The global water cycle as a system with stores and flows, drainage-basin processes and the water budget, the physical and human causes of water insecurity, and the conflicts and management strategies that surround a finite water resource.

What this dot point is asking

Edexcel wants you to explain the global water cycle as a system of stores and flows, explain drainage-basin processes and the water budget, explain the physical and human causes of water insecurity, and evaluate the conflicts and management strategies surrounding a finite water resource.

The global water cycle and drainage-basin system

The water budget balances precipitation against evapotranspiration, runoff and storage, written as

where $P$ is precipitation, $ET$ evapotranspiration, $Q$ runoff and $\Delta S$ the change in storage. It is shown over a year in a soil-water graph, with a winter surplus and recharge, summer utilisation and deficit, and autumn recharge.

Causes of water insecurity

The Aral Sea is the classic case of human-driven insecurity: Soviet diversion of the Amu Darya and Syr Darya rivers for cotton irrigation from the 1960s shrank what was the world's fourth-largest lake by around 90 per cent, leaving a salt desert, collapsed fisheries and a public-health crisis. The Colorado River in the south-western USA is so heavily abstracted by cities and agriculture across seven states and Mexico that it now often fails to reach the sea, and reservoirs Lake Mead and Lake Powell have fallen to record lows.

Conflicts over water

Growing scarcity creates conflict between users (farmers, cities, industry, ecosystems) and between countries that share transboundary rivers and aquifers. The Nile is a flashpoint: Ethiopia's Grand Ethiopian Renaissance Dam (GERD), filling since 2020, gives Ethiopia hydropower but alarms downstream Egypt, which depends on the Nile for over 90 per cent of its freshwater. The Tigris-Euphrates (Turkey's dams reducing flow to Iraq and Syria) and the Colorado show the same upstream-downstream tension. Synoptically, players (upstream and downstream states, farmers, cities) hold competing attitudes, and the futures of shared basins depend on cooperation or conflict.

Managing water sustainably

Hard-engineering schemes (mega-dams, inter-basin water transfers such as China's South-North Water Transfer Project, desalination as in Israel and the Gulf) boost supply but are costly and can damage environments and displace people. Sustainable, soft approaches include water conservation, efficient smart irrigation (drip irrigation in Israel), recycling, restoring catchments and integrated water resource management (IWRM) that coordinates a whole basin and balances competing players.

Examples in context

Example 1: the Aral Sea, Central Asia. Over-abstraction for cotton irrigation collapsed the lake within a few decades, an environmental and human catastrophe (lost fisheries, toxic dust storms, falling life expectancy). It demonstrates economic and human-driven insecurity overwhelming a physical resource, and partial recovery of the North Aral Sea after the Kok-Aral dam shows that management can reverse some damage.

Example 2: Israel and water management. A water-scarce country that has achieved relative water security through a combination of hard and soft strategies: large-scale desalination now supplies a large share of domestic water, while pioneering drip irrigation and high rates of wastewater recycling (over 85 per cent reused in agriculture) cut demand. It shows that wealth, technology and policy can overcome physical scarcity, in sharp contrast to economically water-scarce regions that cannot afford such infrastructure.

Try this

Q1. State the drainage-basin water-budget equation. [2 marks]

• Cue. Precipitation = evapotranspiration + runoff + change in storage.

Q2. Explain why transboundary rivers can cause international conflict. [4 marks]

• Cue. Upstream abstraction or dams reduce downstream flow, so countries compete over a shared, finite resource crossing borders.