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 January 2009 was well below during average, making the recently-concluded month the fifth driest January since a sufficiently dense and reliable climate network began in 1895. 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 January 2009 that were below "normal" (or the averages from the 1971-2000 climatological reference interval), with the southern Plains and the Southwest experiencing the largest departures. Only a few locations across the northern Plains and the northern Rockies had above average precipitation totals for January.
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 14 February 2009) shows a widespread region of moderate to extreme drought across portions of Texas, northern California and southwestern Wyoming, with moderate drought conditions across sections of Florida, coastal North Carolina and northeastern Wisconsin. On the other hand, unusually moist to extremely moist conditions prevail across the Pacific Northwest, the northern and central Plains, sections of the Midwest and much of the Northeast. 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 (17 February 2009) shows extreme to exceptional agricultural and hydrological drought extending across southern Texas, much of California and sections of the Southeast. 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 19 February 2009 by the Climate Prediction Center and valid through May 2009 indicates that drought conditions were expected to develop or persist across the Southeast, including Florida, the southern Plains that include Texas, Oklahoma and New Mexico and sections of the Southwest, primarily across southern and central California and central Nevada. Sections of northern California, the northern Great Basin along with sections of the western Great Lakes and the Appalachians 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.
