4 Water

The Water topic studies the hydrosphere as a natural system and the ways human societies depend on it and change it. It follows water from the global hydrological cycle to the taps, fields, fisheries and sewers of human societies, and back into rivers, lakes and the sea.

Water links almost every other part of the course. It carries energy and nutrients through ecosystems, shapes climate and biomes, supports food production on land and at sea, and moves pollutants from where they are released to where they cause harm. Access to safe water is also one of the clearest measures of inequality between and within societies.

Questions the topic asks

• How does water move between the oceans, atmosphere, land and living things, and how do human activities alter those flows and stores?

• Why do some societies have secure water supplies while others face scarcity and stress, and how can water be shared fairly and used sustainably?

• How are fish and other aquatic foods produced, and to what extent can fisheries and aquaculture be sustainable?

• Where does water pollution come from, how is it measured, and how can it be prevented, reduced or cleaned up?

• How do the perspectives and values of different groups shape decisions about water?

The four subtopics

How the subtopics connect

Water systems come first because the other three subtopics depend on them. The stores and flows of the water cycle set how much fresh water a region can use (water security); the mixing, stratification and upwelling of water bodies control how productive lakes and seas are (aquatic food); and the same flows of run-off and streamflow carry pollutants into rivers, lakes and the sea (pollution).

The subtopics also feed back into each other. Pollution reduces the amount of safe water and so worsens water stress; nutrient pollution and the effluent from fish farms cause eutrophication and dead zones that damage fisheries; and over-abstraction of rivers concentrates pollutants and shrinks lakes, as the Aral Sea shows.

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Perspectives, systems and sustainability in the Water topic

Systems

Every subtopic uses systems diagrams. A lake or aquifer is modelled with a water budget of inputs and outputs; a fishery with inputs of recruitment and growth and an output of catch; a polluted lake with the positive feedback loop of eutrophication. Each model shows where a human activity changes a flow, and so where management can act.

Perspectives

Water problems are rarely only technical. A technocentric response to scarcity is to build dams, desalination plants and water transfers; an anthropocentric response is to manage demand through pricing, licences and efficiency; an ecocentric response is to leave water in rivers and wetlands and to restore the ecosystems that regulate supply. Fishing communities, governments, farmers, conservationists and Indigenous peoples often see the same lake, river or fishery in very different ways.

Sustainability

Water use is sustainable only when abstraction stays within the rate at which stores are replenished, when fish catches stay within what stocks can replace, and when pollutants are released no faster than water bodies can absorb them. Sustainable Development Goals 6 (clean water and sanitation) and 14 (life below water) set targets for the topic, and freshwater change is one of the planetary boundaries. Questions of environmental justice run through all four subtopics: who has safe water, who eats the fish, and who lives downstream of the pollution.

Links to other topics

• Foundation: systems, steady states, feedback and tipping points; perspectives; sustainability, footprints and planetary boundaries.

• Ecology: the water cycle beside the carbon and nitrogen cycles; productivity and energy flow in aquatic food webs; ENSO and climate.

• Biodiversity and conservation: invasive species and biodiversity loss in lakes and seas; marine protected areas as conservation.

• Land: water in soils, irrigation, salinisation and fertiliser run-off from farmland.

• Atmosphere and climate change: the oceans as a heat store and carbon sink, sea-level rise, droughts and floods, and the possible weakening of ocean circulation.

• Natural resources: water as a renewable resource that can be overused, and hydroelectric power.

• Human populations and urban systems: rising demand from growing cities, urban run-off and flood risk.

How SL and HL differ

SL and HL students study the same core content in all four subtopics: the water cycle and water budgets, water security and scarcity, fisheries and aquaculture, and water pollution and eutrophication, each with its own case study. HL students study everything SL students do, and each subtopic then adds sections marked “(HL only)”:

HL students also apply the three HL lenses to water. Environmental law appears in water-sharing treaties, the UN Convention on the Law of the Sea, the high seas treaty and drinking-water standards. Environmental economics appears in water pricing, the valuation of ecosystem services such as clean water and fisheries, and the costs of pollution and who should pay them. Environmental ethics appears in the right to water, the duties owed to downstream communities and future generations, and the debate over hunting marine mammals.

Assessment

The Water topic is examined in both external papers, and water investigations are popular choices for the internal assessment.

• Paper 1 gives an unseen case study, often a river basin, lake, coastal sea or city with water data. Typical questions ask students to construct or complete a systems diagram, calculate a water budget, a percentage change or a rate, interpret graphs of water levels or fish catches, explain the causes of scarcity or pollution, and evaluate a management strategy for the place from several perspectives.

• Paper 2 Section A asks short-answer and data-based questions: defining flows and terms, drawing the hydrological cycle or the eutrophication feedback loop, calculating BOD or a water budget, interpreting a yield–effort curve, and outlining conservation or pollution-management strategies.

• Paper 2 Section B essays often combine water with other topics, such as food security, climate change or urban growth. The final, markband-assessed part rewards named examples, linked systems reasoning, balanced perspectives and a supported judgement; HL answers are also expected to draw on the HL lenses.

• The internal assessment often uses water: measuring water quality along a river, comparing biotic indices above and below an outfall, or analysing database records of rainfall, river flow or fish catches.