11.1 DETERMINING DROUGHT SEVERITY AND EXTENT: THE U.S. NATIONAL DROUGHT MONITOR Michael J. Hayes* and Mark D. Svoboda University of Nebraska, Lincoln, Nebraska 1. INTRODUCTION In August 1999, a new product providing a weekly assessment of drought conditions for the United States was released for the first time. This product is called the Drought Monitor. Since then, the Drought Monitor map and accompanying text have received a large amount of attention. The original goal of the Drought Monitor was to provide a relatively “simple” classification of drought severity over a large scale based on a variety of indicators that could be easily interpreted by users such as the public, media, decision- and policy-makers, and the scientific community. The Drought Monitor does not attempt to capture local conditions that may vary considerably from the larger scale, nor is it a “forecast” but rather a weekly “snapshot” of the current conditions (Svoboda et al. 2002). The Drought Monitor began as a partnership of three agencies. Now, nine authors from four agencies take turns developing the Drought Monitor map each week. The four agencies are: NOAA’s Climate Prediction Center, NOAA’s National Climatic Data Center, USDA, and the National Drought Mitigation Center (NDMC). Each author develops the map for a two- or three-week period, and then it rotates to a different author. In addition, there is a very important group of “local experts” that provide input to the Drought Monitor author each week. This group of experts has now grown to more than 150, and their input is critical in validating the Drought Monitor’s accuracy across the country. 2. DROUGHT M ONITOR MAP Figure 1 shows the Drought Monitor map for August 6, 2002. The map shows four levels or categories of drought severity ranging from moderate drought (D1) up to exceptional drought (D4). There is also a fifth category labeled as “abnormally dry” (D0) that works to 1) alert officials that recent dryness could be sending them into a drought situation, and 2) help identify those areas coming out of a drought event but are still experiencing some lingering impacts. The categories were established to roughly align with possible return periods for droughts: moderate drought might occur every couple of years; severe drought about every 10 years, extreme drought every 20-25 years, and exceptional drought every 50 years or so. The maps are released every Thursday morning before the agricultural markets open in the United States. However, the maps portray the drought conditions up through 8 am Eastern Time Tuesday mornings. Any precipitation that may change a drought * Corresponding author address : Michael Hayes, National Drought Mitigation Center, 241 Chase Hall, Lincoln, NE 68583-0749; e-mail: mhayes2@unl.edu. situation and falls on Tuesday or Wednesday does not get incorporated until the next week’s map. The Drought Monitor maps also try to give an indication of the major impacts that are occurring with each drought event, particularly if one sector is affected more than another. Three impacts qualifiers exist: “A” for agricultural impacts, “W” for hydrological impacts, and “F” for wildfire impacts. Usually these qualifiers are added if one or two of the sectors is being affected more than the other(s). If all three sectors are being affected at approximately the same level, either no qualifier is added, or sometimes all three are added depending on what might make the map clearer for the user. Unfortunately, impacts blur between these qualifiers and so the qualifiers should be viewed as a rather imperfect generalization. The Drought Monitor map in Figure 1 shows that wildfire impacts related to the drought are not as big an issue in southeastern Arizona and southern New Mexico, and so the qualifiers “A” and “A, W” are added in these areas. The Drought Monitor map is available on a web site hosted by the NDMC (http://drought.unl.edu/dm/). This site also contains the narrative that accompanies the text, as well as the indicators that are used to develop the maps and any supplemental information or forecasts that might be helpful for a user interested in the info rmation behind th e creati on of the m ap or how a drought situation might evolve in the coming weeks and months. 3. DROUGHT CLASSIFICATION SCHEME One of the first tasks in creating the Drought Monitor product was to develop a drought classification scheme that identified the important drought indicators used to create the Drought Monitor map and the associated severity levels that correspond to the Drought Monitor severity levels. Table 1 shows the current drought classification table. Several of the indicators, except for the Palmer Drought Severity Index (PDSI) and percent of normal rainfall, are organized by percentiles, which makes them more closely correspond with the rough return periods of the Drought Monitor categories. Additional indicators are also very important in the development of the Drought Monitor maps including snowpack information, radar-estimated rainfall totals, and agricultural statistics from the state and national Agricultural Statistics Service offices such as the condition of rangelands and the top and subsoil moisture levels. 4. ASSOCIATED PRODUCTS As mentioned, the Drought Monitor is an assessment of the current drought conditions. In March 2000, a Climate Prediction Center (CPC) product was first released that provides a 3-month “outlook” of what drought conditions could potentiall y be. This CPC