Defining Drought and Its Causes
Drought is a relative water deficit that develops slowly — caused by atmospheric, oceanic and human factors.
Definition: Drought is a prolonged period of abnormally low precipitation (or high evapotranspiration) causing a water shortage relative to the normal needs of people, plants and animals in that region. Drought is a relative concept — what constitutes drought in the UK (e.g. 3 months without significant rain) would be a normal dry season in the Sahel.
Three types of drought:
| Type | Definition | Time to develop | Primary impact |
|---|---|---|---|
| Meteorological | Precipitation significantly below average for a sustained period | Weeks–months | Reduced soil moisture, lower river levels |
| Agricultural | Soil moisture falls below what crops/livestock need | Weeks–months after meteorological | Crop stress, yield losses, fodder shortage |
| Hydrological | Surface water (rivers, lakes) and/or groundwater levels fall below normal | Months–years after meteorological | Water supply failure, aquatic habitat loss, hydropower reduction |
The three types develop in sequence — meteorological drought triggers agricultural drought, which eventually leads to hydrological drought as longer-term water stores are depleted.
Atmospheric causes:
Blocking anticyclones: A persistent high-pressure system (anticyclone) that remains stationary for days to weeks over a region, deflecting the normal track of rainfall-bearing frontal systems. Blocking anticyclones cause many of Europe and North America's significant droughts — the UK 2022 drought resulted from an unusually persistent anticyclone over Central Europe blocking Atlantic weather fronts for most of spring and summer. The mechanism:
- High pressure → sinking air → air warms as it descends (adiabatic warming) → cloud cannot form → no rainfall.
ENSO (El Niño–Southern Oscillation): Under normal (La Niña-like) conditions: strong trade winds push warm water westward → warm pool in western Pacific → heavy rainfall over SE Asia/Australia. During El Niño: trade winds weaken → warm water shifts east → convection follows warm water → drought in SE Asia, Australia, southern Africa, northeast Brazil (sinking air replaces the convection that had normally delivered rainfall).
El Niño drought impacts:
- SE Asia/Australia: below-average rainfall, crop failures (Indonesia 1997-98; eastern Australia 2002-03).
- Southern Africa: 2015-16 El Niño caused worst drought in 35 years → 14 million people needed food aid.
- NE Brazil: 'Drought Polygon' semi-arid region regularly affected.
Climate change:
- Rising global temperatures increase evapotranspiration — soils and vegetation dry out faster even if rainfall does not change.
- Changes in atmospheric circulation may increase the frequency and persistence of blocking anticyclones.
- Reduced snowfall in mountains → less snowmelt water in spring/summer for irrigated agriculture.
- PNAS (2015) study: human-caused climate change made the eastern Mediterranean drought of 2006-2010 (Syrian drought) 2–3 times more likely.
- Drought = prolonged water deficit relative to regional normal. Develops over weeks-years (slow-onset hazard).
- Meteorological → agricultural → hydrological (in order of time lag).
- Blocking anticyclone: sinking stable air → no cloud formation → no rainfall (UK 2022 example).
- El Niño: trade winds weaken → warm water east → drought in SE Asia, Australia, southern Africa, NE Brazil.
- Climate change: higher temperatures → more evapotranspiration → drier soils even with same rainfall.