2.7 - Causes of Drought
Meteorological factors contributing to droughts
Meteorological factors are linked to weather patterns and climate conditions that influence the occurrence of droughts. These factors often involve disruptions in expected rainfall and atmospheric systems, leading to prolonged dry periods in affected regions.
Key weather and climate influences on droughts
- Reduced rainfall levels - When an area receives significantly less rain than usual, it can trigger drought conditions.
- Atmospheric circulation changes - Shifts in air patterns can result in extended periods without rain, sometimes lasting months or years, especially in regions dependent on seasonal rains.
- Disrupted seasonal patterns - In areas like Southeast Asia, where regular monsoon rains are expected, changes in circulation can cause these rains to fail.
- Impact of ocean currents - Alterations in nearby ocean and air currents can lead to severe droughts, particularly in coastal zones.
- High pressure systems - These weather systems can block rain-bearing low pressure fronts, preventing precipitation from reaching certain areas.
Hydrological factors leading to water scarcity in droughts
Hydrological factors relate to the availability and management of water in natural stores such as rivers, lakes, and underground reserves. When these stores are depleted, the risk of drought increases, often with long-lasting effects on water availability.
Influences of water storage on drought conditions
- Depletion of surface water - In warm and dry climates, water in rivers, lakes, and reservoirs evaporates rapidly, reducing available supplies faster than they can be replenished.
- Slow impact of groundwater loss - Aquifers, which are underground rock layers storing water, deplete more slowly but take much longer—sometimes years—to recover once exhausted.
- Reliance on rainfall - Regions heavily dependent on regular rainfall and surface water stores are particularly susceptible to drought when these sources diminish.
- Prolonged recovery periods - Once water stores are depleted, replenishment can take months or even years, extending the duration of drought impacts.
Human activities that exacerbate drought conditions
Human actions can significantly worsen drought conditions by increasing water demand and altering natural water cycles. These activities often compound the effects of meteorological and hydrological factors, making droughts more severe.
Human-induced causes of water scarcity
- Intensive agriculture - Large-scale farming uses vast amounts of water for irrigation, rapidly depleting local water supplies.
- Livestock farming - Raising animals requires substantial water for drinking, further straining resources in dry areas.
- Dam construction - Building dams to create reservoirs provides water locally but reduces downstream flow, potentially causing droughts in other regions.
- Deforestation effects - Removing trees decreases soil's capacity to retain moisture, leading to faster drying of land.
- Atmospheric drying from deforestation - Trees release water vapour through their leaves; without them, the local atmosphere becomes drier, contributing to meteorological drought causes.
Global distribution and vulnerability to droughts
Certain regions of the world are more prone to severe droughts due to their geographical location and climatic conditions. Vulnerability varies over time and is influenced by global atmospheric patterns and potentially by long-term environmental changes.
Patterns and locations of drought vulnerability
- Geographical hotspots - Severe droughts are most common around 30° north and south of the equator, where cool, dry air descends, creating high pressure zones with minimal rainfall.
- Affected regions - Key areas include northern and southern Africa, the Middle East, parts of Australia, western South America, and sections of North America.
- Atmospheric circulation influence - The pattern of descending dry air at these latitudes, driven by global circulation, explains the consistent drought risk in these belts.
- Temporal changes in drought distribution - Since the mid-20th century, droughts have increased in frequency in Africa, Asia, and the Mediterranean, while they have decreased in the Americas and Russia.
- Future climate concerns - Some research suggests that climate change may lead to more frequent and intense droughts in various regions over the coming decades.