The global water cycle as a system; its major stores and fluxes; the global water budget; and the causes and consequences of water insecurity.

What this dot point is asking

Eduqas wants you to explain the global water cycle as a system, identify its major stores and the fluxes between them, use the global water budget and residence times, and explain the physical and human causes and the consequences of water insecurity.

The answer

Stores and fluxes

At the global scale the water cycle is effectively a closed system: the total amount of water is fixed and only moves between stores. The oceans dominate, holding around $97\%$ of all water; the remaining freshwater is overwhelmingly held in the cryosphere (ice caps and glaciers), with only a tiny share in groundwater, lakes and rivers, soil moisture, the atmosphere and the biosphere. Water moves between these stores by evaporation and transpiration (collectively evapotranspiration), condensation, precipitation, surface runoff, infiltration and groundwater flow, all driven by solar energy and gravity.

The water budget and residence times

The global water budget balances inputs and outputs for each store over time, and at the planetary scale evaporation equals precipitation. Locally and regionally, however, budgets are unbalanced, which produces surpluses (humid regions) and deficits (arid regions). The very different residence times of stores matter for security: water cycled quickly through the atmosphere and rivers is renewable, whereas water held for millennia in ice and fossil aquifers is, in practice, a finite resource that human use can exhaust.

Causes and consequences of water insecurity

Water insecurity exists where the available, usable supply of fresh water fails to meet demand reliably. Physical causes include low or highly variable precipitation, high evaporation, recurrent drought, and dependence on stores (aquifers, glaciers) being depleted faster than they recharge. Human causes include rising population and demand (agriculture uses around $70\%$ of fresh water globally), over-abstraction for irrigation and industry, pollution that reduces usable supply, and inequitable distribution and governance. Geographers distinguish physical scarcity (not enough water in the environment) from economic scarcity (water exists but people lack the infrastructure or wealth to access it). The consequences are severe: threats to food production, health and sanitation, economic loss, ecosystem damage and the potential for conflict over shared resources.

Examples in context

Example 1. The Colorado River basin, USA. The Colorado supplies water to seven US states and Mexico and shows acute water insecurity from combined causes. Physically, the basin has experienced a prolonged drought and declining snowpack (a shrinking cryosphere store) under warming. Humanly, demand from cities (Las Vegas, Phoenix, Los Angeles) and from irrigated agriculture has grown, and the river is so over-abstracted that it often no longer reaches the sea. Reservoirs Lake Mead and Lake Powell have fallen to record lows, threatening supply and hydropower. The Colorado is a standard Eduqas case showing physical and human causes compounding, and the consequences for cities, farming and ecosystems.

Example 2. The Sahel, Africa. The Sahel illustrates insecurity dominated by physical scarcity and economic scarcity together. Precipitation is low and highly variable, droughts are recurrent, and high evaporation limits surface storage, while populations dependent on rain-fed farming and herding have grown. Much of the water that exists is hard to access because of limited infrastructure and wealth (economic scarcity), so the same drought causes far greater hardship than in a richer region. The Sahel demonstrates how the consequences of water insecurity, food shortage, displacement and conflict, fall hardest where physical scarcity meets poverty.

Try this

Q1. Approximately what percentage of the world's water is held in the oceans? [1 mark]

• Cue. About $97\%$, leaving only around $3\%$ as fresh water, most of which is locked in ice.

Q2. Distinguish between physical and economic water scarcity. [3 marks]

• Cue. Physical scarcity is a genuine shortage of water in the environment (low precipitation, depleted stores); economic scarcity is where water exists but people lack the infrastructure or wealth to access and use it.