By January 2018, Cape Town, a city of about 4 million people, had gone through three years of severe drought. Its dams had fallen to around 25% of capacity, and officials set a date, "Day Zero," when they expected to shut off most of the city's taps entirely.
How close did Cape Town actually come to turning off the taps?
What water security means
Water security is having access to sufficient amounts of safe drinking water. It is a significant component of a sustainable society: without it, health, food production and economic activity are all put at risk.
Why access is not equally distributed
Social, cultural, economic and political factors all shape the availability of freshwater and, separately, how equitably it is actually accessed once available. A society can have enough water overall and still leave some groups with far less reliable access than others.
Practice: classify each factor by the type of impact it represents.
Before Cape Town's crisis even began, many residents of the city's informal settlements already relied on shared communal standpipes rather than a household tap, so they had far less slack to cut back further when restrictions arrived.
Wealthier households facing the same restrictions could afford to drill a private borehole or pay for delivered water, while lower-income households facing an identical daily limit had no such option.
In some communities, water used for religious or ceremonial purposes is treated as a priority use that households continue to set aside even during a strict shortage.
The City of Cape Town set a legally enforceable household usage limit and imposed fines and, for repeat offenders, restrictive water-flow devices on households that exceeded it.
Water security (whether a society has, and can rely on, enough safe water) with equitable access (whether that water reaches everyone within the society fairly). Cape Town's dams held enough water to supply the metro overall for most of the crisis; the harder, ongoing question was who inside the city could actually depend on their share.
Rising demand forces a choice
As a society's population grows or its economy develops, it must either increase its water supply or use the water it already has more efficiently. Water is drawn on for domestic use, for irrigation and livestock in agriculture, and for industry, so growth in any one of those sectors adds pressure to the same finite local supply.
Ways to increase supply
Supply can be increased by constructing dams and reservoirs, building rainwater catchment systems, building desalination plants, or restoring and enhancing natural wetlands that store and slowly release water. Desalination removes salt and other minerals from seawater or brackish water to produce freshwater; reverse osmosis, forcing water through a semi-permeable membrane that blocks salt, is the most common method.
- Expanded groundwater abstraction from the Cape Flats and Table Mountain Group aquifers, reducing reliance on rain-fed dams alone
- Built temporary, then planned permanent, seawater desalination capacity using reverse osmosis
- Cleared water-hungry invasive alien vegetation from mountain catchments: in one six-month stretch this alone freed up more than 16 billion liters of water, since these invasive trees can consume tens of billions of liters a year that would otherwise reach the dams
- Began investing in water reuse and reclamation, treating wastewater to a standard where it can supplement supply
Figures: City of Cape Town New Water Programme reporting; IOL/Cape Argus, 2024 and 2026.
The population growth and economic development driving demand here are the same forces Topic 8 studies directly: population growth rates, the demographic transition and consumption patterns. A named country's total water demand cannot be understood from 4.2 alone without also asking what is happening to its population and its economy.
Water scarcity: physical or economic
Water scarcity refers to the limited availability of water to a human society. It comes in two forms. Physical scarcity exists where the actual abundance of water present is too low to meet demand, common in naturally arid regions. Economic scarcity exists where enough water physically exists nearby, but a society lacks the storage, treatment or transport infrastructure needed to deliver it, common where investment in water infrastructure has been limited.
Practice: classify each scenario as physical or economic scarcity.
A region receives less than half its usual annual rainfall for three consecutive years, and the rivers and dams that supply it physically hold far less water than usual.
A city sits beside a substantial river, but decades of underinvestment mean it lacks the pipes, pumps and treatment plants needed to deliver a reliable supply to most households.
Domestic conservation
At a domestic level, water can be conserved through metering (so households can see and manage their own usage), rationing, greywater recycling (reusing water from sinks, showers or washing machines for purposes such as flushing toilets or watering gardens), low-flush toilets, and rainwater harvesting.
Industrial and food-production conservation
At an industrial level, and specifically in food production, conservation techniques include greenhouses that capture and recycle harvested rainwater, aquaponics systems that combine fish and vegetable production in a shared recirculating system, drip irrigation systems that deliver water directly to a plant's roots, drought-resistant crop varieties, and shifting food production away from water-intensive livestock toward more vegetarian food production.
Practice: sort each technique into domestic or industrial/food-production conservation.
Water drained from a bathroom sink is piped to the toilet cistern and reused to flush it.
A commercial farm raises fish and vegetables together in one recirculating system, where nutrient-rich water from the fish tanks feeds the plants.
A household is fitted with a water meter so it can track its own daily usage against a target.
A vineyard installs a network of perforated tubing that delivers water drop by drop directly to each vine's roots instead of spraying the whole field.
If asked to outline or describe conservation strategies, name the actual technique (greywater recycling, drip irrigation, low-flush toilets) rather than writing only "use less water," and state whether it is a domestic or industrial/food-production measure. Vague answers that never name a specific technique tend to lose marks even when the general idea is correct.
Mitigation strategies for water scarcity
Mitigation strategies to address water scarcity are rarely a single fix. They usually combine supply-side measures with demand-side measures, applied together and adjusted as conditions change.
- Demand-side: a citywide target of 50 liters per person per day, the lowest target ever set during a South African drought, backed by tiered tariffs that charged steeply more per liter above a household's allowance
- Supply-side: the New Water Programme's groundwater, desalination and alien-vegetation-clearing measures described earlier in this tab
- Result: daily citywide consumption fell from over 1,200 million liters a day before the crisis to close to the city's target, and combined with the return of average winter rain in June 2018, Day Zero was called off indefinitely
Figures: City of Cape Town crisis reporting; Grist, 2019; Global Citizen, 2020.
It is tempting to treat Cape Town's 2018 recovery as the end of the story. It was not. As recently as February 2026, the City of Cape Town placed itself in an "Early Drought Caution" phase after combined dam storage fell to around 55%, roughly 19 percentage points below the same point the previous year, and daily consumption again crept above target. Mitigation is an ongoing management task, not a one-time fix: a society that avoids one water crisis has not made itself immune to the next one.
Mitigation strategies always have to be evaluated for a specific, named place: what is affordable, what the local geology and climate allow (desalination needs a coastline; groundwater needs a viable aquifer), and what a population is realistically willing to sustain over years, not just weeks, of restriction.
- Cape Town Day Zero timeline, dam levels and rationing plan: Grist, 2019; Global Citizen, 2020; Wikipedia, "Cape Town water crisis."
- New Water Programme, alien vegetation clearing yield, and 2026 dam-level update: City of Cape Town reporting; IOL/Cape Argus, March 2024 and February 2026.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 4, Subtopic 4.2, statements 4.2.1-4.2.8.
Use secondary data sources to investigate water stress
The named application of skills for this subtopic is using secondary data sources, data someone else has already collected and published, to investigate the causes of water stress within a named society. Citable secondary sources for this include the FAO's AQUASTAT database, the World Bank's Open Data portal, the UN-Water SDG 6 Data Portal, and the World Resources Institute's Aqueduct water risk atlas. All four publish country-level renewable freshwater and water-use figures that anyone can look up.
When using a secondary data source in an exam answer or an internal assessment, name the actual source (for example, "FAO AQUASTAT data"), give the figure, and state the year the data is from where possible. Water availability changes year to year, so a figure without a year attached is much weaker evidence.
Try it: the water stress threshold is 1,700 m³ of clean, accessible water per person per year. Enter a renewable-freshwater figure below and check whether it falls above or below that line.
Figures to test it on, all renewable internal freshwater resources per capita, AQUASTAT/World Bank data via Our World in Data, 2022, the latest year available as of 2026:
Jordan
About 61 m³ per person per year, among the lowest in the world.
Egypt
About 9 m³ per person per year of internally generated renewable water; Egypt's total supply, including Nile inflow from upstream countries, is higher but still low.
Global average
About 5,400 m³ per person per year, well above the water stress threshold.
A country can look "water rich" in total volume and still be water stressed per person. Egypt's Nile carries a huge total volume of water, but that volume is shared across a very large population and much of it originates outside Egypt's own borders, which is exactly why per-capita figures, not total national volume, are what the 1,700 m³ threshold actually measures.
Glossary
Every term introduced in this subtopic. Terms marked HL are only required at Higher Level.
- Water security
- Having access to sufficient amounts of safe drinking water; a significant component of a sustainable society.
- Equitable access
- Whether water actually reaches everyone within a society fairly, distinct from whether the society as a whole has enough water overall.
- Water scarcity
- The limited availability of water to a human society, whether physical or economic in origin.
- Physical scarcity
- Water scarcity caused by a low actual abundance of water present in a region.
- Economic scarcity
- Water scarcity caused by a lack of storage, treatment or transport infrastructure, even where enough water physically exists nearby.
- Desalination
- Removing salt and other minerals from seawater or brackish water to produce freshwater.
- Reverse osmosis
- A method of desalination that forces water through a semi-permeable membrane that blocks salt and other dissolved minerals.
- Greywater
- Wastewater from sinks, showers or washing machines, low-contamination enough to be reused for purposes such as flushing toilets or irrigation after little or no treatment.
- Aquaponics
- A food-production system that combines raising fish and growing vegetables in a shared, recirculating water system.
- Drip irrigation
- An irrigation method that delivers water directly to a plant's roots through perforated tubing, minimizing water lost to evaporation or run-off.
- Mitigation strategy (water scarcity)
- A management measure intended to reduce or address water scarcity, typically combining supply-side and demand-side actions for a named place.
- Freshwater planetary boundary HL
- The proposed global limit on freshwater use, beyond which rising water stress risks abrupt and irreversible change to the hydrological system.
- Governance (water) HL
- The local and global rules, regulations and agreements that manage how freshwater is shared and used, needed to keep use at sustainable levels.
- Water footprint HL
- A measure of the water used by an individual, a nation, or to grow a crop, raise livestock or manufacture a product, used to inform decisions about water security.
- Citizen science HL
- Also called community or crowdsourced science; research in which members of the public collect data using a shared protocol, playing a growing role in monitoring water resources.
- Water stress HL
- A measure of the limitation of water supply that accounts for water quality, environmental flows and accessibility, not only scarcity of availability; defined at less than 1,700 m³ of clean, accessible water per person per year.
- Environmental flow HL
- The quantity, timing and quality of water flow required to sustain freshwater and estuarine ecosystems and the human livelihoods that depend on them.
- Transboundary dispute HL
- A conflict over a shared water source that crosses regional or national boundaries, often rooted in historical or political context.
- Brine discharge HL
- The concentrated salt byproduct released back into the sea by a desalination plant, a negative environmental impact of industrial freshwater production.
- Saline intrusion HL
- The movement of salt water into a freshwater aquifer, often caused by over-abstraction of groundwater; one of the impacts industrial freshwater production can have on aquifers.
Test Yourself is coming soon for this subtopic
Practice questions and markschemes for this page are still being written. Check back once they have been added.
Freshwater as a planetary boundary
Freshwater use is one of Earth's proposed planetary boundaries. As demand for a limited freshwater supply rises, water stress increases, and beyond some threshold, the risk grows of abrupt, irreversible change to the hydrological system itself, not just a shortage for the humans depending on it, but a real shift in how the system behaves.
Planetary boundaries were introduced in 1.3, alongside boundaries for climate change, biodiversity loss and biogeochemical flows. Freshwater use is assessed on two separate measures within that framework: green water (soil moisture available to plants) and blue water (water in rivers, lakes and aquifers). Both can be pushed into a zone of increasing risk by the same forces driving demand in this subtopic: population growth, economic development and land-use change.
Governance keeps use sustainable
Local and global governance is needed to maintain freshwater use at sustainable levels. A local example is a municipal bylaw banning garden watering or setting a household usage limit during a drought, exactly what Cape Town did in 2018. A global or regional example is a formal international agreement over a shared water source, such as the Indus Waters Treaty (1960), which has governed how India and Pakistan share the Indus river system for over six decades despite the two countries' broader tensions.
Water footprints
A water footprint is a measure of the water used by an individual or a nation, or the amount needed to grow a crop, raise livestock, or manufacture a product such as textiles or steel. Water footprints can inform decision-making about water security by revealing water use that is otherwise invisible to a consumer, most of a product's water footprint is embedded in its production, not its final use.
Practice: click each item below in order, from smallest water footprint to largest.
A product's water footprint is almost never the water visible in the finished item. Cotton is an extremely thirsty crop to grow and process, which is why a pair of jeans, made mostly of dry fabric, still carries a footprint of thousands of liters: nearly all of it went into growing the cotton, long before the jeans reached a shelf.
Citizen science in water monitoring
Citizen science, also called community or crowdsourced science, is playing an increasing role in monitoring and managing water resources. Its defining features are that anyone can take part, all participants use the same protocol so their data can be combined into one high-quality dataset, and the resulting data is open access.
During the Cape Town crisis, the city published near real-time combined dam-level data that residents could check daily, and independent citizen dashboards tracking usage against target sprang up alongside it. Making the data visible and shared, rather than held only by officials, is credited as part of what drove such a sharp, sustained drop in household consumption.
Citizen science has limits too: volunteer measurements can vary in accuracy between participants, coverage tends to cluster where volunteers already live rather than where data is most needed, and a crowdsourced dataset still needs a coordinating body to check, combine and publish it responsibly.
Water stress: a broader measure than scarcity
Water stress, like water scarcity, measures the limitation of water supply, but it is broader. It accounts not only for the scarcity of availability but also water quality, environmental flows (the water a river or wetland needs to sustain its own ecosystem) and accessibility. A region can have an ample overall supply and still experience water stress because of poor water quality.
The water stress threshold
Water stress is defined as a clean, accessible water supply of less than 1,700 m³ per person per year. The Skills tab has a calculator to test real countries' renewable freshwater figures against this exact threshold.
Water scarcity (4.2.5, a matter of quantity: is there physically or economically enough water) with water stress (a matter of quantity, quality, environmental flow and accessibility combined). Every water-scarce region is under water stress, but a region can be water stressed from poor water quality alone even if it is not, strictly, scarce in quantity.
Causes of water stress depend on context
The causes of increasing water stress depend heavily on a country's socio-economic context. In an emerging economy pursuing rapid industrialization, water stress can arise from growing industrial and urban demand outpacing supply. In a low-income country, water stress can instead arise from over-abstraction driven by population pressure on subsistence agriculture, with little capital available to invest in more efficient irrigation or storage.
Transboundary disputes
Water stress can also arise from transboundary disputes, when a water source crosses regional or national boundaries and the countries or regions sharing it disagree over its use.
- Ethiopia built the GERD on the Blue Nile, completing its reservoir filling in September 2024 and formally inaugurating the dam in September 2025, ending 14 years of construction
- Egypt, almost entirely dependent on the Nile for its freshwater, calls the dam an existential threat to its water supply and filed a formal complaint with the UN Security Council after the 2025 inauguration, accusing Ethiopia of operating it without downstream countries' consent
- Sudan sits between the two and is affected by both the dam's operation and downstream flooding risk
- In October 2025, a sharp rise in Nile water levels, releases estimated at around 2 billion m³ over three weeks, flooded farmland and villages in Egypt and Sudan and reignited the dispute
- Egypt and Sudan have called for joint, legally binding management of the dam's operation; Ethiopia, which sees the GERD as central to its own development and electricity supply, has consistently refused, and the underlying dispute remains unresolved even with the dam now complete
Figures: France 24, July 2025; Foreign Policy Research Institute, September and October 2025; Middle East Council on Global Affairs, 2025.
For a transboundary dispute question, name the specific shared water source and the countries involved, and give the actual historical or political context, not just "two countries disagreed about water." The GERD dispute's context is specific: Ethiopia's need for hydropower and development versus Egypt's near-total dependence on a single river it does not control the source of.
Addressing water stress at an industrial level
Water stress can be addressed at an industrial level through strategies such as dams, water transfer pipelines or tankers, estuary storage with barrages, rainmaking or cloud seeding, desalination, solar distillation, dew harvesting, water treatment plants, and aquifer storage and recovery, deliberately recharging an aquifer to store water underground for later use.
The environmental cost of industrial freshwater production
Industrial freshwater production has negative environmental impacts that can be minimized but usually not eliminated entirely. These include concentrated brine discharges from desalination plants (the leftover salt and minerals, released back into the sea at a much higher concentration than surrounding seawater), noise and air pollution from the plants themselves, saline intrusion into aquifers from heavy groundwater abstraction, and the combustion of fossil fuels to power the process, desalination in the United Arab Emirates is a named example of a country meeting large-scale water demand this way, at significant energy and emissions cost.
This is the same trade-off Topic 7 studies for energy: a technology can solve one resource problem (water supply) while creating another (fossil fuel demand and emissions), so a full evaluation has to weigh both sides rather than treating desalination as a cost-free fix.
Inequitable access, health and development
Inequitable access to drinkable water and sanitation negatively impacts human health and sustainable development. According to the WHO/UNICEF Joint Monitoring Programme's 2025 report, roughly 1 in 4 people worldwide, about 2.1 billion people, still lack access to safely managed drinking water, including 106 million who drink directly from untreated surface water sources. These disparities fall hardest on people in low-income countries, fragile settings, rural communities, and minority ethnic and indigenous groups, exactly the kind of named marginalized groups this statement asks you to be able to discuss with real evidence.
The consequences extend well beyond thirst: unsafe water and inadequate sanitation drive preventable disease, cost time (often disproportionately borne by women and girls who walk to collect water), and limit school attendance and economic participation, all of which slow a community's broader sustainable development.
This equity gap is the HL-depth version of the social and economic factors introduced in 4.2.2: a society's overall water security can look adequate in national statistics while specific marginalized groups within it remain systematically underserved.
- Water footprint figures: Water Footprint Network product gallery; sciencemirror.com, 2025 estimates.
- GERD dispute timeline and facts: France 24, July 2025; Foreign Policy Research Institute, September and October 2025; Middle East Council on Global Affairs, 2025.
- Global drinking water access statistics: WHO/UNICEF Joint Monitoring Programme, 2025 report (2024 data).
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 4, Subtopic 4.2, statements 4.2.9-4.2.19.