Global Water Distribution and Availability

Step 1: Identify the maximum percentage of Earth's water that is fresh.

• Maximum estimated fresh water = 3% (or 0.03 as a decimal).

Step 2: Convert the total volume into millions to match the required answer format.

• $1.4 \text{ billion km}^3 = 1400 \text{ million km}^3$

Step 3: Multiply the total volume by the fresh water percentage.

• $1400 \text{ million km}^3 \times 0.03 = 42 \text{ million km}^3$
• (Note: The estimated range of fresh water is 2.5% to 3%. Because the question asked for the maximum estimate, we use 3%).

Global Variations in Water Availability (Climate)

Why does the Amazon Basin have vast rivers while the Sahara is covered in dry sand dunes? Global water availability is primarily driven by atmospheric pressure belts, known as the Tricellular Model.

At the equator (the ITCZ), rising air creates low pressure systems, leading to intense convectional rainfall of over $2000\text{ mm}$ per year. This heavy precipitation creates a water surplus, where water supply vastly exceeds demand.

Conversely, at the subtropical high-pressure belts (30° north and south of the equator), sinking air prevents cloud formation. This mechanism causes a severe lack of rainfall (often less than $250\text{ mm}$ per year), resulting in physical water scarcity in arid regions like the Sahel or the Australian Outback. Mid-latitudes, such as the UK, experience frontal rainfall from the Ferrel Cell, providing a consistent supply.

The Water Balance Equation

Geographers calculate whether an area has a surplus or a water deficit (where demand exceeds supply) using the water balance equation:

Where:

• $P$ = Precipitation (Input)
• $Q$ = Runoff/Discharge (Output)
• $E$ = Evapotranspiration (Output)
• $\Delta S$ = Change in storage (Groundwater/Soil)

If precipitation is greater than the combined outputs of runoff and evapotranspiration ($P > Q + E$), the region is in surplus.

National Variations in Water Availability (Wealth)

Turning on a tap in the UK costs less than a penny per litre, but in other parts of the world, clean water is an expensive luxury. This difference is caused by wealth and infrastructure rather than climate, creating economic water scarcity. Over 1 billion people, particularly in Sub-Saharan Africa and South Asia, live in areas where water is physically present but inaccessible because their countries lack the monetary means to build dams, treatment plants, or pipe networks.

Consumption patterns also vary drastically between nations. High-Income Countries (HICs) use the majority of their water for industry (54%), whereas Low-Income Countries (LICs) use a massive 85% for agriculture. Globally, poor infrastructure means 45 million cubic metres of water is lost daily through leaky pipes.

Even within a single country, spatial imbalances exist. In the UK, the North and West experience high rainfall but have low populations (a surplus). The South and East, including London, have high populations but low rainfall, leading to water stress.

Calculating Water Scarcity

A country has an annual renewable water supply of $50 \text{ billion m}^3$ and a population of 60 million people. Calculate the water supply per person and determine if the country is experiencing water stress or water scarcity.

Step 1: State the definitions for stress and scarcity.

• Water stress: Annual supply falls below $1700\text{ m}^3$ per person.
• Water scarcity: Annual supply falls below $1000\text{ m}^3$ per person.

Step 2: Set up the calculation (Total Supply ÷ Population).

• $50,000,000,000\text{ m}^3 \div 60,000,000 \text{ people}$

Step 3: Calculate the final value and state the condition.

• $833\text{ m}^3$ per person.
• Because $833\text{ m}^3$ is less than $1000\text{ m}^3$, the country is experiencing water scarcity.

Local Variations in Water Availability (Geology & Pollution)

Understanding the rocks beneath our feet explains why some cities are literally sinking into the ground. Local geology dictates water storage: permeable rocks like chalk and sandstone allow water to infiltrate, forming reliable underground aquifers. In contrast, impermeable rocks like clay and granite do not allow infiltration, causing surface runoff and forming rivers.

When cities pump water from aquifers faster than rain can naturally replenish them, it causes over-abstraction. This rapidly lowers the water table and can cause the land above to collapse or subside. For example, a supply deficit in Beijing has caused land subsidence of up to $11\text{ cm}$ in some areas, while extraction by a Coca-Cola plant in Rajasthan, India, dropped the water table by $15\text{ m}$ in just ten years.

Pollution also severely limits local water availability by making existing supplies unusable. Agricultural runoff containing nitrates causes eutrophication in places like East Anglia (UK), depleting oxygen in rivers. Industrial mining has leaked lead into water sources in Zambia, while untreated domestic sewage spreads water-borne diseases in Mumbai's Dharavi slum.