IB ESS HLTopic 4 — Water Access, Use & SecurityPaper 1 & 2Core skill~10 min read
Ways to Increase Water Supply
When a society grows or gets richer, it needs more water than nature happens to deliver. There are only really four things you can do about that: catch more, make more, waste less, or move it from somewhere else. Every strategy in this topic is one of those four.
📘 What you need to know
Growing populations and developing economies force societies to increase supply or use water more efficiently.
The main demands are domestic, agricultural and industrial.
Supply strategies include dams and reservoirs, rainwater harvesting, desalination, wetland restoration, better irrigation, recycling and reuse, artificial recharge and redistribution.
Every strategy has costs and drawbacks as well as benefits — be ready to evaluate, not just describe.
Wetlands do a supply job for free: they filter water and recharge groundwater.
Real water management uses a combination of strategies, not one silver bullet.
Four things you can actually do
Sorting strategies this way stops you writing a list. It gives an essay a structure: what each group achieves, what it costs, and where each one suits.
The strategies in detail
Strategy
How it works
Real example and catch
Dams and reservoirs
A barrier across a river stores wet-season flow and releases it in controlled amounts, often generating hydroelectricity too
The Hoover Dam supplies several US states. Catch: floods land upstream and disrupts river ecology
Rainwater catchment
Rain running off roofs and hard surfaces is collected and stored, usually for non-drinking uses
Rooftop harvesting is widely used in Chennai, India. Catch: supply stops when the rain does
Desalination
Salt and impurities are removed from seawater, usually by forcing it through membranes under pressure (reverse osmosis)
Large plants supply Gulf cities such as Dubai. Catch: very energy-hungry and produces brine
Restoring wetlands
Healthy wetlands filter pollutants, slow flood water and let it soak down to recharge aquifers
Everglades restoration in Florida. Catch: slow, and needs land to be given back to nature
Better irrigation
Drip systems deliver water straight to roots, cutting evaporation and run-off losses
Israel is known for advanced drip technology. Catch: equipment costs are beyond many farmers
Recycling and reuse
Wastewater is treated to a standard suitable for industry, irrigation or even drinking
Singapore’s reclaimed water scheme cuts reliance on imports. Catch: expensive plants and public reluctance
Artificial recharge
Surface water is directed into the ground through wells or basins to top up an aquifer
Managed recharge in California offsets over-extraction. Catch: polluted surface water contaminates the aquifer
Redistribution
Canals and pipelines carry water from surplus regions to areas short of it
The Central Arizona Project moves Colorado River water to arid areas. Catch: high cost and harm to the source region
How desalination works
Desalination gets asked about often, so it is worth knowing the sequence rather than just the word.
Remembering that brine is a product, not a leftover, is what turns a description of desalination into an evaluation of it.
Notice that increasing supply and reducing demand end up in the same place. Fixing a leaking pipe network can deliver more usable water than a new reservoir, for a fraction of the cost and damage.
Wetlands: the strategy that costs nothing
Wetlands are easy to overlook because they look like wasted land. They are not. A functioning wetland:
Filters water naturally, as plants and sediments trap pollutants and nutrients.
Slows flood water, giving it time to soak in instead of racing downstream.
Recharges groundwater, topping up the aquifers that wells rely on.
Stores carbon and supports biodiversity at the same time.
Drain a wetland and you lose all of that, then pay to replace it with treatment works and flood defences. Protecting one is often the cheapest water strategy available.
Using a combined approach
No single strategy solves a water shortage. Building supply without managing demand just means the new supply fills up with new demand. A sensible plan usually mixes:
Storage or new supply, to raise the amount available.
Efficiency and conservation, to reduce the amount needed.
Recycling and recharge, to use the same water more than once.
Protection of natural systems, so the free services keep working.
Worked examples
WORKED EXAMPLE 1
How much can a roof collect?
A house has a roof area of 120 m2 in a region receiving 600 mm of rain per year. About 80% of the rain that lands can be captured. Calculate the annual harvest, and the fraction of a four-person household’s needs it covers at 150 litres per person per day.
Step 1: rainfall volume on the roof
600 mm = 0.6 m, so 120 × 0.6 = 72 m3Step 2: allow for losses72 × 0.8 = 57.6 m3 = 57 600 litres per yearStep 3: household demand4 × 150 × 365 = 219 000 litres = 219 m3Step 4: fraction covered57.6 ÷ 219 = 0.263About 26% of household demanduseful but not a whole solution — and it arrives in the wet season, when it is least needed
WORKED EXAMPLE 2
The energy cost of desalination
A city uses 600 000 m3 of water per day. A desalination plant supplies 15% of this. If producing fresh water takes about 3.5 kWh per m3, calculate the plant’s daily energy use.
Step 1: volume supplied600 000 × 0.15 = 90 000 m3 per dayStep 2: energy needed90 000 × 3.5 = 315 000 kWh per day315 000 kWh, or 315 MWh, every dayif that electricity comes from fossil fuels, solving a water problem creates a climate one — exactly the trade-off an evaluation question wants
WORKED EXAMPLE 3
Evaluate building a large dam [6]
How to structure a six-mark evaluation.
Benefits (two or three)
Stores wet-season water for the dry season, gives controlled release, provides hydroelectric power and flood control
Costs (two or three)Floods habitat upstream, displaces communities, blocks fish migration and traps sediment that farmland downstream relies on
Add a condition
Value depends on location and scale — and on whether affected people were consulted or compensated
Judgement
Justified where storage is the limiting factor and the flooded area is small, but a poor fit where the river supports downstream farming and fisheries
Benefits, costs, condition, judgementan evaluation without a judgement at the end loses the top marks, however good the lists are
💡 Exam tip
Learn two or three named schemes. One dam, one desalination plant, one recycling scheme is plenty.
For evaluate and discuss, always give drawbacks as well as benefits, then finish with a judgement.
Match the strategy to the place. Desalination suits a wealthy coastal city; rainwater harvesting suits a monsoon region; recharge suits somewhere with depleted aquifers.
Mention who pays and who benefits — the two are often different, and that is an equity point.
Remember that demand management counts. Using less has the same effect as supplying more.
Wetlands deserve a mention as natural infrastructure providing free ecosystem services.
⚠ Common mix-up
Treating desalination as a clean fix. It is energy-hungry and produces brine.
Confusing artificial recharge with aquifer storage and recovery. Recharge replenishes an aquifer; storage and recovery deliberately banks water for later withdrawal.
Assuming a dam creates water. It only stores and reschedules water that was already flowing.
Forgetting the source region when discussing redistribution. Water moved away is water someone else no longer has.
Describing when the question says evaluate. Describing earns the lower half of the marks only.
Ignoring maintenance. A pipeline or treatment plant that nobody can afford to repair stops being a supply.
Up next: Tackling Water Scarcity — the difference between not having water and not being able to get it, and what conservation looks like in homes and on farms.
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