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What is the U.S. Drought Monitor?

August 24, 2016
Figure 1: The U.S. Drought Monitor (USDM) is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the U.S. Department of Agriculture (USDA), and the National Oceanic and Atmospheric Administration (NOAA). Map courtesy of NDMC-UNL. The most up to date version of the California-Nevada USDM can be found here.

Near to above normal precipitation accumulations observed during the Water Year (Oct 1-Sept 30) 2016 to-date eased drought conditions in some parts of California and Nevada. However, a combination of near-average Sierra snowpack, above normal temperatures, below expected precipitation accumulations in southern California with a strong ENSO, and lingering drought impacts have allowed drought to persist in the region. As of July 19, 2016, 83.59% of California and 34.25% of Nevada, were observing drought conditions according to the U.S. Drought Monitor (figure 1). This is an improvement to this time last year, where 97.35% of California and 97.78% of Nevada were in drought. The current D3-D4 percent area has decreased to levels not seen since late 2013-early 2014 (figure 2). However, long term extreme to exceptional drought and long term impacts remain in areas of southern California, south Central Coast, southern Central Valley, and southern Sierras. The USDM provides a way to track drought in our region, and this post addresses the questions: What is the U.S. Drought Monitor? How does it work?

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Figure 2: Time series of percent of area of California and Nevada by USDM drought category from 2011 to present.

The U.S. Drought Monitor (USDM) began in 1999 and is a collaborative effort between the National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Agriculture (USDA). Every Thursday, they release a USDM map that is used by state, local, and basin-level decision makers to trigger drought response, as well as by the USDA.

What is shown on a USDM map?

The map shows the drought intensity for whatever geographic region is selected. On the USDM Website you can select the US as a whole, or select a state or watershed. This CA-NV version is provided for the regional California-Nevada Drought Early Warning System (DEWS). In the upper right corner, the cutoff date for data is shown (here, July 19th) followed by the date the map was released (here, July 21st). Below that, you can find the percent of area in the selected region in each drought category and the USDM color classification scheme of drought intensity, followed by the USDM author and partners.

What is the USDM Classification Scheme?

The yellow to red colors represent the classification of drought intensity. The USDM uses five classifications: D0 Abnormally Dry, D1 Moderate Drought, D2 Severe Drought, D3 Extreme Drought, and D4 Exceptional Drought. Additionally, on some versions of the USDM (such as the national map), black lines define areas of short- (S = past 6 months) and long-term (L = 6+ months) drought impacts.

How are the USDM Categories determined?

There are 13 USDM authors from NDMC, NOAA (NCEI, CPC, WRCC), and USDA. Each week, the Drought Monitor author compiles a variety of information and input from local and regional experts and observers to localize the information as much as possible. Inputs they consider are drought indices and other climatological inputs, satellite observations of vegetation health, and other observations and models of soil moisture and hydrologic data such as snowpack. The map draft goes through multiple iterations each week, taking in more input from observers from emails, state and regional-level drought conference calls, and other helpful contacts. More information on the classification scheme inputs can be found here. The USDM focuses on broad-scale conditions assessing drought down to the county level, and local conditions may vary.

How can I stay up-to-date on the USDM?

Current and archived USDM maps can be found at the USDM, while a CA-NV regional map can be found at the California-Nevada Drought Early Warning System (DEWS).

How can I get involved in the USDM process?

You can provide Drought Monitor input by emailing, contacting your local state climatologist, becoming a CoCoRaHS observer, or submitting a Drought Impact Report.

Want more information? Check out NDMC, this brochure, or send Amanda Sheffield an email at Thank you to Nina Oakley and Dave Simeral at the WRCC for reviewing this blog!


From → Drought, NIDIS

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