IB DP Environmental Systems and Societies – Water systems, security, aquatic food production and pollution Revision Notes

For full explanations and worked examples, read the Topic 4 Water study guide first. These notes are for the final weeks before your exams.

They cover Topic 4, Water, of IB Diploma Programme Environmental Systems and Societies: syllabus sections 4.1 (water systems), 4.2 (water access, use and security), 4.3 (aquatic food production systems) and 4.4 (water pollution). The topic is studied at SL (12 hours) and HL (25 hours), so HL students study some topics in extra depth. It follows the IB ESS subject brief for first assessment 2026 — the course examined in the May and November 2026, 2027 and 2028 sessions.

Useful links: IB DP ESS course hub, printable ESS checklist, Topic 4 practice questions, ESS assessment revision notes and, for systems vocabulary, the Topic 1 Foundations revision notes.

Key definitions

• Hydrological cycle: the system of storages and flows that moves water between the oceans, atmosphere, land and living things, driven by solar energy and gravity.
• Storage: a place water is held (ocean, ice, groundwater, lake, soil, atmosphere, river, biomass).
• Transfer: a flow that moves water without a change of state (infiltration, runoff, advection).
• Transformation: a flow that changes the state of water (evaporation, condensation, melting).
• Residence time: the average time water stays in a storage.
• Aquifer: permeable rock that stores and transmits groundwater.
• Thermohaline circulation: deep ocean circulation driven by differences in temperature and salinity.
• Upwelling: rising of cold, nutrient-rich deep water to the surface.
• Water security: reliable access to enough safe water for people and ecosystems, with acceptable risk from floods and droughts.
• Physical scarcity / economic scarcity: not enough water in the region / water exists but people cannot afford or reach it.
• Water footprint: total freshwater used to produce what a person or country consumes.
• Virtual water: water embedded in traded goods.
• Maximum sustainable yield (MSY): the largest catch that can be taken every year without shrinking the stock.
• Catch per unit effort (CPUE): catch divided by fishing effort; a proxy for stock size.
• Aquaculture: farming aquatic organisms.
• Point / non-point source: pollution from one identifiable outlet / from many diffuse sources.
• BOD: the oxygen used by microorganisms to break down organic matter in a water sample over a set time.
• Indicator species: an organism whose presence or absence shows the level of pollution.
• Eutrophication: nutrient enrichment of water leading to algal blooms, oxygen depletion and loss of life.
• Bioaccumulation / biomagnification: build-up of a persistent pollutant within one organism / rising concentration at each higher trophic level.

Calculations table

Quantity	Method	Units
Residence time	storage ÷ flow rate	years (or days)
Water per person	renewable water (m³) ÷ population	m³ per person per year
CPUE	catch ÷ effort	e.g. t per boat-day
Percentage change	(new − old) ÷ old × 100	%
Feed conversion ratio	feed mass ÷ increase in body mass	no units
Biomagnification factor	concentration higher level ÷ concentration lower level	no units

Remember: 1 km³ = 10⁹ m³.

Availability thresholds: 1,700 m³ or more, no stress; 1,000 to 1,699, stress; 500 to 999, scarcity; below 500, absolute scarcity.

Method in steps

Evaluating a water-security strategy

1. Say what it does (increase supply or reduce demand).
2. Give one benefit, with who gains.
3. Give one environmental cost and one social or economic cost.
4. Judge: under what conditions would it work best?

Interpreting fishery data

1. Calculate CPUE for each year.
2. Compare the trend in CPUE with the trend in total catch.
3. Falling CPUE with rising effort means a shrinking stock, even if the catch looks steady.
4. Link to MSY and suggest a named control (quota, mesh size, closed season, no-take zone).

Explaining eutrophication Nutrients in → algal bloom → light blocked, plants die → decomposers use oxygen → animals die → more nutrients released → positive feedback.

Describing a river pollution profile Quote the upstream (control) value, the lowest value and its distance, and the distance where values return to normal. Then explain using BOD.

Small worked reminders

Residence time. An aquifer holds 150 km³ and is recharged at 0.5 km³ per year. Residence time = 150 ÷ 0.5 = 300 years. Pumping it faster than 0.5 km³ per year mines it like a non-renewable resource.

Biomagnification. A metal rises from 0.2 ppm in small fish to 5.0 ppm in a predatory fish. Factor = 5.0 ÷ 0.2 = 25.

Human impacts on the water cycle

Activity	Main change to flows	Consequence
Urbanisation	Less infiltration, more surface runoff	Shorter lag time, higher flood peaks
Deforestation	Less interception and transpiration	More runoff and soil erosion
Irrigation	Water moved from rivers and aquifers to fields	More evaporation; falling water tables
Dams	Water held back in reservoirs	More evaporation; reduced, steadier flow downstream
Aquifer pumping	Abstraction faster than recharge	Falling water table; saltwater intrusion near coasts

Managing fisheries: strengths and limits

Measure	How it works	Limit
Quota (total allowable catch)	Caps the mass landed each year	Needs honest reporting; discards may rise
Mesh size	Lets young fish escape to breed	Does not cut overall effort
Closed season	Protects spawning fish	Effort may simply shift to other months
No-take marine protected area	Stock recovers and spills over into nearby waters	Needs patrols; fishers lose grounds
Eco-label	Consumers reward sustainable fleets	Only works where buyers pay attention to labels

Linking Topic 4 to the key concepts

• Systems: model a lake, river or fishery as storages and flows; identify feedback, such as the positive feedback in eutrophication.
• Sustainability: compare the rate of use with the rate of renewal, whether that is aquifer recharge or a fish stock’s yearly growth.
• Perspectives: farmers, city users, fishing communities and conservation groups value water differently; say whose view a strategy favours.

Must-know distinctions

• Transfer vs transformation: a change of state is the test.
• Physical vs economic scarcity: lack of water vs lack of access to it.
• Total catch vs CPUE: catch can hold steady while the stock falls, if effort rises.
• Wild fishery vs aquaculture: harvesting a common resource vs farming a managed one; each has different impacts.
• Point vs non-point source: a single pipe can be licensed and monitored; farm runoff cannot.
• Direct vs indirect measurement: a snapshot of chemistry vs a record of conditions over time.
• BOD vs dissolved oxygen: BOD is the demand; dissolved oxygen is the supply left in the water. High BOD usually goes with low dissolved oxygen.
• Bioaccumulation vs biomagnification: within one organism vs across trophic levels.
• Three levels of pollution management: change the activity, control the release, clean up and restore.

Where marks are usually lost

• Describing a river data table without quoting figures and distances.
• Classifying precipitation or runoff as a transformation.
• Leaving km³ unconverted, so per-person figures are out by a factor of a billion.
• Stating a per-person figure but not naming its category (stress, scarcity).
• Calling a fishery sustainable because the catch is steady, without checking effort or CPUE.
• Blaming algae directly for the fall in oxygen instead of decomposers.
• Listing management options without evaluating them or linking them to a level in the three-level model.
• Treating aquaculture as automatically sustainable, without naming its impacts.
• Giving one-sided evaluations in “to what extent” answers, with no judgement at the end.

Quick self-test

1. Is condensation a transfer or a transformation?
2. Give two effects of urbanisation on the hydrological cycle.
3. A country has 9 km³ of renewable freshwater a year and 10 million people. Calculate water per person and name the category.
4. Distinguish physical and economic water scarcity.
5. Why is upwelling linked to productive fisheries?
6. A fishery lands 3,600 t using 1,200 boat-days. Calculate CPUE.
7. Why is MSY taken at about half the carrying capacity in the logistic model?
8. A fish farm uses 1.8 t of feed and the fish gain 1.2 t. Calculate the FCR.
9. Name two indicator species of clean, oxygen-rich water.
10. Give one measure at each of the three levels of pollution management for nitrate pollution.

Answers

1. Transformation: water vapour changes to liquid.
2. More surface runoff and less infiltration because of impermeable surfaces; shorter lag time and higher flood peaks as drains speed water to rivers.
3. 9 × 10⁹ ÷ 10 × 10⁶ = 900 m³ per person per year: water scarcity (500 to 999).
4. Physical: not enough water in the region for demand. Economic: water is there but people lack the money or infrastructure to use it.
5. Upwelling brings nutrients from deep water to the sunlit surface, boosting phytoplankton growth and so food for fish.
6. 3,600 ÷ 1,200 = 3 t per boat-day.
7. The population grows fastest there, so the largest harvest can be replaced by new growth each year.
8. 1.8 ÷ 1.2 = 1.5.
9. Stonefly larvae and mayfly larvae.
10. Change the activity: apply less fertiliser at the right time. Control the release: buffer strips along rivers, or nitrate removal at treatment works. Clean up: dredge sediment or restore wetlands that absorb nutrients.

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief, Environmental systems and societies, first assessment 2026. These notes cover Topic 4, Water: sections 4.1, 4.2, 4.3 and 4.4. The brief lists subtopics for Topic 1 only; the subtopic numbering on this page follows the printable ESS checklist.

The brief gives Topic 4 Water 12 teaching hours at SL and 25 at HL. It does not list the subtopics or learning outcomes for this topic, so the numbered subtopics and outcomes on this page follow the syllabus numbering used in the printable ESS checklist, not the brief itself.