Drought has been a common feature of the American landscape during the past several decades. These droughts have resulted in significant economic, environmental, and social impacts, underscoring our continuing and possibly increasing vulnerability to this natural hazard. Drought should not be viewed as merely a physical phenomenon. It is the result of an interplay between a natural event (precipitation deficiencies due to natural climatic variability on varying timescales) and the demand placed on water supply by human-use systems. Drought is among the most complex and least understood of all natural hazards, affecting more people than any other hazard. The droughts of the early to mid-1980s in Africa, for example, affected more than 40 million people. Although drought is a recurring and inevitable feature of climate, we have rarely planned for its occurrence. Instead, most governments have reacted to drought through the process commonly referred to as crisis management. Recognizing this, the American Meteorological Society recommends that appropriate government institutions initiate or increase drought planning efforts. Before this can take place, however, governments must understand the phenomenon of drought, its effects, mitigation technologies, and preparedness strategies.
Drought differs from other natural hazards in several ways. First, it is a "creeping phenomenon," making its onset and end difficult to determine. The effects of drought accumulate slowly over a considerable period of time and may linger for years after the termination of the event. Second, the absence of a precise and universally accepted definition of drought adds to the confusion about whether or not a drought exists and, if it does, its severity. Third, the societal impacts of drought are less obvious and extend over a larger geographical area than damages that result from other natural hazards. Drought seldom results in structural damage. For these reasons the quantification of impacts and the provision of disaster relief is a far more arduous task than it is for other natural hazards.
Drought is a normal, recurring feature of climate; it occurs in virtually all climatic regimes. It is a temporary aberration, in contrast to aridity, which is a permanent feature of regional climate. Drought should be considered relative to some long-term average condition of balance between precipitation and evapotranspiration (ET) in a particular area, a condition often perceived as "normal." Common to all types of drought is the fact that they originate from a deficiency of precipitation that results in water shortage for some activity or for some group. It is also commonly recognized that other meteorological elements, such as temperature, wind, and relative humidity, may aggravate the severity and impacts of drought in some instances.
Definitions of drought reflect many disciplinary perspectives and therefore incorporate different physical, biological, and socioeconomic variables in their definitions. Most definitions are region specific, reflecting differences in climatic characteristics. For this reason, it is usually difficult to transfer definitions derived for one region to another.
Drought definitions and types are often grouped into four types: meteorological or climatological, agricultural, hydrological, and socioeconomic. Meteorological and climatological drought is defined in terms of the departure from normal and the duration of the event. Drought is a slow-onset phenomenon that usually takes at least three months to develop and may last for several seasons or years. Agricultural drought links the various characteristics of meteorological drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential ET, soil-water deficits, and so forth. Agricultural drought is largely the result of a deficit of soil moisture. A plant's demand for water is dependent on prevailing weather conditions, biological characteristics of the specific plant, its stage of growth, and the physical and biological properties of the soil.
Hydrological droughts are concerned with the effects of periods of precipitation shortfall on surface or subsurface water supply, rather than with precipitation shortfalls directly. Hydrological droughts are typically out of phase or lag the occurrence of meteorological and agricultural droughts. More time elapses before precipitation deficiencies show up in these components of the hydrological system. As a result, impacts are out of phase with those in other economic sectors. Socioeconomic drought associates the supply and demand of some economic good with elements of meteorological, agricultural, and hydrological drought. This approach to defining drought suggests that the time and space processes of supply and demand are the two basic processes that should be included in an objective definition of drought.
It is rare for drought not to occur somewhere in North America each year. Each region has its "drought of record," commonly used by water managers and planners to design water supply facilities. The most notable drought of the last decade occurred in 1988, although the 1996 drought in the Southwest and southern Great Plains states also resulted in serious consequences for many sectors. According to the Palmer Drought Severity Index,1 nearly 40% of the country was affected by severe to extreme drought in 1988; an additional 30% of the country experienced moderate drought conditions. The impacts of this drought were widespread, affecting such diverse sectors as agriculture, transportation, energy, and recreation. The direct impacts of the 1988 drought have been estimated at nearly $40 billion. The environmental and social costs of this drought were also substantial.
Droughts differ in three essential characteristics: intensity, duration, and spatial coverage. Intensity refers to the degree of the precipitation shortfall and/or the severity of impacts associated with the shortfall. Intensity is generally measured by the departure of some climatic index from normal and is closely linked to duration in the determination of impact. Impacts are also closely related to the timing and effectiveness of rainfall. The most widely used index in the United States is the Palmer Drought Severity Index, although other indices such as the Surface Water Supply Index2 and the Standardized Precipitation Index3 have become quite popular in recent years in monitoring regional climatic conditions.
Another distinguishing feature of drought is its duration. Drought develops slowly and impacts accumulate as conditions persist for seasons or years. Impacts are usually first apparent in agriculture, but gradually ripple to other sectors such as transportation, energy, recreation and tourism, and urban water supplies. For example, hydrologic storage systems (e.g., reservoirs, lakes, and rivers) are often used for multiple and competing purposes. Competition for water in these storage systems escalates during drought, and conflicts between water users increase significantly. Impacts continue well beyond the end of the meteorological event because the recovery time for water stored in surface and subsurface systems is quite long in many cases.
Each drought also has unique spatial characteristics. Drought occurs somewhere in the United States almost every year. The percentage of the total area for the contiguous United States affected by severe to extreme drought has been highly variable over the past century. The largest area affected by drought occurred in 1934, when more than 65% of the nation experienced severe or extreme drought. Other significant drought episodes, measured by the percent area affected, occurred in the 1890s, 1910, 192526, 193140, mid-1950s, 196465, 197677, 1983, 198892, 1994, and 1996. The spatial characteristics of these droughts differed substantially.
Droughts are manifestations of persistent large-scale disruptions in the global circulation pattern of the atmosphere. Empirical studies conducted over the past century have never shown that drought is exclusively the result of a single cause. Drought typically results from a synergistic interaction between regional and remote influences. Forecast model experiments during the past few years indicate that drought conditions themselves may play a role in the perpetuation of the drought through a feedback between the land surface and the overlying atmosphere that reinforces the drought sustaining circulation features. In a global context, extensive research during the past two decades clearly indicates the central role of tropical Pacific sea surface temperature variations, associated with the El NiñoSouthern Oscillation (ENSO) phenomenon, in year- to-year global climate variations. The effect of these ocean variations is transmitted to remote areas of the globe through recurrent, seasonally varying patterns of atmospheric circulation anomalies referred to as teleconnections. These teleconnections affect the precipitation regime over much of the Tropics, and over large areas of the extratropics as well, including Australia, eastern Asia, southern Africa, and regions of both North and South America. Observational studies and model experiments have also demonstrated a significant link between Atlantic sea surface temperatures and precipitation over the drought-prone areas of the African Sahel and northeast Brazil.
Experiments with coupled atmosphereocean forecast models, that is, models that predict the simultaneous evolution of the ocean and atmosphere, provide promising evidence that the ENSO cycle fluctuations may exhibit a useful degree of predictability for up to a year in advance.
For most parts of the world, drought remains a threat that may occur with little or no warning. However, many countries and some regions have developed drought early warning systems that monitor numerous components of the hydrologic system. These systems offer the opportunity for governments and others to track trends and patterns of precipitation and other climatic variables, as well as streamflow, groundwater and reservoir levels, snowpack, and soil moisture. Vegetation conditions can be monitored using satellite-derived data. Critical information can thus be provided to decision makers in a timely manner. Our ability to monitor and disseminate critical drought-related information has been enhanced by new technologies such as automated weather stations, satellites, computers, and improved communication techniques. Some of the deficiencies of previous drought response efforts have been associated with the lack of adequate early warning systems and insufficient information flow within and between levels of government.
The economic, social, and environmental impacts suffered because of drought are the product of both the natural event (i.e., meteorological event) and the vulnerability of society to extended periods of precipitation deficiency. The impacts of future drought occurrences will be determined not only by the frequency and intensity of meteorological drought, but also by the number of people at risk and their degree of risk. The degree of risk is a function of exposure, vulnerability, and response. As demand for water and other shared natural resources increases as a result of population growth and regional shifts in population, future droughts can be expected to produce greater impacts, with or without any increase in the frequency and intensity of meteorological drought. If projected changes in climate because of increasing concentrations of greenhouse gases do occur, there will be concomitant changes in regional hydrology, further aggravating the nation's already high sensitivity to climate variability.
The impacts of past droughts have been exacerbated by the absence of preparedness plans. Plans can improve the coping capacity of local, state, and federal governments, reducing impacts and the need for government intervention. Since 1982, the number of states with drought plans has increased from 3 to 28 and several states are in the plan development process. Generally these plans are aimed at providing a more organized, better coordinated response rather than reducing long-term vulnerability to future drought episodes. These plans, however, represent an important first step in recognizing our current inability to effectively cope with drought. Drought plans should include the development of an integrated climate monitoring and delivery system for distributing information to decision makers in a timely manner. Such a plan also should include development of a drought monitoring system, based largely on meteorological and climatic information but also including improved methodologies for early assessment of drought impacts. Policies that promote the development and implementation of regionally appropriate drought mitigation measures today will help to reduce the future costs of drought, whether or not future changes in climate alter the frequency and intensity of meteorological drought.
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© 1997 American Meteorological Society