During the past year, drought conditions across some areas of the West and the nation's midsection have improved, while sections of the Southeast have begun to experience severe drought conditions. Significant monetary losses were felt across the nation last year, with lost crops and numerous wildfires. Drought impacts the soil moisture available to growing crops, an important factor as we approach the end of the 2010 agricultural season. Precipitation across the nation during September 2010 was below during average. One could inspect the tables of monthly precipitation (in hundredths of an inch) that are furnished by the National Weather Service several days after the conclusion of each month for nearly 250 selected U.S. cities. Many of the cities across the Southeast, the Northeast and the Southwest had monthly precipitation totals in September 2010 that were below "normal" (or the averages from the 1971-2000 climatological reference interval). Sections of the western Gulf Coast, the Plains, the middle and upper Mississippi Valley and the Middle Atlantic States reported above average precipitation totals for September.
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 25 September 2010) shows a widespread region of moderate to extreme drought across the Mid-South, the Southeast and the Middle Atlantic States, along with sections of the central Rockies and the eastern slopes of the Sierras. On the other hand, unusually moist to extremely moist conditions prevail across the most of the Plains, the upper Mississippi Valley and western Great Lakes. Near normal soil moisture conditions prevailed over the remainder of the nation. (NOTE: Some sections of the Southeast and the Middle Atlantic States received torrential rain during the last several days of September that helped alleviate the drought in many of these regions. Changes in the Drought Index should be anticipated this coming week that should reflect these rains. EJH)
In the last several years, the National Drought Mitigation Center, 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 (28 September 2010) shows severe to extreme hydrological drought extending across the Gulf Coast and sections of the Middle Atlantic States, while moderate to severe agricultural and hydrological drought was found across scattered areas of the Great Basin and central Rockies. 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 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 September 2010 by the Climate Prediction Center and valid through December 2010) indicates improvement in the drought conditions across the northern tier of states, especially in the northern Great Lakes and New England, along with some marginal improvement in scattered areas across the Gulf Coast, the Ohio Valley and the Atlantic Seaboard. Drought conditions were anticipated to persist or develop across the Great Basin, the lower Mississippi and Ohio Valleys and the Southeast.
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, 2010, The American Meteorological Society.
