Recent efforts to improve drought monitoring and early warning in the United States and other countries have provided new early warning and decision-support tools and methodologies in support of drought preparedness planning and policy development, tte lessons learned can be helpful models for other countries to follow as they try to reduce the impacts of future droughts. An effective monitoring, early warning, and delivery system continuously tracks key drought and water supply indicators and climate-based indices and delivers this information to decision makers, ttis allows for the early detection of drought conditions and timely triggering of mitigation and emergency response measures, key ingredients of a drought preparedness plan.
Until recently, a comprehensive, integrated drought monitoring, early warning, and delivery system did not exist in the United States. Until the late 1990s, the country relied principally on the Palmer Drought Severity Index (Palmer 1965) to track moisture conditions and anomalies across the country, ttis index, while having some advantages, has a number of key disadvantages, including the fact that it is slow to detect emerging drought conditions. A series of severe drought years between 1987 and 1992 highlighted both the vulnerability of the agricultural and other sectors to drought and the inability of the PDSI to adequately monitor these conditions.
Although drought is a normal part of the climate of all regions of the United States and occurs somewhere in the country each year, drought conditions have been especially severe in the period from 1996 to 2006. However, the drought episodes from 1996 to 1999 brought considerable attention to the drought issue because of the magnitude of the impacts experienced, the largely ineffective response of state and federal government, and the deficiencies of the nation's drought monitoring system. A partnership emerged in 1999 between the NDMC at the University of Nebraska-Lincoln, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Agriculture (USDA) with the goal of improving the coordination and development of new drought monitoring tools, tte U.S. Drought Monitor (USDM) became an operational product on August 18,1999 (Svoboda et al. 2002). tte USDM is a weekly product that is posted on the website of the NDMC (http://drought.unl.edu/monitor/monitor.html). ttis website has become a web-based portal for drought and water supply monitoring.
tte development of the USDM was timely because many regions of the country have been affected over several consecutive years since 1999 and on more than one occasion. Some regions of the country experienced as many as 5 to 7 consecutive drought years, and drought conditions are ongoing today in many parts of the country, tte USDM successfully integrates information from multiple parameters (i.e., climate indices and indicators) and sources to assess the severity and spatial extent of drought in the United States on a weekly basis. It is a blend of objective analysis and subjective interpretation, ttis map product has been widely accepted and is used by a diverse set of users to track drought conditions across the country. It is also used for policy decisions on eligibility for drought assistance, tte USDM represents a weekly snapshot of current drought conditions. It is not intended to be a forecast and is illustrated in Fig. 2.2.
ttis assessment includes the 50 U.S. states, Pacific possessions, and Puerto Rico, tte product consists of a color map, showing which parts of the United States are suffering from various degrees of drought, and accompanying text, tte text describes the drought's current impacts, future threats, and prospects for improvement. tte USDM is by far the most user-friendly national drought monitoring product currently available in the United States. Currently, the Internet is the pri-
mary distribution vehicle, although the map also appears in local and national newspapers and on television. A single weekly map illustrates the drought pattern in each year. All USDM maps since 1999 are archived on the website and available to users for comparison.
Because no single definition of drought is appropriate in all situations, agricultural and water planners and others must rely on a variety of data or indices that are expressed in map or graphic form, tte authors of the USDM rely on several key indicators and indices, such as the Palmer Drought Severity Index, the Standardized Precipitation Index, stream flow, vegetation health, soil moisture, and impacts. Ancillary indicators (e.g., Keetch Byram Drought Index, reservoir levels, Surface Water Supply Index, river basin snow water equivalent, and pasture and range conditions) from different agencies are integrated to create the final map. Electronic distribution of early drafts of the map to field experts throughout the country provides excellent ground truth for the patterns and severity of drought illustrated on the map each week.
tte USDM classifies droughts on a scale of one to four (D1-D4), with D4 reflecting an exceptional drought event (i.e., 1 in 50 year event). A fifth category, DO, indicates an abnormally dry area, tte USDM map and narrative identify general drought areas, labeling droughts by intensity from least to most intense. DO
areas (abnormally dry) are either heading into drought or recovering from drought but still experiencing lingering impacts.
tte USDM also shows which sectors are presently experiencing direct and indirect impacts, using the labels A (agriculture-crops, livestock, range, or pasture) and W (water supplies). For example, an area shaded and labeled as D2 (A) is in general experiencing severe drought conditions that are affecting the agricultural sector more significantly than the water supply sector, tte map authors are careful to not bring an area into or out of drought too quickly, recognizing the slow-onset characteristics of drought, the long recovery process, and the potential for lingering impacts.
tte methodology associated with the USDM has now been applied to the production of the North American Drought Monitor (NADM), a collaborative project between the United States, Mexico, and Canada, tte partnership began in 2002 in an attempt to map drought severity and spatial pattern across the North American continent. Multiple indices and indicators are used to map drought conditions, similar to the procedure used to create the USDM. Responsibility for this product is shared by NOAA's National Climatic Data Center, the U.S. Department of Agriculture, and the National Drought Mitigation Center at the University of Nebraska in the United States; the National Water Commission in Mexico; and Environment Canada and Agriculture Canada, ttis product is prepared on a monthlybasis and is an excellent example of international cooperation on drought monitoring.
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