Agricultural Drought Indices - US Department of Agriculture

indices that are combined using a simple D0-D4 scheme and a percentile ranking methodology
(Table 1) to look at addressing both short- and long-term 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 subjective
experience and guidance 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. Nearly 300 local experts from across the country 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 Drought Indicator Blends (OBDI) is 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.
Figure 4. The USDM for March 1, 2011.
Table 1. 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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