Drought causes and locations (Edexcel GCSE Geography A): Revision Notes
Drought causes and locations
Understanding drought vs arid conditions
Drought and arid conditions are both characterised by lack of water, but they differ in important ways.
Understanding the Key Difference:
Arid regions experience permanently dry conditions as part of their normal climate pattern, while drought conditions represent temporary periods when an area receives significantly less rainfall than normal for that location.
Arid regions typically have high atmospheric pressure systems that prevent cloud formation and rainfall. Examples include the Sahara Desert (completely arid) and the Kalahari Desert (semi-arid). Arid regions generally receive between of precipitation annually.
In the UK, drought is officially defined as 15 consecutive days without any rainfall. Unlike arid regions, drought can occur anywhere in the world when normal weather patterns are disrupted.
The key factors determining whether an area experiences drought include how it normally receives precipitation and when that precipitation typically occurs during the year. Some regions are more vulnerable because they rely heavily on seasonal rainfall patterns or have limited water storage capacity.
Natural causes of drought
Meteorological drought
Meteorological drought occurs when a region receives less precipitation than its long-term average. This happens when high pressure weather systems become established over an area for extended periods.
Critical Concept: High Pressure Systems
High pressure systems are characterised by descending air that warms and becomes less likely to form clouds or produce rainfall. These systems can persist for weeks or months, preventing normal precipitation patterns.
In the UK, meteorological drought often results from blocking anticyclones that push away the low pressure systems normally responsible for bringing rain. When these high pressure systems persist, they can prevent rainfall across entire regions for weeks or months, leading to drought conditions even in typically wet climates.
Hydrological drought
Hydrological drought develops when the water cycle receives less input than normal, affecting groundwater supplies, rivers, and reservoirs. This type of drought occurs when reduced precipitation means that water stores are not being replenished at their usual rate.
The Drought Feedback Loop
The hydrological cycle becomes disrupted because less water evaporates from surface sources, reducing atmospheric moisture and further decreasing the likelihood of precipitation. This creates a feedback loop where drought conditions become self-reinforcing - the drier the land becomes, the less moisture is available to generate future rainfall.
Global circulation patterns and drought vulnerability
Global atmospheric circulation makes certain locations particularly vulnerable to drought conditions. The position of major circulation cells, including Hadley and Ferrel cells, influences where descending dry air creates high pressure zones.
Africa's Sahel region exemplifies how global circulation affects drought risk. When Hadley cell circulation shifts, it can cause the descending dry air to affect areas that normally receive seasonal rainfall. This movement of circulation patterns explains why some regions experience recurring drought cycles.
Areas located under descending air currents in the global circulation system typically experience little precipitation during certain seasons. If the expected wet season fails to materialise due to shifts in these circulation patterns, drought conditions quickly develop.
Human causes of drought
Human activities can significantly contribute to drought conditions through two main processes:
Deforestation reduces an area's ability to maintain moisture in the local environment. Trees play a crucial role in the water cycle through transpiration - the process of releasing water vapour from their leaves into the atmosphere. When forests are cleared, this natural source of atmospheric moisture is removed. Additionally, tree roots normally store water in the soil and release it gradually, but without this vegetation, water runs off more quickly rather than being retained in the local ecosystem.
Critical Impact: Dam Construction
Dam construction can create drought conditions downstream by restricting natural water flow patterns. When rivers are dammed, water that would normally flow to lower areas becomes trapped in reservoirs, reducing water volume downstream and lowering groundwater levels in surrounding areas.
Communities and ecosystems that depend on regular river flow may experience drought-like conditions even when overall regional precipitation remains normal.
Both human activities can interact with natural drought causes to make conditions more severe and longer-lasting than they would be from natural factors alone.
Key Points to Remember:
- Arid regions are permanently dry, while drought represents temporary dry conditions in areas that normally receive more rainfall
- Meteorological drought occurs when precipitation falls below average, often due to persistent high pressure systems
- Hydrological drought develops when the water cycle becomes disrupted, preventing normal replenishment of water stores
- Global circulation patterns make some locations naturally vulnerable to drought, particularly where descending air creates high pressure zones
- Human activities like deforestation and dam building can worsen drought conditions by disrupting local water cycles and natural water flow patterns