Drought is difficult to define and needs different definitions to explain specific situations.
It is important that those involved in drought preparedness and mitigation share a common
understanding of the ways in which drought may be defined and the assumptions and
constraints involved in using particular definitions.
In most cases, drought is temporary. A month-long drought may occur in an area that normally
experiences alternating wet and dry periods. Defining a temporary reduction of water/moisture
availability as a drought is extremely difficult and depends upon the time period being considered.
Box 2.1: Drought
Drought most generally is defined as a temporary reduction in moisture availability
significantly below the normal for a specified period.
The key assumptions of this definition are:
■ the reduction is temporary (for permanent reductions, terms such as “dry” and
“arid” conditions are more appropriate),
■ the reduction is significant,
■ the reduction is defined in relation to normal expectation,
■ the period of the normal expectation is specified.
Normal expectation may be defined:
■ technically – a reduction of water availability might qualify as a drought when it falls
below about 80% of the average availability of the preceding 30 (or more) years.
■ culturally – drought definition depends on local perception.
It is essential to understand local perception of drought. For example, after
experiencing a run of ten years with above average rainfall, a society may become
used to the wetter conditions and perceive a year of average rainfall as a drought.
10CLIMATE VARIABILITY AND CHANGE: ADAPTATION TO DROUGHT IN BANGLADESH
Droughts of similar severity may have dramatically different impacts on livelihoods because of
ecological, socio-economic and cultural differences. Thus, it is difficult to define drought solely
with regard to reduction in water/moisture availability. Invariably, the definition has to consider
how the physical event impacts upon society.
Causes of drought in Bangladesh are related to climate variability and non-availability of surface
water resources. The immediate cause of a rainfall shortage may be due to one or more factors
including absence of moisture in the atmosphere or large-scale downward movement of air
within the atmosphere which suppresses rainfall. Changes in such factors involve changes in
local, regional and global weather and climate. While it may be possible to indicate the
immediate cause of a drought in a particular location, it often is not possible to identify an
underlying cause.
Short-term drought episodes can be linked to global atmospheric and oceanic circulation
features. For example, the El Nino/southern oscillation (ENSO) phenomenon, which results from
development of warm surface water off the Pacific coast of South America, affects the levels of
rainfall in many parts of the world, including monsoon rainfall in Bangladesh. On a larger scale,
the link between sea surface temperature and rainfall has been suggested as a possible cause of
long, dry regimes.
Increasing levels of carbon dioxide and other greenhouse gasses have been suggested as causes of
rainfall changes, which are, in turn, attributed as climate change. There is strong evidence that
climate change will alter the rainfall pattern and as a result more frequent droughts are
expected. Among the local-level causes are human-induced changes resulting from vegetation
loss due to over exploitation of resources and deforestation.