What constitutes a drought? The answer depends upon whom you ask. At least four types of drought can be defined. To the farmer, an agricultural drought represents an extended interval with a serious soil moisture deficiency during critical crop growth periods. A hydrologist would classify a hydrological drought as an extended interval containing abnormally low stream flow, lake levels and ground water reservoirs. Most meteorologists would consider a meteorological drought to occur when the accumulated precipitation is well below a prescribed amount that would depend upon the region or season. A fourth type would be socioeconomic drought, where the shortage of water affects humans, typically in terms of economic activities. Economic goods that may be affected may include water, agricultural food products and hydroelectric power.
Typically, the severity of a drought depends upon the lack of soil moisture, which is influenced by one or more of the following factors to include a lack of precipitation, low atmospheric humidity, high air temperature, strong winds, a lack of clouds and intense sunlight. In addition, the drought severity depends upon its duration and the size of the affected area.
The start of a drought usually is subtle in that few can tell when a spell of dry weather really constitutes the incipient phase of a drought. Similarly, the end of a drought is also difficult to assess, since one rain event does not necessarily "break a drought". The National Weather Service uses several indices to assess the severity of a drought. One of the most frequently used drought indices is the Palmer Drought Severity Index developed by Wayne Palmer in the 1960s. This Palmer Index incorporates temperature and rainfall information in a formula to determine abnormal dryness or wetness over prolonged time intervals, such as a month to years. The National Weather Service and U.S. Department of Agriculture jointly compute the Drought Index weekly for each of 344 climatological divisions across the United States. A map of the current Drought Index is available that shows those divisions experiencing drought with negative index values and varying shades of red, while those regions with excess precipitation have positive values and varying shades of green.
The most recent map (weekly index values ending 11 February 2012) shows moderate to extreme drought across sections of the Southwest including west Texas and New Mexico, in the Southeast especially across Florida, Georgia and South Carolina and over the upper Mississippi Valley primarily in Minnesota. On the other hand, unusually moist to extremely moist conditions prevailed across sections of the Midwest and the Northeast along with scattered areas of the Northwest, the Intermountain West and the northern Plains. Near normal soil moisture conditions according to the long-term Palmer Drought Severity Index prevailed over the remainder of the nation.
In the last several years, the National Drought Mitigation Center (NDMC), a group consisting of several governmental agencies along with the University of Nebraska-Lincoln, has maintained a Drought Monitor site that provides weekly updates of current drought information and forecasts of the potential for drought across the nation. Their current summary map of drought conditions attempts to improve upon the Palmer Drought Severity Index and synthesize five other indices, together with a certain amount of subjectivity to arrive at six drought severity categories. They attempt to show the impacts of the drought upon agriculture and wildfire potential. Their most recent map (14 February 2012) shows moderate to exceptional drought extending across many areas of the southern tier of states, especially over the southern Plains from Texas and southeastern New Mexico extending northward into southern Kansas as well as across the Southeast, running across northern Florida along with southern sections of Alabama, Georgia and South Carolina. An accompanying narrative entitled "National Drought Summary" also provides a five-day forecast and a 6- to 10-day outlook for precipitation and temperature across the country. This site also includes animated Drought Monitor maps for the prior six and twelve weeks. The NDMC Drought Impact Reporter is an interactive tool that permits exploration of the reported drought impacts across the nation. The goal is to help in risk management that could ultimately help shape drought related policy at the state and federal levels.
The US Seasonal Drought Outlook released on 16 February 2012 by NOAA's Climate Prediction Center and valid for the last two weeks of meteorological winter (February 2012) and all of meteorological spring (March through May) 2012 indicates many areas of the nation should experience the continuation or development of drought conditions, primarily across the southern tier of states from southern California eastward across the southern Rockies to the southern Plains and into the Southeast. Persistent drought conditions across the upper Mississippi Valley were also expected to expand westward. Improvement in the drought conditions was anticipated across the interior Northwest, with some slight improvement across scattered sections of the Mid-South, primarily along the central Gulf Coast.
Palmer, W.C., 1988 (12 Jul): The Palmer Drought Index: When and how it was developed. Weekly Weather and Crop Bulletin, 75 (28), 5.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2012, The American Meteorological Society.
