Droughts are often predictable: periods of unusual dryness are normal in all weather systems. Advance warning is possible (WHO).
By 2025, 1.8 billion people will experience absolute water scarcity, and 2/3 of the world will be living under water stressed conditions (UNCCD).
Drought can be defined according to meteorological, agricultural, hydrological and socio-economic criteria.
- Meteorological, when precipitation departs from the long-term normal
- Agricultural, when there is insufficient soil moisture to meet the needs of a particular crop at a particular time. Agricultural drought is typically evident after meteorological drought but before a hydrological drought
- Hydrological, when deficiencies occur in surface and subsurface water supplies
- Socio-economic, when human activities are affected by reduced precipitation and related water availability. This form of drought associates human activities with elements of meteorological, agricultural, and hydrological drought (FAO).