The Water Cycle
Portions reprinted courtesy of the NOAA National Weather Service Office of Meteorology

 

While much of the weather that we experience is brief and short-lived, drought is a more gradual phenomenon, slowly taking hold of an area and tightening its grip with time. In severe cases, drought can last for many years, and can have devastating effects on agriculture and water supplies.

Nationwide losses from the U.S. drought of 1988 exceeded $40 billion, exceeding the losses caused by Hurricane Andrew in 1992, the Mississippi River floods of 1993, and the San Francisco earthquake in 1989. In some areas of the world, the effects of drought can be far more severe. In the Horn of Africa the 1984–1985 drought led to a famine which killed 750,000 people.

NDVI Anomaly
One of the worst droughts of the Twentieth Century occurred in the Horn of Africa in 1984 and 1985. This image shows Normalized Difference Vegetation Index (NDVI) anomaly for August 1984. NDVI anomaly indicates the vigor of vegetation relative to the long-term average. Dark red indicates the most severe drought, light yellow areas are normal, and green areas have denser than normal vegetation.

In general, drought is defined as an extended period–a season, a year, or several years–of deficient rainfall relative to the statistical multi-year average for a region. However, dozens of more specific drought definitions are used around the world that are defined according to the lack of rain over various time periods, or measured impacts such as reservoir levels or crop losses. Because of the various ways drought is measured, an objective drought definition has yet to be produced upon which everyone can agree.

Drought can be defined according to meteorological, hydrological, or agricultural criteria.

Meteorological drought is usually based on long-term precipitation departures from normal, but there is no consensus regarding the threshold of the deficit or the minimum duration of the lack of precipitation that make a dry spell an official drought.

Hydrological drought refers to deficiencies in surface and subsurface water supplies. It’s measured as stream flow, and as lake, reservoir, and ground water levels.

Agricultural drought occurs when there is insufficient soil moisture to meet the needs of a particular crop at a particular time. A deficit of rainfall over cropped areas during critical periods of the growth cycle can result in destroyed or underdeveloped crops with greatly depleted yields. Agricultural drought is typically evident after meteorological drought but before a hydrological drought.
 

 

by Steve Graham
August 28, 2000

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Drought: the Creeping Disaster
Introduction
Drought Planning and Drought Indices
Improved Monitoring
Improved Forecasting

Related Links:
Measuring Vegetation (NDVI & EVI)

For the purposes of this Web site, our definition most closely resembles that of an agricultural drought. The regions displayed as "in drought" on the maps on this site show areas of reduced plant growth, relative to the historical average, caused by reduced precipitation.

The underlying cause of most droughts can be related to variations in large-scale atmospheric circulation patterns and the locations of anticyclones, or high-pressure systems. Sometimes, whirling masses of air separate from the main westerly airflow (analogous to whirlpools that form in rapidly flowing rivers) and effectively prevent the usual west-to-east progression of weather systems. When these "blocking systems" persist for extended periods of time, weather extremes such as drought, floods, heat waves, and cold snaps can occur.

In addition, analyses of climate records and experiments with climate models have led climatologists to believe that many of the droughts in tropical regions are associated with global patterns of sea surface temperature anomalies (differences from normal) such as El Niño. During an El Niño, the anomalously warm equatorial Pacific Ocean warms the overlying atmosphere, which leads to changes in large-scale atmospheric circulation patterns, and increased probabilities of drought in many parts of the world.

next: Drought Planning and Drought Indices

  Drought Stricken Oranges
One type of drought occurs when natural vegetation and agricultural crops don't receive enough water. (Photograph courtesy U.S. Department of Agriculture)

 Drought:  The Creeping Disaster

Drought Planning and Drought Indices

Drought planning usually involves picking or creating an index to identify and quantify anomalies. Drought indices assimilate thousands of bits of data on rainfall, temperature, snow pack, stream flow, and other water supply indicators into a comprehensible big picture that is far more useful than raw data for decision-making.

A sophisticated system for measuring drought was developed by National Weather Service meteorologist Wayne Palmer in 1965. Now called the Palmer Drought Severity Index (PDSI), it uses temperature and rainfall information in a formula to determine dryness and has become the semi-official drought index. The PDSI is most effective in determining long-term drought–a matter of several months–and is not as effective with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of negative numbers; for example, -2 is moderate drought, -3 is severe drought, and -4 is extreme drought. In mid August 2000, sections of many western states were in the -4 range.

Palmer Drought Index, August 19, 2000

Image courtesy NOAA Drought Information Center.

next: Improved Monitoring
back: Introduction

  pullquote

Drought: the Creeping Disaster
Introduction
Drought Planning and Drought Indices
Improved Monitoring
Improved Forecasting

Related Links:
Measuring Vegetation (NDVI & EVI)

 Drought:  The Creeping Disaster

Improved Monitoring

Careful monitoring of drought can ease its impacts, allowing people to take early actions that prevent harsh impacts later. Both U.S. government and private meteorologists are doing their part by producing better monitoring and forecast products. At the federal level, an inter-agency consortium involving the United States Department of Agriculture (USDA), the National Weather Service, and the National Drought Mitigation Center in Lincoln, Nebraska, began producing the Drought Monitor in 1999. The Monitor includes a weekly national map displaying dryness divided into five categories, or levels of intensity. The categories are based on readings from a number of different drought indices, giving the user a composite picture of many indicators. Drought information is updated daily through use of the thousands of observations available from cooperative weather observers. These three agencies also consult with numerous experts from other agencies and offices across the country so that the map can be tweaked to more effectively depict what is happening in the real world. The drought analysts also indicate on the map if forecast conditions over the next 2 weeks will result in significant changes to the drought situation.

U.S. Drought Monitor August 22, 2000

Image courtesy NOAA Drought Information Center.

next: Improved Forecasting
back: Drought Planning and Drought Indices

  pullquote

Drought: the Creeping Disaster
Introduction
Drought Planning and Drought Indices
Improved Monitoring
Improved Forecasting

Related Links:
Measuring Vegetation (NDVI & EVI)

 Drought:  The Creeping Disaster

Improved Forecasting

Improved remote sensing from satellites and radar, as well as the use of thousands of daily in-situ precipitation measurements, has dramatically improved drought-monitoring capabilities. However, the most exciting developments in mitigating drought impacts may be advances made in forecasting the conditions that result in drought. Meteorologists at the National Oceanic and Atmospheric Administrations’ (NOAA) Climate Prediction Center (CPC) are using medium-range forecast models to predict soil moisture two weeks into the future. For the longer term, meteorologists are using statistical techniques and historical drought information to construct analogues to current conditions. They then create forecasts up to several seasons ahead of time based on past events. CPC is also using sophisticated computer models that link ground and ocean conditions to the overlying atmosphere to create forecasts of temperature, precipitation, and soil moisture months ahead of time.

Drought Outlook

The drought outlook depicts general, large-scale trends based on subjectively derived probabilities guided by numerous indicators, including short and long-range statistical and dynamic forecasts. Short-term events—such as individual storms—cannot be accurately forecast more than a few days in advance. Image courtesy NOAA Drought Information Center.

back: Improved Monitoring

  pullquote

Drought: the Creeping Disaster
Introduction
Drought Planning and Drought Indices
Improved Monitoring
Improved Forecasting

Related Links:
Measuring Vegetation (NDVI & EVI)