Agricultural Drought Indices - US Department of Agriculture

In cases such as these, it is important to utilize and incorporate a consolidation of indices and
indicators into one comprehensive “composite index”. A composite index approach allows for the
most robust way of detecting and determining the magnitude (duration + intensity) of droughts as
they occur. Through a convergence of evidence approach, one can best determine (for a
particular state, country, or region for a particular time of the year) which indices and indicators do
the best job of depicting and tracking all types of droughts. The users can then determine which
indicators to use and how much weight to give each indicator/index in a “blended approach” that
incorporates a multiple parameter and weighting scheme. Such approaches have been used in
the U.S. Drought Monitor (USDM) and North American Drought Monitor (NADM), as described
below, and as part of a series of Objective Blend products, which are produced for the USDM.
The U.S. Drought Monitor
Created in 1999, the weekly U.S. Drought Monitor (USDM) (Figure 1) was the first to use a
composite index/indicator approach (Svoboda et al. 2002). The product is not an index in and of
itself, but rather a combination of indicators and indices that are combined using a simple D0-D4
scheme and a percentile ranking methodology (Table 7) to look at addressing both short- and longterm
drought across the United States. The key indicators/indices revolve around monitoring
precipitation, temperature, streamflow, soil moisture, snowpack, and snow water equivalent.
Various indices, such as the SPI and PDSI, are incorporated and integrated with remotely sensed
vegetation indices to come up with a “blended convergence of evidence” approach in dealing with
drought severity. The ranking percentile approach allows the user to compare and contrast
indicators originally having different periods of record and units into one comprehensive indicator
that addresses the customized needs of any given user. The approach also allows for flexibility
and adaptation to the latest indices, indicators, and data that become available over time. It is a
blending of objective science and art through the integration of impacts and reports from local
experts at the field level. The impacts covered and labeled on the map are (A) for agricultural and
(H) for hydrological drought. Some 275 local experts from across the country are allowed to view
the draft maps and provide their input, data, and impacts to either support or refute the initial
depiction. An iterative process works through all the indicators, indices, data, and field input until a
compromise is found for the week. The process then repeats itself the next week and so on. In
addition, a set of Objective Blends are used to help guide the process. This method combines a
different set of indicators to produce separate short- and long-term blend maps that take various
indices with variable weightings (depending on region and type of drought) to produce a composite
set of maps, which are updated weekly. More details and information on the USDM, its
classification scheme, and the Objective Blends can be found at http://drought.unl.edu/dm.
Table 7. The U.S. Drought Monitor classification and ranking percentile scheme.
Category Description Ranking Percentile
D0 Abnormally dry 30
D1 Moderate 20
D2 Severe 10
D3 Extreme 5
D4 Exceptional 2
Source: National Drought Mitigation Center, USDA, NOAA.
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