Tremendous progress in drought early warning has taken place since the first edition of Drought and Water Crises: Science, Technology, and Management Issues was released in 2005. At that time, the US drought monitor (USDM) product was relatively new and was just beginning to be used as a decision-making tool. The famine early warning systems network (FEWS NET) is another example of an early warning system targeted at addressing food security issues for specific locations around the world, and drought was, and still is, an important component within this system. More drought indicators and indices were being developed, but relatively few were available for people to use or access. As illustrated in Chapter 8 of this book, there are now more than 50 indicators and indices available for use by decision makers. This section of the chapter reviews some of the recent advancements in drought monitoring and early warning that have taken place since that first edition.
US Drought Monitor
One of the tools highlighted in the first edition of this book was the USDM (Svoboda et al. 2002). The USDM product has now been produced and released every week since it became operational as a weekly assessment of drought conditions in August 1999 (http://drought.unl.edu/dm). The consistency and reliability of the product has led to it becoming the "state- of-the-science" for drought monitoring in the United States; it is a major tool for decision-making by resource managers and policy makers, a communication tool for the media, and a resource for teachers at all levels of educational instruction.
Several of the fundamental characteristics originating when the USDM process began still apply today and have likely helped make the process as successful as it is. Authorship of the map rotates between the National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln, the US Department of Agriculture (USDA), and the National Oceanic and Atmospheric Administration (NOAA). Within NOAA, authors specifically come from the Climate Prediction Center, National Centers for Environmental Information, and Western Regional Climate Center. In addition, the USDM incorporates information from approximately 420 scientists and local experts around the country. This number was close to 150 in 2005, highlighting the growth in collaboration and awareness that has taken place since then. The USDM continues to seek corroborative monitoring and impact data, and information from this group of participants in order to provide added confidence in the initial assessments gained from purely quantitative information describing the physical environment. This kind of "ground truth" is important, and it increases broad-based credibility and trust in the product.
The USDM is not a forecast; rather, it was designed to be a comprehensive drought assessment that reflects the current drought situation (i.e., snapshot) across the country. Because multiple physical conditions may be present at one time and no preferred scale exists for assessing drought, the USDM also depends on, incorporates, and weights human expertise and judgment in the assessment of the associated impacts.
A key strength of the USDM product is that it is based on multiple indicators. One indicator is not adequate to represent the complex characteristics of drought across a region. Therefore, it is important for a product like the USDM to use a variety of quantitative and qualitative indicators. The key indicators used in creating the weekly USDM map include streamflow, measures of recent precipitation, drought indices, remotely sensed products, and modeled soil moisture. Many other ancillary indicators are also used, depending on the region and the season. For example, in the western United States, indicators such as snow water content, reservoir information, and water supply indices are important for evaluating the current and future availability of water. These indicators inherently incorporate the effects of hydrological lag and relationships across space and time between climate and the surface or groundwater system.
The USDM defines four categories of drought severity based on increasing intensity (D1-D4), with a fifth category (D0) indicating abnormally dry areas (possible emerging drought conditions or an area that is recovering from drought but may still be seeing lingering impacts). The drought categories represented by this scale are moderate (D1), severe (D2), extreme (D3), and exceptional (D4). Another of its strengths is that the five categories are based on a percentile approach, where D0 is approximately equal to the 30th percentile; D1, the 20th; D2, the 10th; D3, the 5th; and D4, the 2nd (Svoboda et al. 2002).
Recent improvements to the USDM have focused on providing value- added products and tools for assistance in decision-making. For example, USDM maps and weekly statistics for various regions, states, tribal reservations, and river basins are now available. A user can incorporate census data and make an approximate estimate of the number of people affected by various drought categories for each of these regions. Change maps (i.e., a comparison to the prior week) and animations (i.e., multiple weeks) are also available to the public, as well as all of the shape files related to the map each week, thus providing an opportunity for researchers and decision makers to tailor the information to their needs. Improvements like these continue to be made as the data availability and technology evolve.
As a tool for decision-making, the USDM provides a great example of how science can motivate policymaking. The USDM was first formally incorporated into the 2008 US Farm Bill for several livestock-related drought relief programs. The 2014 Farm Bill expanded the USDM's use for agricultural drought relief programs, and it is also a trigger to assist in fast-tracking USDA Secretarial Drought Disaster designations. Other federal agencies that use the USDM for decisions include the Internal Review Service, the National Weather Service, the Environmental Protection Agency, the Centers for Disease Control and Prevention, and the Bureau of Land Management. Multiple states and regional or local organizations also use the USDM for triggering various activities or as an information source.
Building upon the USDM experience in the United States, other nations have either experimented with or adapted the USDM process for drought early warning in their countries. Brazil, Mexico, and the Czech Republic also have operational USDM-like tools for drought early warning. The monthly North American Drought Monitor (NADM) continues to be produced by drought scientists in Canada, Mexico, and the United States (http://www. ncdc.noaa.gov/oa/climate/monitoring/drought/nadm/). The development of the NADM represented an important step in a cooperative, multinational effort to improve monitoring and assessment of climate extremes throughout the continent (Lawrimore et al. 2002).