Cambridge IGCSE0680

Global water distribution

Environmental Management 0680 Chapter Notes

What this chapter covers

Global water distributionThe water cycleWater supplyWater usageWater quality and availabilityMultipurpose dam projectsWater pollution and its sourcesImpact of water pollutionManaging pollution of fresh waterManaging water-related disease
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In this chapter

  1. 1Earth's Water: A Global Inventory
  2. 2The Global Water Cycle
  3. 3Water Scarcity and Availability
  4. 4Access to Safe Drinking Water

1. Earth's Water: A Global Inventory

Although over 75% of the Earth's surface is covered in water, its usability for humans is extremely limited. The vast majority, about 97%, is saltwater found in oceans and is unsuitable for drinking or agriculture. Only 3% of all water is fresh water. However, most of this fresh water is not easily accessible. Over two-thirds (approximately 69%) is locked away as ice in polar ice caps and glaciers. Another 30% is stored underground as groundwater. This leaves only about 1% of all fresh water (or about 0.03% of all Earth's water) on the surface in lakes, rivers, and swamps, readily available for human use.

Key term

Freshwater: Water that contains low concentrations of dissolved salts, making it suitable for human consumption and other uses.

Examiner insight

Examiners reward answers that use specific data, such as quoting the 3% freshwater figure and explaining that most of this is frozen in ice caps.

Common pitfall

Confusing the 3% of total water that is fresh with the amount of water that is actually available for human use. Most of that 3% is frozen and inaccessible.

Fun fact

If all the world's water were collected into a 4-litre jug, the readily available freshwater for human use would amount to only about one tablespoon.

Worked example 13 marks

The total volume of fresh water on Earth is approximately 41.6 million km³. If 68.7% of this is in ice caps and glaciers, and 30.1% is groundwater, calculate the volume of surface fresh water in km³.

  1. 1

    Step 1: Find the total percentage of fresh water that is NOT surface water. This is the sum of ice caps and groundwater percentages: 68.7% + 30.1% = 98.8%.

  2. 2

    Step 2: Calculate the percentage of fresh water that is on the surface. Total fresh water is 100%, so surface water is 100% - 98.8% = 1.2%.

  3. 3

    Step 3: Calculate the volume of surface water. This is 1.2% of the total fresh water volume: 0.012 * 41,600,000 km³.

  4. 4

    Step 4: The final answer is 499,200 km³.

Recap

  • Approximately 97% of Earth's water is saltwater, and 3% is freshwater.
  • The majority of freshwater (about 69%) is frozen in ice caps and glaciers.
  • Groundwater accounts for about 30% of all freshwater.
  • Less than 1% of the world's freshwater is easily accessible in rivers and lakes.
  • The tiny fraction of accessible freshwater must support the entire global population and ecosystems.

Quick check

  1. What percentage of the Earth's total water is freshwater?1 mark
  2. Where is the majority of the Earth's freshwater stored?1 mark

2. The Global Water Cycle

The water cycle describes the continuous movement of water on, above, and below the Earth's surface. It is a closed system, meaning the total amount of water is constant. The sun's energy drives the cycle by causing evaporation, turning liquid water from oceans and lakes into water vapour. Plants also release water vapour in a process called transpiration. This vapour rises, cools, and undergoes condensation to form clouds. When the water droplets in clouds become heavy enough, they fall back to Earth as precipitation (rain, snow, sleet). On the ground, water can be intercepted by plants, infiltrate the soil to become groundwater, or flow over the land as surface run-off, eventually returning to rivers, lakes, and oceans.

Key term

Water Cycle: The continuous circulation of water on, above, and below the Earth's surface, driven by solar energy and gravity.

Examiner insight

Marks are often awarded for using precise scientific terms for each process (e.g., 'precipitation' instead of just 'rain', 'transpiration' for water loss from plants).

Common pitfall

Confusing infiltration (water soaking into the ground) with surface run-off (water flowing over the land surface).

Worked example 14 marks

Describe the journey of a water molecule from the ocean to a river on land, naming four key processes of the water cycle.

  1. 1

    Step 1: Evaporation. The sun heats the surface of the ocean, causing the water molecule to change from a liquid to a gas (water vapour) and rise into the atmosphere.

  2. 2

    Step 2: Condensation. As the water vapour rises, it cools and changes back into tiny liquid water droplets, forming a cloud with other droplets.

  3. 3

    Step 3: Precipitation. The water droplets in the cloud combine and become heavy enough to fall back to Earth over land as rain.

  4. 4

    Step 4: Surface Run-off. The raindrop lands on the ground and, instead of soaking in, flows over the land's surface, eventually joining a stream or river.

Recap

  • The water cycle is the constant movement of water between the Earth and the atmosphere.
  • Evaporation turns liquid water into water vapour.
  • Condensation is water vapour cooling to form liquid water droplets (clouds).
  • Precipitation is water falling from clouds as rain, snow, or hail.
  • Infiltration is water soaking into the ground, while run-off is water flowing over it.
  • Transpiration is the release of water vapour from plants.

Quick check

  1. What is the term for water vapour turning into clouds?1 mark
  2. Name two ways water on land can return to the ocean.2 marks

3. Water Scarcity and Availability

Water is not distributed evenly across the globe. Some countries receive abundant rainfall, while others are deserts. This leads to issues of water scarcity, which can be categorized into two types. Physical water scarcity occurs when there is not enough water to meet demand due to natural reasons like low rainfall and high evaporation; this affects arid regions like the Middle East. Economic water scarcity occurs when a country has sufficient water resources but lacks the financial investment and infrastructure to extract, treat, and distribute it to the population. Many countries in sub-Saharan Africa face this challenge. A country's water security is not just about total rainfall; it is also determined by population size. A country with vast water resources but a massive population (like China) may have less water available per person than a drier country with a small population.

Water Availability per Capita (m³/person/year) = Total Renewable Water Resources (m³) / Total Population

Key term

Water Scarcity: The lack of sufficient available water resources to meet the demands of water usage within a region.

Examiner insight

Examiners look for a clear distinction between physical and economic water scarcity, often rewarding answers that provide a named example for each type.

Common pitfall

Assuming that a country with high total rainfall cannot experience water shortages. High population density or economic scarcity can still cause major problems.

Worked example 14 marks

Country A has 500 km³ of water available annually for its 25 million people. Country B has 80 km³ of water for its 2 million people. Calculate the per capita water availability for each country and determine which is more 'water wealthy' per person.

  1. 1

    Step 1: Convert populations to the same unit. 25 million = 25,000,000. 2 million = 2,000,000.

  2. 2

    Step 2: Convert water volume to m³. 1 km³ = 1,000,000,000 m³. So, Country A has 500,000,000,000 m³ and Country B has 80,000,000,000 m³.

  3. 3

    Step 3: Calculate per capita for Country A: 500,000,000,000 m³ / 25,000,000 people = 20,000 m³/person/year.

  4. 4

    Step 4: Calculate per capita for Country B: 80,000,000,000 m³ / 2,000,000 people = 40,000 m³/person/year.

  5. 5

    Step 5: Conclude that Country B is more 'water wealthy' per person, with double the per capita availability of Country A, despite having less total water.

Recap

  • Global water resources are unevenly distributed.
  • Physical water scarcity is a lack of water due to climate.
  • Economic water scarcity is a lack of investment and infrastructure to access existing water.
  • Water availability per person depends on both total water resources and population size.
  • A country rich in water can still have low per capita availability if its population is very large.

Quick check

  1. A country has rivers but no money for pipes or treatment plants. What type of scarcity is this?1 mark

4. Access to Safe Drinking Water

The presence of water does not guarantee it is safe to drink. Potable water is water that is free from harmful contaminants and microorganisms and is safe for human consumption. Ensuring water is potable relies on two main principles: sanitation and water treatment. Sanitation involves the safe management and disposal of human waste (sewage) to prevent it from contaminating drinking water sources. Water treatment is the process of purifying raw water to make it potable. Despite global progress, billions of people, particularly in developing countries, lack access to both basic sanitation and safe drinking water. This leads to widespread waterborne diseases like cholera and typhoid. The burden of collecting water often falls on women and girls, who may spend hours each day on this task, preventing them from attending school or earning an income.

Key term

Sanitation: The conditions necessary for public health, such as providing clean drinking water and the safe disposal of sewage.

Examiner insight

Students who can clearly link the lack of sanitation to the contamination of specific water sources (e.g., groundwater, rivers) and subsequent spread of named diseases demonstrate a strong understanding of the topic.

Common pitfall

Thinking that visually clear water is always safe to drink. Dangerous microorganisms are invisible to the naked eye.

Worked example 14 marks

Explain why building a new sanitation system with toilets and sewers is as important for public health as building a new water treatment plant.

  1. 1

    Step 1: State the purpose of a water treatment plant: It cleans incoming water to make it safe to drink (potable).

  2. 2

    Step 2: State the purpose of a sanitation system: It safely removes and treats wastewater and sewage, preventing it from contaminating the environment.

  3. 3

    Step 3: Link the two systems. Without a sanitation system, even if you treat water, the sources (rivers, groundwater) can be constantly re-contaminated by untreated sewage. This puts a huge strain on the treatment plant and increases the risk of system failure.

  4. 4

    Step 4: Conclude that sanitation prevents contamination at the source, while treatment cleans up what's left. Both are essential for a safe and sustainable water supply and to break the cycle of waterborne disease.

Recap

  • Potable water is water that is safe to drink.
  • Sanitation systems prevent drinking water sources from being contaminated by human waste.
  • Water treatment processes remove impurities and kill harmful microbes to make water potable.
  • Lack of access to safe water and sanitation is a major cause of disease and death worldwide.
  • The task of collecting water often has significant negative social impacts on women and girls.

Quick check

  1. What is the term for water that is safe for human consumption?1 mark
  2. Name one waterborne disease often linked to poor sanitation.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the percentage of the Earth's water that is saltwater, and the percentage that is fresh water.2 marks
  2. Name and briefly describe three processes in the water cycle.3 marks
  3. If 3% of the Earth's water is fresh, and 30.1% of that fresh water is groundwater, what percentage of the Earth's total water is groundwater? Show your working.3 marks
  4. Explain the difference between physical water scarcity and economic water scarcity. For each, suggest a country or region that might experience it.4 marks
  5. Describe how the lack of adequate sanitation in a community can lead to an outbreak of a waterborne disease like cholera.4 marks
  6. A country has a total renewable water resource of 60 km³ and a population of 25 million people. Calculate the annual water availability per capita in m³.4 marks
  7. Evaluate the statement: 'A country with high annual rainfall will not have problems with its water supply.' In your answer, consider population, economic factors, and pollution.6 marks
  8. Imagine a water molecule in a farmer's field in Brazil that has just been taken up by the roots of a corn plant. Describe a possible journey it could take to end up in the Arctic Ocean, naming and explaining at least five distinct processes from the water cycle.6 marks
  9. Define the term 'potable water' and explain why it is a more precise term than 'clean water'.2 marks
  10. Discuss the global distribution of fresh water. Explain why, despite there being enough total fresh water for everyone on Earth, billions of people face water shortages.6 marks

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