Agricultural Drought Indices - US Department of Agriculture

WMO’s Perspectives on Drought
Given the extensive number of activities undertaken, it will be difficult to provide an exhaustive
description of WMO’s perspectives on droughts in this short paper. Hence, a short description of
different perspectives is provided below, using the material from the different publications listed
above.
Understanding of Droughts and Drought Definitions
WMO’s early efforts placed emphasis on many meteorological facets of drought, including its
definition and early recognition; its effect on plants, animals, and diseases; and methods of
surviving under its influence. Clarifications were provided on the distinction between drought and
aridity; the linkage between drought and water balance (soil water, precipitation, dew and fog, and
surface runoff); fire hazards; drought, ecological imbalance, and soil erosion; the space and time
characteristics of droughts; causes of droughts; and forecasting of droughts (WMO 1975). Some
detailed studies of past droughts suggested that changes in the surface albedo, existence of a
deep dust layer in the atmosphere, changes in sea surface temperatures, and an increase of
carbon dioxide may lead to changes in the general circulation features, which may cause drought
(WMO 1987).
Drought definitions used over time vary from region to region and according to the purpose for
which they are defined. A general survey of drought definitions has indicated that they can be
classified according to the criteria used. A classification of drought definitions was given (WMO
1975) under each of the following subheadings:
a) Rainfall
b) Rainfall with mean temperature
c) Soil water and crop parameters
d) Climatic indices and estimates of evapotranspiration
e) General definitions and statements
As an example, looking more closely into definitions based solely on rainfall, it was shown that a
number of these refer to short period “droughts” or “dry spells” (WMO 1975).
• Less than 2.5 mm in 48 hours
• Rainfall half of normal or less for a week
• 10 days with rainfall not exceeding 5 mm
• 15 days with no rain
• 15 consecutive days, none with 0.25 mm
• 15 consecutive days, none with 1 mm
• 21 days or more with rainfall less than 30% of normal
• 21 days with precipitation less than one-third of normal
These appeared to be geared mainly to climatic experience in the British Isles or perhaps the
northeastern United States, where rainfall is received at fairly frequent intervals and crop and
animal husbandry and water-storage operations are not geared to the long spells of rainless
weather that are seasonally normal in the semiarid regions (WMO 1975).
Agricultural Drought and Drought Index
Agriculture is often the first sector to be impacted by drought because access to water resources
and soil moisture reserves determine crop productivity. Drought in the agricultural sense does not
begin with the cessation of rain, but rather when available stored water will support actual
evapotranspiration at only a small fraction of the potential evapotranspiration rate (WMO 1992).
The rate of transpiration by a crop depends largely upon the availability of soil water as determined
by the root systems of crops. In a drought situation, the dearth of soil water is often aggravated by
an increased heat load imposed on the plant by net radiation because of less cloudiness and
possibly lower albedo. The deficiency may result either from an unusually small moisture supply or
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