IB ESS SLTopic 4 — Water Access, Use & SecurityPaper 1 & 2Core skill~11 min read
Ways to Increase Water Supply
When a society grows or gets richer, its water demand rises. There are only two honest responses: find more water, or waste less of what you already have. Every strategy in this sub-topic is one of those two, and the best answers in an exam are the ones that combine both.
📚 What you need to know
Societies undergoing population growth or economic development must increase supply or use water more efficiently.
Water is needed for domestic use, agriculture (livestock and irrigation) and industry.
Supply strategies include dams and reservoirs, rainwater catchment, desalination, wetland enhancement, better irrigation, recycling and reuse, artificial aquifer recharge and redistribution.
Each has costs as well as benefits — financial, environmental and social.
Wetlands are worth knowing as more than habitat: they purify water and recharge groundwater.
Sustainable management needs a combination of strategies, not one big project.
Where the water actually goes
Before deciding how to get more, it is worth knowing who is using it. Globally the split is lopsided, and it explains why agricultural strategies dominate this topic.
Domestic saving matters and is easy to campaign about, but it is the smallest slice. A small percentage improvement in irrigation efficiency saves more water than every low-flush toilet in a country.
The strategies
Strategy
How it works
Example
Dams and reservoirs
Structures that store water, regulate river flow and reduce flooding, holding water from the wet season for use in the dry one
The Hoover Dam in the USA created Lake Mead, supplying several states and generating hydroelectric power
Rainwater catchment
Collecting and storing run-off from rooftops and other surfaces, usually for non-potable uses such as irrigation, flushing and cleaning, which takes pressure off the mains supply
Rooftop harvesting in Chennai, India, which also reduces storm run-off, flooding and erosion
Desalination
Removing salt and minerals from seawater, most often by reverse osmosis, to produce fresh water
The Jebel Ali plant supplies a large share of Dubai’s water
Wetland enhancement
Restoring wetlands so they filter pollutants naturally and help recharge groundwater
Restoration work in the Everglades, Florida, to improve water flow and quality
Better irrigation
Efficient techniques such as drip irrigation that cut the water wasted in agriculture
Israel’s advanced drip irrigation technology, developed to maximise efficiency in a dry climate
Recycling and reuse
Treating wastewater so it can be used again in industry or for irrigation
Singapore’s NEWater scheme, which reduces reliance on imported water
Artificial aquifer recharge
Directing surface water into the ground to replenish aquifers, preventing depletion and keeping wells and springs viable
Managed recharge projects in California, USA, offsetting over-extraction
Redistribution
Canals and pipelines moving water from water-rich regions to areas of scarcity, balancing supply and demand
The Central Arizona Project, carrying Colorado River water to arid parts of Arizona
Wetlands earn their place here. They are not only habitat. They provide ecosystem services that societies depend on — purifying water and recharging groundwater — which is why restoring one counts as a water supply strategy, not just a conservation project.
Desalination, close up
Desalination comes up constantly because it looks like the perfect answer: the ocean never runs out. Knowing roughly how reverse osmosis works lets you explain its drawbacks properly.
The two standard evaluation points sit in that last line. Desalination needs a lot of energy, which usually means a carbon cost, and returning hot, concentrated brine to the sea harms marine life near the outfall.
If a question asks you to evaluate any water strategy, run the same three checks: what does it cost to build and run, what does it do to the ecosystem, and who benefits or loses. That structure works for dams, desalination and everything else on the list.
Two directions, not one
The source material tends to list strategies as one long menu. It is far easier to remember, and far better in an evaluation answer, if you sort them into supply-side (get more water) and demand-side (need less water).
Supply-side projects are visible, expensive and popular with governments. Demand-side measures are cheaper and often more effective, but they need people to change what they do, which is the harder half.
Why a combination is needed
Dams, reservoirs, catchment systems, desalination plants and restored wetlands all raise availability. On their own they tend to be expensive, environmentally damaging, or both. They work far better when complemented by conservation, recycling and reuse, aquifer recharge and more sustainable agriculture.
A balanced approach also protects against a specific trap: build a big new supply and demand often rises to meet it, leaving you no better off than before. Managing demand at the same time is what stops that happening.
Worked examples
EXAM Q1
Evaluate the use of desalination to increase a city’s water supply. [4]
Strength 1: reliability
It does not depend on rainfall, so it keeps working through droughts.
Strength 2: scale
A large plant can supply a substantial share of a coastal city’s water.
Limitation 1: energy
Reverse osmosis is energy-intensive, so it is costly and often adds greenhouse gas emissions.
Limitation 2: waste
Concentrated brine returned to the sea damages marine ecosystems near the outfall, and only coastal places can use it.
Reliable but expensive — best as one part of a wider strategy“evaluate” needs both sides and a judgement at the end
EXAM Q2
Explain two disadvantages of building a large dam and reservoir. [4]
Disadvantage 1: ecological
Flooding a valley destroys habitat, and the dam blocks fish migration and traps sediment that used to enrich land downstream.
Disadvantage 2: social and political
People are displaced from the flooded land, and downstream countries or regions receive less water, which can cause conflict.
Environmental damage upstream and downstream, plus displaced communitiestwo developed points beat four one-line assertions
EXAM Q3
A dry farming region wants to improve its water situation. Suggest a combined strategy. [3]
Supply sideRainwater catchment to capture the wet season, and artificial aquifer recharge to store it underground where it will not evaporate.
Demand side
Switch to drip irrigation and drought-resistant crops, since agriculture is the largest user.
Management side
Meter and price water so the efficiency gains are not simply used up by irrigating more land.
Store more, use less, and manage the demand so savings sticknaming both a supply and a demand measure is what makes it “combined”
💡 Exam tip
Learn three named examples you can reuse: Hoover Dam, Singapore’s NEWater, Israeli drip irrigation. That is usually enough for any question here.
Sort strategies into supply-side and demand-side in your head before writing. It gives an evaluation answer instant structure.
For any strategy, be ready with one cost and one benefit. Never present a strategy as purely good.
Mention agriculture whenever scale matters — roughly 70% of use is there.
Say wetlands provide ecosystem services such as purification and recharge, not just habitat.
Finish evaluation answers with a judgement. “A combination is needed” is a legitimate and creditworthy conclusion here.
⚠ Common mix-up
Treating desalination as a universal fix. It needs a coastline and a lot of energy, and it produces brine.
Assuming dams are purely beneficial. They store water and generate power, but they also displace people and wreck river ecosystems.
Confusing recycling with desalination. Recycling treats wastewater; desalination removes salt from seawater.
Forgetting that new supply can increase demand. Cheap, plentiful water tends to get used.
Listing strategies without evaluating them when the command word asks for evaluation. A list caps your marks.
Ignoring aquifer recharge. It is the direct answer to the depletion problem from the last sub-topic.
Up next: Tackling Water Scarcity — the difference between not having enough water and not being able to reach the water that exists, and what actually works in each case.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
