During the past year, drought conditions across the Southeastern States have moderated, but many locations across the West, such as across California and Texas, continue to experience severe drought conditions. Significant monetary losses were felt across a large portion of the nation last year, with lost crops and numerous wildfires. Drought impacts the soil moisture available to growing crops, an important factor as we enter the 2009 agricultural season. Precipitation across the nation during September 2009 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 nation had monthly precipitation totals in September 2009 that were below "normal" (or the averages from the 1971-2000 climatological reference interval), with coastal sections of the Southeast experiencing the largest departures. Sections of the Southeast and the Mid-South 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 26 September 2009) shows a widespread region of moderate to extreme drought across much of the nation west of the Rocky Mountains, along with sections of the Midwest bordering Lake Superior, south Texas and scattered areas in the Appalachians. On the other hand, unusually moist to extremely moist conditions prevail across the central and northern Plains, the lower and middle Mississippi Valley and southern New England. Near normal soil moisture conditions prevailed over the remainder of the nation.
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 (29 September 2009) shows extreme to exceptional hydrological drought extending across southern Texas, while moderate to extreme agricultural and hydrological drought was found across northern Wisconsin and adjacent sections of Minnesota and Michigan's Upper Peninsula. 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 1 October 2009 by the Climate Prediction Center and valid through December 2009 indicates that drought conditions were expected to develop or persist across sections of the Northwest, primarily in Washington, Oregon and northern California. The regions of the Southeast, south Texas, the Great Lakes States that have been experiencing drought should experience definite improvement in drought conditions. Other drought-stricken areas in California, Nevada and northern Montana east of the Divide could experience varying degrees of improvement.
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, 2009, The American Meteorological Society.
