Topic 4: Water
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4.1 Water systems
4.1.1Water movement drivers
• Solar radiation and gravity • Heat for evaporation, gravity for drainage
4.1.2Hydrological cycle
• System with stores and flows • Stores: boxes; flows: arrows
4.1.3Hydrological cycle stores
• Oceans (96.5%), glaciers/ice caps (1.7%), groundwater (1.7%) • Surface freshwater (0.02%), atmosphere (0.001%), organisms (0.0001%)
4.1.4Hydrological cycle flows
• Transpiration, sublimation, evaporation, condensation, advection, precipitation • Melting, freezing, run-off, infiltration, percolation, streamflow, groundwater flow
4.1.5Human activity impacts
• Agriculture, deforestation, urbanization alter flows and stores • Reduced evapotranspiration, increased run-off → flash floods
4.1.6Water body steady state
• Flow diagrams of inputs and outputs • Calculate sustainable harvesting rates
4.1.7(HL)—Water properties
• Polarity, cohesion, adhesion, solvent properties • Transparency, high specific heat capacity • Density variations with temperature • Gas solubility variations
4.1.8(HL)—Oceans as carbon sink
• Absorb atmospheric CO₂ and sequester it • Moderates atmospheric CO₂ increases • Saturation point may be reached
4.1.9(HL)—Ocean carbon sequestration
• Short-term: dissolved CO₂ → ocean acidification • Long-term: biomass accumulates on seabed → fossil fuels
4.1.10(HL)—Water temperature and density
• Cold water below, warmer above • Density differences restrict mixing → stratification • Water densest at 4°C → freezing from surface down
4.1.11(HL)—Stratification
• Occurs in deeper lakes, coastal areas, seas, ocean • Thermocline: transition layer between warm surface and cold deep water • Differences in dissolved oxygen and mineral nutrients
4.1.12(HL)—Ocean stratification increase
• Global warming and salinity changes • Most pronounced in upper 200m
4.1.13(HL)—Upwellings
• Bring cold, nutrient-rich waters to surface • Mass vertical movement from depths • Seasonal cycles in stratified lakes and ENSO
4.1.14(HL)—Thermohaline circulation
• Driven by temperature and salinity differences • Ocean conveyor belt distributes heat globally • Affects climate
4.2 Water access, use and security
4.2.1Water security definition
• Access to sufficient safe drinking water • Component of sustainable societies
4.2.2Factors affecting water availability
• Social, cultural, economic, political factors • Impact availability and equitable access
4.2.3Water supply increase needs
• Population growth or economic development • Uses: domestic, irrigation, livestock, industry
4.2.4Increasing water supplies
• Dams, reservoirs, rainwater catchment, desalination plants • Natural wetland enhancement
4.2.5Water scarcity types
• Physical scarcity: limited abundance • Economic scarcity: limited storage/transport systems
4.2.6Domestic water conservation
• Metering, rationing, grey-water recycling • Low-flush toilets, rainwater harvesting
4.2.7Industrial water conservation
• Food production systems • Greenhouses, aquaponics, drip irrigation, drought-resistant crops
4.2.8Water scarcity mitigation strategies
• Country-specific management strategies
4.2.9(HL)—Freshwater planetary boundary
• Increasing demand causing water stress • Risk of abrupt/irreversible hydrological system changes
4.2.10(HL)—Governance for sustainability
• Local and global governance needed • Examples: local regulations, international agreements
4.2.11(HL)—Water footprints
• Measure of water use by individuals/nations/products • Informs decision-making about water security
4.2.12(HL)—Citizen science role
• Monitoring and managing water resources
4.2.13(HL)—Water stress definition
• Considers scarcity, quality, environmental flows, accessibility
4.2.14(HL)—Water stress threshold
• < 1,700 m³ per year per capita
4.2.15(HL)—Water stress causes
• Depend on socio-economic context
4.2.16(HL)—Transboundary disputes
• Water sources crossing regional boundaries
4.2.17(HL)—Industrial-level water stress solutions
• Dams, pipelines, desalination, water treatment plants • Aquifer storage and recovery (ASR)
4.2.18(HL)—Freshwater production impacts
• Negative environmental impacts • Brine discharges, noise, air pollution, aquifer impacts
4.2.19(HL)—Inequitable access impacts
• Negatively impacts health and sustainable development • Affects marginalized groups
4.3 Aquatic food production systems
4.3.1Aquatic primary producers
• Phytoplankton: microscopic photosynthetic plankton • Macrophytes: visible aquatic plants
4.3.2Human aquatic food consumption
• Organisms from freshwater and marine environments
4.3.3Increasing demand
• Human population growth, dietary preference changes
4.3.4Unsustainable harvesting practices
• Bottom trawling, ghost fishing, poisons, explosives
4.3.5Fishery collapse
• Dramatic, lasting stock decrease • Example: Newfoundland cod
4.3.6Maximum sustainable yield (MSY)
• Highest annual catch sustainable over time • Used to set fishing quotas
4.3.7Climate change impacts
• Ocean acidification • May cause population collapse in aquatic ecosystems • Example: coral bleaching
4.3.8Mitigation strategies
• Policy legislation for fishing industry • Consumer behaviour changes • Permits, quotas, seasons, mesh size, zones, labelling
4.3.9Marine Protected Areas (MPAs)
• Support food chains, maintain sustainable yields • Provide shelter and spawning grounds
4.3.10Aquaculture
• Farming of aquatic organisms • Expanding for food supply and economic development • Impacts: habitat loss, pollution, disease spread, escapees
4.3.11(HL)—Water system interconnections
• Productivity, thermal stratification, nutrient mixing/loading • Highest productivity near coastlines and shallow seas
4.3.12(HL)—Fish stock assessment
• Required for conservation and sustainable use • Monitoring harvest rates
4.3.13(HL)—MSY risks
• Estimated value, inevitably inaccurate • Exceeding rate → positive feedback → rapid decline
4.3.14(HL)—Overexploited species recovery
• Requires cooperation: governments, industry, consumers, NGOs • Measures: fishing bans, license limits, bycatch prevention
4.3.15(HL)—UNCLOS
• Exclusive economic zone: 370 km from coast • High seas (60% of ocean): limited regulation • Equity issues when countries sell fishing access
4.3.16(HL)—Marine mammal harvesting ethics
• Seals, whales, dolphins • Animal rights vs. indigenous group rights
4.4 Water pollution
4.4.1Water pollution sources
• Sewage, agricultural run-off, industrial effluent • Urban run-off, solid waste disposal, oil spills
4.4.2Plastic debris accumulation
• Marine environments • Oceanic gyres, microplastics enter food chain
4.4.3Water quality measurement
• Chemical, physical, biological characteristics • Water quality index (WQI) • Parameters: dissolved O₂, pH, temperature, turbidity, nitrates, phosphates, metals
4.4.4Biochemical oxygen demand (BOD)
• Dissolved oxygen required by microorganisms for decomposition • mg O₂ per litre in 5 days at 20°C
4.4.5Eutrophication
• Mineral nutrient inputs (nitrates, phosphates) • Excessive phytoplankton growth (algal blooms)
4.4.6Eutrophication sequence
• Impacts and changes to aquatic system
4.4.7Eutrophication and ecosystem services
• Eutrophication can substantially impact ecosystem services • Eutrophication can affect services associated with fisheries, recreation, aesthetics and health
4.4.8Eutrophication management levels
• Eutrophication can be addressed at three different levels of management • These three levels of management are: • the reduction of human activities that produce pollutants—e.g., alternatives to current fertilizers and detergents • the reduction of the release of pollution into the environment—e.g., treatment of wastewater to remove nitrates and phosphates
4.4.9(HL)—Water pollutants
• There is a wide range of pollutants that can be found in water • These pollutants may include organic matter (e.g., sewage), dissolved substances (e.g., tributyltin, an endocrine-disrupting chemical), persistent chemicals
4.4.10(HL)—Harmful algal blooms
• Algal blooms may produce toxins that threaten the health of humans and other animals • Harmful algal blooms (HABs) contain a variety of organisms, including cyanobacteria, protists, algae and dinoflagellates • A small number of these organisms produce potentially fatal toxins • In freshwater, cyanotoxins are the most common toxins
4.4.11(HL)—Anoxic and hypoxic waters
• The frequency of anoxic/hypoxic waters is likely to increase due to the combined effects of global warming, freshwater stratification, sewage disposal and eutrophication • Hypoxic conditions can arise from a variety of causes, resulting in aquatic dead zones
4.4.12(HL)—Sewage treatment stages
• Sewage is treated to allow safe release of effluent by primary, secondary and tertiary water treatment stages • Consider: what is achieved in each of the sewage treatment stages, including both biological and chemical processes • Also consider the challenges of implementing sewage treatment equitably in all societies
4.4.13(HL)—Indicator species
• Some species are sensitive to pollutants or are adapted to polluted waters, so these can be used as indicator species
4.4.14(HL)—Biotic index
• Indirect water quality measure • Based on species tolerance, abundance, diversity • Example: Trent biotic index
4.4.15(HL)—Water Quality Index (WQI)
• Single weighted average of multiple test parameters • Represents contamination degree
4.4.16(HL)—Water quality guidelines
• WHO drinking water quality guidelines • Local government statutory standards
4.4.17(HL)—Citizen action
• Reduce water pollution • Consumption changes, protest, data collection, lobbying