humid regions or where the rainy season in seasonally dry areas does not occur or is shortened.

The absence of precipitation when it is normally expected creates variable problems. For example, the absence of precipitation for one week in an area where daily precipitation is the norm would be 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 areas that have one rainy season, a 50 percent reduction in precipitation would be considered a drought. In regions that have two rainy seasons, the failure of one could lead to drought conditions. Thus, the word “drought” is a relative term, as it has different meanings in different climatic regions.

User demands also influence drought definition. Distinctions are often made among climatological, agricultural, hydrologic, and socioeconomic drought. Climatological, or meteorological, drought occurs at irregular periods of time, usually lasting months or years, when the water supply in a region falls far below the levels that are typical for that particular climatic regime. The degree of dryness and the length of the dry period are used as the definition of drought. For example, drought in the United States has been defined as occurring when there is less than 2.5 millimeters of rain in a forty-eight hour period. In Great Britain, drought has been defined as occurring when there are fifteen consecutive days with less than 0.25 millimeter of rain for each day. In Bali, Indonesia, drought has been considered as occurring if there is no rain for six consecutive days.

Agricultural drought occurs when soil moisture becomes so low that plant growth is affected. Drought must be related to the water needs of the crops or animals in a particular place, since agricultural systems vary substantially. The degree of agricultural drought also depends on whether shallow-rooted or deep-rooted plants are affected. In addition, crops are more susceptible to the effects of drought at different stages of their development. For example, inadequate moisture in the subsoil in an early growth stage of a particular plant will have minimal impact on crop yield as long as there is adequate water available in the topsoil. However, if subsoil moisture deficits continue, then the yield loss could become substantial.

Hydrologic drought definitions are concerned with the effects of dry spells on surface flow and groundwater levels. The climatological factors associated with the drought are of lesser concern. Thus, a hydrological drought for a particular watershed is said to occur when the runoff falls below some arbitrary value. Hydrological droughts are often out of phase with climatological and agricultural droughts and are also basin-specific; that is, they pertain to the particular watersheds that they affect.

Socioeconomic drought includes features of climatological, agricultural, and hydrological drought and is generally associated with the supply and demand of some type of economic good. For example, the interaction between farming (demand) and naturally occurring events (supply) can result in inadequate water for both plant and animal needs. Human activities, such as poor land-use practices, can also create a drought or make an existing drought worse. The Dust Bowl in the Great Plains provides a ready example of the symbiotic relationship between drought and human activities.

In a sense, droughts differ from other major geophysical events such as volcanic eruptions, floods, and earthquakes because they are actually nonevents—that is, they result from the absence of events (precipitation) that should normally occur. Droughts also differ from other geophysical events in that they often have no readily recognizable beginning and take some time to develop. In many instances, droughts are only recognized when plants start to wilt, wells and streams run dry, and reservoir shorelines recede.

There is wide variation in the duration and extent of droughts. The length of a drought cannot be predicted, as the irregular patterns of atmospheric circulation are not fully known and remain unpredictable. A drought ends when the area receives sufficient precipitation and water levels rise in the wells and streams. A study by the Woods Hole Research Center found that drought recoveries took increasingly longer amounts of time over the twentieth century and that shorter intervals between droughts would likely become the norm in the twenty-first. By the third decade of the twenty-first century, disruption to the global climate connected to human activities such as the burning of fossil fuels had been scientifically argued to have resulted in extreme high temperatures over consecutive years, less precipitation in certain regions, greater evapotranspiration, more rainfall than snowfall, and drier winters, thus exacerbating drought conditions in drought-prone regions, such as the American West. In the summer of 2021, measurements taken using both the PDI and the US Drought Monitor indicated that the Western United States, considered the site of a megadrought that had stretched over decades, was experiencing the largest percentage of widespread drought in several decades. As temperatures had consistently reached record highs throughout the late 2010s and into the 2020s, experts using the PDI method reported that 100 percent of this region of the United States was affected by drought, contended to be particularly intense due mostly to human-driven climate change and global warming, by mid-2021. Such excessively dry conditions had only worsened the devastating, widespread wildfires that had been occurring for prolonged periods in several parts of the state of California. Drastic reductions in water supplies within the Western region, especially sources such as the Colorado River, sparked even greater discussion about how to best react and adapt to these kinds of drought conditions as they had become more common.

Meanwhile, 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 rise 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. There were also a high number of droughts in 2024. Additionally, the National Centers for Environmental Information reported that global temperatures between January and November 2024 were the warmest since global records began.

Principal Terms

adiabatic: a change of temperature within the atmosphere that is caused by compression or expansion without transfer of heat into or out of the system

desertification: the relatively slow, natural conversion of fertile land into arid land or desert

evapotranspiration: the combined water loss to the atmosphere from both evaporation and plant transpiration

Palmer Drought Index: a widely adopted quantitative measure of drought severity that was developed by W. C. Palmer in 1965

potential evapotranspiration: the water needed for growing plants, accounting for water loss by evaporation and transpiration

precipitation: any form of liquid water or ice that falls from the atmosphere to the ground

Sahel: the semiarid southern fringe of the Sahara in West Africa that extends from Mauritania on the Atlantic coast to Chad in the interior

soil moisture: water that is held in the soil and that is therefore available to plant roots

subsidence: in meteorology, the slow descent of air that becomes increasingly dry in the process, usually due to an area of high pressure

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