Meteorological Drought

(Adopted by AMS Council on 23 December 2003)
Bull. Amer. Met. Soc., 85

 

The American Meteorological Society recommends that appropriate institutions at the local, state, regional, federal, and international levels initiate or increase drought planning, drought preparedness, drought warning, and drought mitigation efforts. Efforts must be made to increase knowledge and information about climate variability, drought impacts, mitigation technologies, societal response such as conservation, and preparedness strategies. The following statement provides a brief background and overview on drought and the challenge it presents.

Introduction

Drought is a common feature of the American landscape and a phenomenon that quietly wreaks havoc in many portions of the globe. It is the unavoidable result of our climate’s variability —– variability that sometimes leaves areas far short of their average water supplies for months or years at a time. It is more than just a moisture deficit, however. It is the result of a complex interplay between natural precipitation deficiencies on varying time and space scales and can be exacerbated by human water demand and inefficiencies in water distribution and usage. Drought affects various sectors of society in different ways, and thus can be defined in many ways. One person’s drought is another’s fair weather. Yet, it is the most far-reaching climate-related disaster year in and year out causing hardship to millions of people. According to the National Climatic Data Center’s “Billion Dollar U.S. Weather Disasters” compilation, losses from drought and related wildfires exceeded 150 billion dollars during the period 1980–2003, accounting for roughly 40 percent of all losses from major weather events, including floods, hurricanes, and severe storms.

Drought as a Natural Disaster

Most natural hazards are singular events that cause structural damage and human injury. Drought is much different in that it is the cumulative effect over time of deficient precipitation and water supply that is followed by a trail of impacts that occur on varying time and space scales. Some agricultural and wildfire impacts can be swift and severe when moisture is lacking at critical times. Other impacts such as wind erosion and desertification take place more gradually. Surface and ground water shortages also develop gradually but can still result in sudden and profound impacts when water levels drop below critical thresholds. Droughts typically don’t damage structures (except for the collateral phenomena of wildfires), and their diverse and diffuse impacts are usually spread over time and space. For these reasons, the provision of disaster relief is a far more complex task than it is for other natural hazards.

Drought Types and Definitions

Drought is often grouped into four basic types: 1) meteorological or climatological, 2) agricultural, 3) hydrological, and 4) socioeconomic. Meteorological and climatological drought is defined in terms of the magnitude of a precipitation shortfall and the duration of this shortfall event. Agricultural drought links the various characteristics of meteorological drought to agricultural impacts, focusing on precipitation shortages, differences between actual and potential evapotranspiration, and soil moisture deficits. Agricultural drought is largely the result of a deficit of soil moisture and is most commonly applied to non-irrigated agricultural regions. A plant's demand for water is dependent on prevailing weather conditions, biological characteristics of the specific plant and its stage of growth, as well as the physical and biological properties of the soil.

Hydrological droughts are related to the effects of periods of precipitation shortfall on surface or subsurface water supply, rather than to precipitation shortfalls directly. Hydrological droughts are typically out of phase with 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. Where irrigation is necessary for agriculture, agricultural drought is really determined by hydrological drought.

Socioeconomic drought is associated with 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 scales of supply and demand should be included in an objective definition of drought.

Characteristics of Drought

Definable characteristics of drought include intensity, duration, spatial extent, and timing. Intensity commonly refers to the magnitude of the precipitation deficit and how quickly it develops. History shows us that each drought is unique, but common features of the most severe droughts include long duration, and large moisture deficits with a large areal extent, particularly during a climatological wet season. These are the droughts with the most far-reaching human and ecological impacts.

Impacts

The impacts from drought tend to follow predictable progressions that vary as a function of societal wealth and socioeconomic activities. In the past, and in less developed regions of the world, the primary impacts were crop failures followed by food shortages, clean drinking water shortages and eventual related health problems, famine, energy shortages, mass migrations, and political unrest. In the developed nations of the world, food shortages and severe health hazards are less of a problem. Instead, the impacts are more economic–related, such as crop production losses, higher food costs, higher costs of transportation and energy as well as reduced recreational opportunities, and domestic and industrial water restrictions.

Ecological impacts also are very important but more difficult to track and quantify. Wildfire is the one drought impact that is most like other natural disasters in that the impacts are immediate and structural and can affect both rich and poor in similar ways.

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 migration to drought-prone areas, urbanization, environmental degradation, government policies, land use changes, technology, and other factors, 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 or other factors do occur, there will be concomitant changes in regional hydrology, possibly aggravating the nation's sensitivity to climate variability. Indeed, the 2001 Third Assessment Report of the Intergovernmental Panel on Climate Change states that it is likely that the frequency and intensity of droughts will increase during the 21st Century, especially over mid-latitude continental interiors. In addition, the 2001 U.S. National Assessment of Climate Change finds that reduced water runoff in summer and increased winter runoff coinciding with increased water demands are likely to compound current stresses, including those to agriculture, water-based transportation, water supplies and ecosystems.

Monitoring

Routine monitoring of all components of the hydrologic cycle is the basis for objective recognition of drought and preparing to deal with impacts. Tracking precipitation departures from average over long periods of time is an important first step. Unfortunately, the precipitation observational record is barely more than a century long in most populated regions of the U.S., and much shorter in remote and mountainous locations. This limits our ability to characterize trends and variations in average precipitation over long time scales. Use of “proxy” data that are related to precipitation variations, such as tree rings, has been successful in extending the record up to several thousand years in some areas. Monitoring other climatic variables, as well as streamflow, groundwater and reservoir levels, snowpack, and soil moisture, provides a more comprehensive perspective. Vegetation conditions can often 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 simply been associated with the lack of adequate monitoring.

Prediction and Warning

Intelligent monitoring of drought precursors and historical perspective remains the best tool for drought prediction and warning. However, progress in understanding large-scale global and regional atmospheric–oceanic phenomena continues to provide hope for drought prediction and warning with longer lead times. Droughts are manifestations of persistent large-scale variations in the global circulation pattern of the atmosphere. 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 drought-sustaining circulation features.

In a global context, extensive research during the past two decades clearly indicates one important influence to be tropical Pacific sea surface temperature variations, associated with the El Nińo–Southern 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. Experiments with coupled atmosphere–ocean 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. 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. In addition, ocean-atmosphere oscillations at longer time scales have recently been recognized as leading to extended decadal and longer periods of wetter or drier conditions in some areas.

Preparedness

We cannot avoid drought, and our predictions will never be perfect, but we can reduce its impacts. One way is to plan ahead. 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 36 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 that our ability to effectively cope with drought is currently limited. 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, climatic, and hydrologic information. An effective monitoring system will aid in the development of improved drought assessment methodologies by providing early warning of drought impacts, and well as a context for planning for drought events against the backdrop of longer-term climate trends and variations. 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.

[This statement is considered in force until September 2013 unless superseded by a new statement issued by the AMS Council before this date.]

 







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