Agricultural Drought Indices - US Department of Agriculture

Limitations of Agricultural Drought Indices Used in the GHA Subregion
Agricultural drought is by far the most complex and difficult type of drought to determine, compared
to other drought types (meteorological and hydrological). In the sub-region, agricultural drought
indices are determined only by rainfall data, despite recommendations that several parameters be
considered. Although SPI can be computed for different time scales, provide early warning of
drought, and help assess drought severity, its values are based on preliminary data, which may
change. With these limitations, the SPI is the best option for monitoring droughts for agriculture,
given the nature of rainfall data available in the sub-region.
Summary and Conclusions
Meteorological drought occurs when the seasonal or annual precipitation falls below its long-term
average. Hydrological drought develops when the meteorological drought is prolonged and
causes shortage of surface and groundwater in the region. Agricultural drought is detected when
continuous and intense soil moisture stress leads to significant crop yield reduction. Finally, socioeconomic
drought is a manifestation of continued drought of severe intensity that causes economic
and sociopolitical instabilities in a region or country. Whereas meteorological drought is just an
indicator of precipitation deficiency, hydrological and agricultural droughts can be considered the
physical manifestations of meteorological drought, and socioeconomic drought results from the
impacts of hydrological and agricultural droughts on the society.
This evaluation focused on all information related to drought assessment and monitoring,
considering any time scale (10-day, monthly, and seasonal) and any format of publication (online
data and maps, bulletins, advisories, etc). Rainfall anomaly data and maps, satellite information
(NDVI), and meteorological and agricultural drought indices are considered in monitoring and
disseminating drought information.
The SPI has been used operationally to monitor conditions across the sub-region since 1989,
when ICPAC was established as a regional drought monitoring center (DMC) in Nairobi. Dekadal,
monthly, and seasonal maps of the SPI for the sub-region are provided operationally on the 8 th and
28 th of the month and at the beginning of every season, respectively. The products are always
posted on the ICPAC website (www.icpac.net) and also sent to National Meteorological and
Hydrological Services (NMHSs) of the ICPAC member countries.
References
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multiple time scales. Climatology Report Number 97–2, Colorado State University, Fort Collins,
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ICPAC. 1999. Homogeneous Climatological Zoning. DMC Lecture Notes, chapter 3, 29-43.
IPCC. 2007a. Summary for Policymakers. In Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment Report (AR4) of the Intergovernmental
Panel on Climate Change (S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt,
M.Tignor, and H.L. Miller, eds.). Cambridge University Press, Cambridge.
IPCC. 2007b. Fourth Assessment Report, Climate Change 2007 (AR4), Working Group II summary for policy
makers.
McKee, T.B., N.J. Doesken, and J. Kleist. 1993. The relationship of drought frequency and duration to time
scales. Preprints, 8th Conference on Applied Climatology, pp. 179–184. January 17–22, Anaheim,
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Wilhite, D. 2000. Drought Preparedness in the US. Pages 119-132 in Drought and Drought Mitigation in
Europe (J.V. Vogt and F. Somma, eds.). Kluwer, The Netherlands.
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