Drought continued across the West through the 2015 agricultural season, although other areas across the 48 contiguous United States experienced some expansion through late summer, followed by shrinkage of drought conditions during the fall. Welcome summer rains helped ease or eliminate drought conditions in many areas. According to the NOAA's National Centers for Environmental Information, 29 percent of the contiguous United States was experiencing moderate to exceptional drought conditions at the start of 2015, but at the end of the year approximately 19 percent of the the nation was under drought conditions.
As we approach the beginning of the 2016 agricultural season, drought continues across a large section of the West. This drought will impact the soil moisture available to growing crops, an important factor during this upcoming agricultural season. 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. Numerous cities across the nation had monthly precipitation totals in January 2016 that were below "normal" (or the averages from the 1981-2010 climatological reference interval). Only a few cities across the Arizona, New Mexico and west Texas reported above average precipitation totals for January. The above average precipitation during December 2015 across many areas helped minimize the moisture deficit accumulated during the meteorological winter season.
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, with unit-less values ranging from below -4 (severe drought) to above +4 (extremely moist),. 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 13 February 2016) shows a widespread region of moderate to extreme drought across the northern Rockies of Montana and Wyoming extending eastward across the northern Plains of Montana and North Dakota to northwestern Minnesota in the Midwest. A few scattered areas across the central Appalachians in West Virginia and the upper Ohio Valley were reporting moderate to severe drought. On the other hand, large sections of the Northwest along with the central Rockies, the central Plains, the Midwest and the Southeast had unusually moist to very moist conditions. Near normal soil moisture conditions prevailed over the remainder of the nation.
Beginning in 2000, the National Drought Mitigation Center, a group consisting of several governmental agencies along with the University of Nebraska-Lincoln, has maintained a US 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 short-term impacts of the drought upon agriculture and wildfire potential and long-term impacts on hydrology and ecology. (A description of the categories used in the drought classification scheme appearing on the National Drought Monitor map is available.) Their most recent map (16 February 2015) shows extreme to exceptional (D3 and D4) drought extending across a large section of California and adjacent areas of Nevada. Moderate to severe (D1 and D2) drought was also found across most of the western third of the nation. The drought across most of these areas would have both short and long-term consequences. Short-term drought that would typically have durations of less than six months affect agriculture and grasslands, while long-term drought exceeds six months and would affect rivers, lakes and groundwater (or "hydrology"), along with trees and other natural perennial vegetation (or "ecology"). Scattered areas across the northern and southern Plains and in the Northeast were reporting short-term abnormally dry (DO) and moderate drought (D1) conditions. 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 18 February 2016 by the Climate Prediction Center and valid through May 2016) indicates that the current drought conditions would persist across a large section of the interior Northwest, extending from northern California and Nevada into Oregon, Idaho and western Montana, as well as across sections of the Southwest running from southern California eastward across southern Nevada and into Arizona. Sections of central California and Nevada, along with scattered areas of Utah and the Rockies in Wyoming could experience improvement in drought conditions, with a few areas possibly being removed from drought classification.
Palmer, W.C., 1988 (12 July): 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@aos.wisc.edu
© Copyright, 2016, The American Meteorological Society.
