CDC's Tracking Network uses data from the United States Drought Monitor (USDM) and the Standardized Precipitation Evapotranspiration Index (SPEI) to track drought trends in the United States.
The SPEI captures the effects of precipitation and increased temperatures on drought conditions. The SPEI is presented as the number of standard deviations from a normal climate period—in this case, 1981-2000—and ranges from +3 (wet) to -3 (dry), with drought designated as any value less than 0. A benefit of the SPEI is its historical record: it allows trends to be tracked over time starting as early as 1895.
The USDM was established in 1999, and is a weekly map of drought conditions produced jointly by the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture, and the National Drought Mitigation Center at the University of Nebraska-Lincoln. USDM values are based on measurements of climatic, hydrologic, and soil conditions as well as reported impacts and observations from more than 350 experts around the country. It is a composite index that includes many indicators: values range from D0 (abnormally dry, transitioning in or out of drought) to D4 (exceptional drought). Data are available from the week of January 4, 2000 through the present.
Although many factors influence how drought will affect a community, drought trend data and other related indicators can be used to prepare for and prevent potential health risks. Understanding drought trends is important for public health professionals, water and sanitation officials, and policy makers for community planning purposes, allocating drought relief, and for other drought discussions. For more information about what drought and health data are available on the Tracking Network and how they can be used, visit Drought Indicators.