Changes

* Drought periods can vary from seasons or years to much shorter periods, as is the case with agriculture. For instance, short dry spells in Africa, lasting 2-4 weeks, occur almost every rainy season, negatively affecting crop growth and potential crop productivity. This means that poor rainfall distribution over time is a more common cause of crop failure than low cumulative annual rainfall.

Research by Wilhite and Glantz (1985) of the National Center for Atmospheric Research, in the early 1980s uncovered more than 150 published definitions of drought, reflecting differences in regions, needs, and disciplinary approaches. They categorized the definitions in terms of four basic approaches to measuring drought (see diagram on right):<br>