The global water cycle as a system, drainage basin processes, and the causes, impacts and management of water insecurity.

What this dot point is asking

WJEC wants you to describe the global water cycle and its stores and flows, explain drainage-basin processes, and analyse the causes, impacts and management of water insecurity, with located examples.

The answer

The global water cycle

About $97$ per cent of water is saline ocean; most of the remaining freshwater is locked in ice and deep groundwater, leaving under $1$ per cent as accessible surface freshwater. Flows include evaporation, transpiration, condensation, precipitation, cryospheric processes and runoff. Stores have very different residence times, from days in the atmosphere to millennia in deep ice and aquifers, which matters because slowly recharging stores cannot be exploited sustainably at high rates.

Drainage-basin processes

Factors such as relief, geology, soil, vegetation and land use change how quickly water reaches the channel, shaping the storm hydrograph (lag time, peak discharge). Urbanisation, which adds impermeable surfaces and drains, and deforestation both shorten lag times and raise peak discharge and flood risk, while afforestation and wetlands lengthen lag times and reduce flooding.

Causes of water insecurity

Physical causes include climate variability, drought, seasonal and unreliable rainfall, and salinisation. Human causes include population and economic growth, irrigation demand (agriculture uses around $70$ per cent of freshwater withdrawals globally), industrial and domestic pollution, and over-abstraction of rivers and aquifers faster than they recharge. Water stress is uneven, so some regions face chronic water insecurity while others have surplus, and the gap between supply and demand defines a country's water security.

Impacts and management

Insecurity threatens drinking water and sanitation, food production, industry and ecosystems, and can drive transboundary tension over shared rivers and aquifers, such as competition on the Colorado River between US states and Mexico, or on the Nile between Egypt, Sudan and Ethiopia over the Grand Ethiopian Renaissance Dam. Management splits into supply schemes (dams and reservoirs, inter-basin transfers, desalination, groundwater) and demand-side sustainable approaches (conservation, recycling, leakage reduction, smart irrigation and integrated water-resource management).

Examples in context

Example 1. The Colorado River basin (USA and Mexico). The Colorado is the classic transboundary water-insecurity case study. Over-allocation under the 1922 Colorado River Compact, decades of drought and rising urban and agricultural demand have shrunk the river so that it frequently dries up before reaching the Gulf of California. Reservoir levels at Lake Mead and Lake Powell have repeatedly hit record lows, forcing emergency cuts to states water shares. The basin shows physical and human causes combining, supply-side engineering reaching its limits, and the need for cross-border cooperation and demand management.

Example 2. The Nile and the Grand Ethiopian Renaissance Dam. The Nile is shared by eleven countries, with Egypt historically dependent on it for almost all its water. Ethiopia's construction of the Grand Ethiopian Renaissance Dam upstream, to generate hydroelectricity, has raised acute tension with downstream Egypt and Sudan over how quickly the reservoir is filled and how flow is guaranteed in drought years. The case illustrates transboundary conflict over a shared basin, the trade-off between upstream development and downstream security, and why treaties and integrated water-resource management matter where one river crosses many borders.

Try this

Q1. Define the water balance of a drainage basin. [2 marks]

• Cue. Precipitation minus evapotranspiration minus the change in storage, governing runoff and discharge.

Q2. Explain one human cause of water insecurity. [3 marks]

• Cue. Over-abstraction of rivers or aquifers for irrigation and supply removes water faster than it is replenished, lowering water tables and reducing availability.