Definitions

The climate and weather systems of the earth are constantly changing. As part of these dynamic processes, extremes of temperature, rainfall, and air movement will naturally occur. Periods of unusual dryness, i.e. droughts, are therefore a normal feature of climate and weather systems in all countries, including those generally regarded as being "wet" and "cold" as well as those areas usually associated with the term "drought" - the semi-arid areas of the tropics. While droughts may be regarded as unusual in that they do not occur all the time, or in some areas for most of the rime, droughts should not be regarded as being "abnormal" and, in fact, should be planned for in all countries.

While droughts may be regarded as unusual in that they do not occur all the time, or in some areas for most of the time, droughts should not be regarded as being "abnormal" and, in fact, should be planned for in all countries.

Drought is notoriously difficult to define and different definitions abound. Nevertheless, it is important that those involved in drought preparedness, mitigation, and declaration activities share a common understanding of the ways in which drought may be defined and the assumptions and constraints involved in using particular definitions.

Among the factors contributing to the difficulties involved in defining drought are:

· In most cases the drought phenomenon is temporary. A "drought" lasting a month may occur in an area which is known to experience cycles of alternating wet and dry periods (say every 5 years) and which is also known to be experiencing an increasingly drier climate for the past 50 years. (See Box 1.). Defining a temporary reduction of water/moisture availability as a drought given such dynamic processes is extremely difficult as much depends upon the length of the time period being considered.

· Droughts of similar severity may have dramatically different impacts on different societies as a result of ecological, socio-economic and cultural differences. This, in turn, affects how drought is perceived and how the term is used. Thus, it is difficult to define the term solely with regard to the physical event, i.e. the reduction in water/moisture availability. Invariably the definition has to take some account of how the physical event impacts upon society.

DROUGHT

A generally accepted definition of drought is a temporary reduction in water or moisture availability significantly below the normal or expected amount (norm) for a specified period.

The key assumptions of such a definition are:

1. the reduction is temporary (if the reduction were permanent then terms such as "dry" and "arid" would be more appropriate)

2. the reduction is significant

3. the reduction is defined in relation to a "norm" i.e. normal expectation

4. the period taken as the basis for the norm is specified.

How the "norm" is defined is of critical importance. Assumptions 3 and 4, therefore, require more detailed clarification. The "norm" may be defined either:

· technically - a reduction of water availability might qualify as a "drought" when it falls below about 80% of the average availability over the preceding 20 years. However, the period selected as the basis for estimating the average may prove misleading. (See Box 1) or

· culturally - in terms of the level of water availability the society has come to expect. Thus, after a run of ten years with above average rainfall a society may have become used to the wetter state and perceive the first year of average rainfall as a drought.

Q. Choose a drought that you are personally familiar with and briefly describe it using each of the key assumptions described above.
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Figure 1 - Schematic presentation of fluctuations occurring on different time scales.