Monitoring Drought in the United States: Status and Trends - Monitoring and Predicting Agricultural Drought: A Global Study - page 125

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Table 9.1 Classification of drought categories for the standardized precipitation index
(SPI) (according to McKee et al., 1993)

SPI values Drought category Time in category

0 to -0.99 Mild drought 34.1%

-1 .00 to -1.49 Moderate drought 9.2%

1. 50 to -1.99 Severe drought 4.4%

≤ -2.00 Extreme drought 2.3%

∼

50%

ga nizations, and nongovernmental organizations. It is now being used in
bo th operational and research modes in more than 50 countries and has
be en proven quite effective as part of a comprehensive, integrated early
w arning system.
The NDMC's experience has been that the SPI detects emerging drought
co nditions more quickly than the PDSI, a characteristic that is extremely
cr itical in the timely implementation of mitigation and response actions by
individuals and governments (Hayes et al., 1999). Many states are using the
SPI as part of their efforts to monitor drought and trigger various drought-
related mitigation and response actions.
Developing an effective drought monitoring system presents many
unique challenges because of the slow onset nature of drought, its spa-
tial extent and duration, and the requirement that multiple indicators and
indices be used to properly characterize its severity and potential impacts.
In addition to these challenges, the ineffectiveness of drought monitoring
systems in the United States and elsewhere is also associated with inad-
equacies in the systems themselves. First, monitoring systems have often
depended on an inadequate network of weather stations. Data from these
stations may be reported infrequently (e.g., monthly) so that information is
not readily available to decision makers at critical times or decision points.
Second, drought monitoring systems are often based on a single parameter
or index. Because of the complexities of drought, no single parameter or
index can adequately capture the intensity and severity of drought and its
potential impacts on a diverse group of users. Each index has strengths and
weaknesses, which often vary spatially. Third, the delivery of information
products to assess drought severity is often untimely. And fourth, informa-
tion products are often developed without a clear understanding of user
needs, or users are confused about how to apply this information when
m aking critical climate-based decisions.
A comprehensive drought monitoring system has been recommended for
many years in the United States (Wilhite et al., 1986; Riebsame et al., 1991;
Wilhite and Wood, 1994), but no action on these recommendations had
taken place until recently. In 1999 it became apparent that a new approach
to drought monitoring was needed to address many of the inadequacies
noted above.

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