Drought: Causes and Impacts

This lesson examines the causes, types and impacts of drought, using case studies from different global contexts. It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5, Enquiry Question 2: What factors influence the hydrological system over short- and long-term timescales? and Enquiry Question 3: How does water insecurity occur and why is it becoming such a global issue?

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Defining Drought

Drought is not simply a lack of rain. There are several types of drought, each defined differently:

Type of Drought	Definition	Timescale
Meteorological drought	Precipitation significantly below the long-term average for a region (typically defined as <75% of normal for a sustained period)	Weeks to months
Hydrological drought	Below-normal water levels in rivers, lakes, reservoirs and groundwater stores	Months to years (lags behind meteorological drought)
Agricultural drought	Soil moisture insufficient to meet the demands of crops, leading to crop stress or failure	Weeks to months
Socio-economic drought	Water supply insufficient to meet the demands of society — affects water supply, industry, energy production	Months to years

These types are linked in a cascade: meteorological drought → agricultural drought → hydrological drought → socio-economic drought. Each type takes progressively longer to develop and to recover from.

Exam Tip: In the exam, always specify which type of drought you are discussing. A meteorological drought (below-average rainfall) does not necessarily cause a socio-economic drought if water storage and management are effective. This distinction is crucial for higher-level analysis.

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Natural Causes of Drought

1. Atmospheric Circulation Patterns

Drought is fundamentally caused by persistent anomalies in atmospheric circulation that suppress precipitation.

Cause	Mechanism	Regions Affected
Blocking anticyclones	A persistent high-pressure system remains stationary for weeks, deflecting rain-bearing weather systems. Descending air inhibits cloud formation.	UK (summer 1976, 2018); Europe; mid-latitudes generally
Subtropical high-pressure belts	The descending limb of the Hadley Cell creates permanent dry conditions at ~30°N and 30°S. During certain years, these belts shift, extending drought zones.	Sahara, Arabian Peninsula, Australia, Kalahari
Rain shadow effect	Moist air is forced to rise over mountains, losing moisture as orographic rainfall on the windward side. The leeward side receives very little precipitation.	Atacama (behind the Andes), Gobi (behind the Himalayas)

2. El Niño–Southern Oscillation (ENSO)

ENSO is the most important driver of interannual drought variability globally.

During El Niño events:

• The warm pool of surface water shifts eastward across the Pacific
• The Walker Circulation weakens or reverses
• Australia, Southeast Asia, India and East/Southern Africa experience reduced rainfall and drought
• Western South America experiences increased rainfall

During La Niña events:

• Cooler-than-normal SSTs in the eastern Pacific
• Enhanced rainfall in Australia and Southeast Asia
• Drought conditions in the western Americas (California, the US Southwest)

The 2015–16 El Niño caused severe drought in:

• Southern Africa (36 million people food insecure in 2016)
• Ethiopia (10 million people required food aid)
• Central America (3.5 million affected, the "dry corridor")

3. Indian Ocean Dipole (IOD) and Other Teleconnections

The Indian Ocean Dipole is a similar oscillation affecting eastern Africa and Australia. A positive IOD brings drought to western Australia and Indonesia but increased rainfall to East Africa. A negative IOD contributes to Australian drought.

The North Atlantic Oscillation (NAO) affects winter drought risk in Western Europe. A negative NAO phase can push storm tracks southward, reducing winter rainfall in the UK.

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Human Causes of Drought

While drought is fundamentally a climatic phenomenon, human activities can exacerbate its onset, severity and impacts.

Human Cause	Mechanism
Over-abstraction of groundwater	Lowers water tables; reduces baseflow to rivers; depletes reserves that would buffer drought impacts. The Ogallala Aquifer has dropped >30 m in parts of Kansas and Texas.
Deforestation	Reduces transpiration and moisture recycling. In the Amazon, modelling shows that 40%+ deforestation could trigger a permanent drying transition ("dieback") across 60% of the basin (Lovejoy & Nobre, 2018).
Over-grazing	Removes vegetation cover, compacts soil, reduces infiltration and soil moisture retention — can trigger desertification.
Poor irrigation practices	Flood irrigation wastes 40–60% of water to evaporation; salinisation reduces soil fertility and water-holding capacity.
Climate change	Shifting precipitation belts; increasing evapotranspiration; reducing snowpack; lengthening dry seasons in many regions. The Mediterranean region is projected to become 20–30% drier by 2100 under RCP8.5.
Population growth	Increasing water demand amplifies the impact of any supply shortfall. Global water demand has increased 6× over the past century.

Exam Tip: For A-Level, always consider both natural and human causes of drought. The best answers explain how human activities can amplify or trigger drought even when natural precipitation variability is moderate. This demonstrates understanding of coupled human-physical systems.

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Case Study 1: California Drought (2011–2017)

Context

• California is the most populous US state (39 million people) and the largest agricultural producer ($50 billion/yr)
• Mediterranean climate: 75% of precipitation falls November–March; dry summers
• Relies on snowpack in the Sierra Nevada as a natural water store