Water: real-world examples

4.1.1Movements of water in the hydrosphere are driven by solar radiation and gravity.

• On the Mer de Glace glacier in the French Alps, solar radiation drives summer meltwater into the Arve and Rhône and winter sublimation off the ice, while gravity drives the ice downhill at ~75 m/year; since 1850 it has retreated ~2.5 km and thinned ~200 m, forcing repeated relocation of access from the 1909 Montenvers station.

  Shows both drivers of the hydrosphere at one site (solar-driven melting and sublimation versus gravity-driven ice and meltwater flow), with outputs now exceeding inputs.

  Mer de Glace, France Water Systems

4.1.5Human activities, such as agriculture, deforestation and urbanization, can alter these flows and stores.

• During the Valencia DANA of 29 October 2024, over 490 mm of rain fell in 8 hours onto a heavily sealed and deforested floodplain, so infiltration dropped near zero and run-off surged; the barranco del Poyo rose to a peak discharge of ~2,300 m³/s in under two hours, producing flash floods that killed over 230 people, Spain's deadliest in modern history.

  Shows how urbanisation reduces infiltration and evapotranspiration while increasing surface run-off, converting rainfall into a rapid, deadly flood peak.

  Valencia, Spain Water Systems

• Intensive irrigated agriculture (sugar beet, apples, wheat, grapes) in the catchment of Lake Urmia, north-western Iran, involving over 50 dams and more than 88,000 wells, cut river streamflow and depleted groundwater so severely that by 2015 the lake had lost about 88% of its surface area and 95% of its volume, leaving a 5,000 km² salt desert whose dust storms harm about 6 million people.

  Demonstrates how agricultural diversion of inflows and groundwater abstraction shrinks a surface freshwater store and reduces streamflow and groundwater flows.

  Lake Urmia, Iran Water Systems

4.1.6The steady state of any water body can be demonstrated through flow diagrams of inputs and outputs.

• Lake Geneva (Lac Léman) holds ~89 km³ of freshwater and stays roughly in steady state with annual inputs and outputs both around 7.1 km³/year (Rhône inflow 5.7, outflow 6.6, evaporation 0.4, abstraction 0.1), supplying drinking water to about 1 million people.

  Quantifies steady state as balanced inputs and outputs in a real water budget, letting sustainable abstraction limits be calculated.

  Lake Geneva Water Systems

• The freshwater aquifer beside the Salar de Atacama in northern Chile receives ~180 million m³/year of Andean recharge against outputs of ~165 million m³/year (120 seepage to flamingo wetlands and Atacameño farms, 30 evaporation, 15 town abstraction), so a lithium mining request for an extra 40 million m³/year would push outputs above inputs and deplete the store.

  Shows how a flow diagram of inputs and outputs sets the maximum sustainable harvest and reveals when abstraction breaches steady state.

  Salar de Atacama, Chile Water Systems

4.2.2Social, cultural, economic and political factors all have an impact on the availability of, and equitable access to, the freshwater required for human well-being.

• During Cape Town's 2015–2018 drought, dam levels fell to ~23% of capacity and authorities imposed a 50 L/person/day limit and rising block tariffs; wealthy suburbs drilled private boreholes while ~20% of residents in informal settlements shared communal taps, and agricultural water rights were cut by 60% amid rivalry between city and national government and the legacy of apartheid-era infrastructure.

  Demonstrates how social, cultural, economic and political factors together produced deeply unequal water access within one city.

  Cape Town, South Africa Water Security and Scarcity

4.2.4Water supplies can be increased by constructing dams, reservoirs, rainwater catchment systems, desalination plants and enhancement of natural wetlands.

• In Morocco's semi-arid Aït Baamrane region, the NGO Dar Si Hmad runs the world's largest fog-harvesting project, with ~1,700 m² of mesh nets on Mount Boutmezguida collecting on average ~35,000 litres per day and piping water to over 1,000 people across ~16 villages, cutting women's daily water-collection time from around 3 hours.

  Shows a low-energy rainwater/fog catchment system delivering village-scale supply where fog is reliable but not at city scale.

  Aït Baamrane, Morocco Water Security and Scarcity

• The Grand Ethiopian Renaissance Dam on the Blue Nile, inaugurated in 2025 with a ~74 km³ reservoir and ~5,150 MW capacity (costing ~US$5 billion), more than doubles Ethiopia's electricity but is fiercely opposed by Egypt, which draws ~97% of its freshwater from the Nile, and displaced 5,000–20,000 people.

  Shows how dams and reservoirs increase supply while creating evaporation losses, displacement and international disputes.

  Grand Ethiopian Renaissance Dam, Ethiopia Water Security and Scarcity

4.2.5Water scarcity refers to the limited availability of water to human societies.

• Haiti receives around 1,400 mm of rain a year yet roughly 70% of its population lacks safely managed drinking water and two-thirds lack basic sanitation, because political instability and disasters such as the 2010 earthquake left treatment plants, pipes and sewage systems under-built.

  Shows economic water scarcity, where abundant rainfall exists but weak infrastructure and governance prevent safe delivery.

  Haiti Water Security and Scarcity

• Jordan is one of the most water-scarce countries on Earth: in 2021 each Jordanian had only ~61 m³ of renewable freshwater per year (against the 500 m³ absolute-scarcity threshold), its Jordan and Yarmouk river flows are shared with Syria and Israel, and about half of households receive piped water only once a week despite near-universal connection.

  Demonstrates physical water scarcity where extensive infrastructure cannot compensate for a genuine climatic and geographic lack of water.

  Jordan Water Security and Scarcity

4.2.6Water conservation techniques can be applied at a domestic level.

• Chennai's "Day Zero" arrived in June 2019 when all four city reservoirs ran dry and families queued for 25 L of water per day, prompting mandatory dual plumbing for grey-water reuse in many new buildings.

  Illustrates rationing and grey-water recycling as domestic conservation responses forced by acute urban scarcity.

  Chennai, India Water Conservation Strategies

• Under NSW's BASIX scheme all new Australian homes since 2004 must meet water-saving targets, usually via rainwater tanks, and ~26% of Australian households now have a tank (rising to ~50% in Adelaide).

  Shows regulation-led domestic rainwater harvesting scaled through building codes.

  New South Wales, Australia Water Conservation Strategies

4.2.7Water conservation strategies can be applied at an industrial level in food production systems.

• India championed the 2023 UN International Year of Millets, expanding drought-resistant crops such as pearl millet, ragi and sorghum (viable on ~350 mm rainfall/year versus ~1,200 mm for rice) to 13.7 million ha by 2024, and highlighting that 1 kg of beef needs ~15,000 L of water against ~1,800 L for wheat.

  Shows drought-resistant crops and diet as low-infrastructure industrial strategies that cut agricultural water demand.

  India Water Conservation Strategies

• Sundrop Farms at Port Augusta, South Australia is a 20 ha greenhouse using solar-desalinated seawater to grow ~17,000 tonnes of tomatoes per year with no fresh water and no fossil fuels, built for ~A$200 million.

  Demonstrates a capital-intensive greenhouse system that eliminates freshwater use in industrial food production.

  Port Augusta, South Australia Water Conservation Strategies

4.2.8Mitigation strategies exist to address water scarcity.

• Australia's national response to Millennium Drought scarcity combined near-universal metering with volumetric pricing, Melbourne's Stage 3a rationing (April 2007–April 2010) which nearly halved per-capita use from ~458 L/day in 1996 to 236 L/day in 2011, compulsory dual-flush toilets (all new toilets since 1993, saving ~32,000 L/household/year), and A$13 billion of Murray-Darling drip/sprinkler conversion saving ~700 GL/year by 2024.

  Demonstrates how a high-income country combines domestic and industrial techniques into a coherent national mitigation strategy against drought-driven scarcity.

  Australia Water Conservation Strategies

• India's response to physical and economic water scarcity blends traditional and regulatory measures: Tamil Nadu made rainwater harvesting compulsory for all buildings in 2003 (~5 million homes retrofitted in three years, raising Chennai's water table by up to 50%), Bengaluru's 2024 rationing banned drinking water for car washes and construction with ₹5,000 fines, and the PMKSY scheme put ~8 million ha under subsidised drip/sprinkler irrigation by 2024.

  Shows a middle-income country's subsidy-led national mitigation strategy drawing on both domestic and agricultural techniques to address recurrent scarcity.

  India Water Conservation Strategies

4.3.2Humans consume organisms from freshwater and marine environments.

• Lake Geneva perch (Perca fluviatilis), served as filets de perche, is the region's signature dish, but while the lake yields only ~400 tonnes a year, lakeside restaurants sell ~7,300 tonnes annually, the rest imported frozen (mainly from Estonia); a 2017 Swiss investigation found 6 of 20 restaurants falsely claiming imported perch were locally caught.

  Demonstrates a local freshwater species consumed by humans and quantifies how demand outstrips local stocks even before global trade.

  Lake Geneva From plankton to plate

• The Atlantic salmon (Salmo salar) is an anadromous high-trophic-level predator whose wild populations have collapsed across most of its North Atlantic range, so that most salmon eaten globally is now farmed, with Norway the world's largest producer followed by Chile, Scotland, the Faroe Islands and Canada.

  Shows a globally significant food species and links its high trophic level to the energetic inefficiency of producing it as food.

  Norway From plankton to plate

4.3.4The increasing global demand for seafood has encouraged use of unsustainable harvesting practices and overexploitation.

• In the Baltic Sea WWF and Healthy Seas estimate several hundred tonnes of fishing gear are lost each year, and recovery dives off the German and Polish coasts have pulled up Soviet-era nets some still actively catching cod, demonstrating how ghost gear kills marine life for decades.

  Shows ghost fishing as an unsustainable method causing prolonged mortality because synthetic gear does not biodegrade.

  Baltic Sea From plankton to plate

• The Dogger Bank in the North Sea is one of the most intensively bottom-trawled seabeds in the world with largely degraded seafloor habitats; the UK banned bottom trawling across four offshore Marine Protected Areas including it in 2022, though enforcement remains a challenge.

  Illustrates bottom trawling as a destructive unsustainable method and the gap between banning and enforcing.

  Dogger Bank, North Sea From plankton to plate

4.3.5Overexploitation has led to the collapse of fisheries.

• The Monterey Pacific sardine recovered through the 1990s but collapsed a second time, falling ~97% after 2006, prompting regulators to close the fishery from 2015, again because fishing had been allowed to continue too far into a known decline.

  Shows that repeated overexploitation during natural declines reproduces collapse even after an apparent recovery.

  Monterey Bay, California, USA Collapse, limits and warming waters

• The Pacific sardine (Sardinops sagax) fishery off Monterey Bay, California, once the largest in the western hemisphere at ~700,000 tonnes/year in the 1930s–40s, collapsed after its 1946 peak, falling ~90% in five years (from ~234,000 to ~24,000 tonnes) because the fleet kept fishing hard through a natural, temperature-linked decline; a ban ran 1967–1986 and the stock never fully recovered.

  Demonstrates how overexploitation combined with a natural downturn drives a fishery to a collapse from which the commercial stock cannot recover.

  Monterey Bay, California, USA Collapse, limits and warming waters

4.3.6The maximum sustainable yield (MSY) is the highest possible annual catch that can be sustained over time, so it should be used to set caps on fishing quotas.

• The Monterey sardine fleet illustrates fishing well to the right of the MSY peak: because fleet capacity far exceeded the effort needed to fish at MSY, boats kept working while the stock was already falling, pushing catch onto the downhill side of the yield–effort curve rather than staying at the sustainable maximum.

  Demonstrates the syllabus point that real fisheries often fish at far higher effort than the MSY, running the stock down.

  Monterey Bay, California, USA Collapse, limits and warming waters

4.3.7Climate change and ocean acidification are having impacts on ecosystems and may cause collapse of some populations in freshwater or marine ecosystems.

• Australia's Great Barrier Reef, the world's largest reef system, has suffered repeated mass coral bleaching as ocean temperatures rise, with corals expelling their symbiotic algae and dying, threatening the thousands of species dependent on living coral.

  Shows warming waters causing marine ecosystem stress and potential collapse that fishing quotas cannot address.

  Great Barrier Reef, Australia Collapse, limits and warming waters

• Lake Geneva (Léman) has failed to mix fully since the cold winter of 2012 because milder winters leave surface water too warm to sink, and monitoring by CIPEL and EPFL shows deep-water oxygen has fallen ~90% in ten years, below the level most aquatic life needs.

  Demonstrates climate warming stressing a freshwater ecosystem via reduced deep mixing and deoxygenation, disrupting the food web.

  Lake Geneva Collapse, limits and warming waters

4.3.8Unsustainable exploitation of freshwater and marine ecosystems can be mitigated through policy legislation addressing the fishing industry and changes in consumer behaviour.

• Sustainable seafood certification schemes shift consumer demand: the Marine Stewardship Council (MSC) blue label certifies wild-caught fish, the Aquaculture Stewardship Council (ASC) certifies farmed fish, and the Marine Conservation Society Good Fish Guide advises shoppers, so producers gain a reason to fish and farm more sustainably even though labelling bans nothing.

  Illustrates individual-level mitigation through consumer behaviour and food labelling working via demand rather than legal prohibition.

  Rules, reserves and fish farms

4.3.9Marine protected areas (MPAs) can be used to support aquatic food chains and maintain sustainable yields.

• In 2014 the ~280 km² South Arran Marine Protected Area was designated around Lamlash Bay, with scallop dredging banned from 2016; surveys show adult scallop density falls with distance from the boundary, a spillover sign the benefit reaches surrounding fished waters, while recovering seagrass, kelp and maerl now shelter juvenile fish including young cod.

  Shows how a protected area seeds and supplies the wider sea via spillover and nursery habitat, maintaining sustainable yield beyond the reserve boundary.

  South Arran Marine Protected Area, Scotland Rules, reserves and fish farms

• Scotland's first no-take zone was created in Lamlash Bay, Isle of Arran, in 2008 (about 2.67 km²) after over ten years of campaigning by COAST; University of York surveys found king scallop density rose from about 6 to about 23 per 100 m² between 2010 and 2019, and legal-sized lobsters became around four times more abundant and larger than in nearby fished areas.

  Quantifies how protecting one patch of sea allows adults to grow larger and denser, demonstrating the recovery mechanism behind MPAs.

  Lamlash Bay, Isle of Arran, Scotland Rules, reserves and fish farms

4.3.10Aquaculture is the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants. The industry is expanding to increase food supplies and support economic development, but there are associated environmental impacts.

• Scotland is the world's third largest producer of farmed Atlantic salmon (after Norway and Chile), producing around 163,000 tonnes in 2016 with a target of 300,000–400,000 tonnes by 2030; open net sea cages cause seabed dead zones from waste feed and faeces, copper anti-fouling toxicity, antibiotic and sea-lice pesticide pollution, sea lice spread to wild salmon, and escapees that cause genetic introgression in wild stocks.

  Demonstrates aquaculture's role in expanding food supply and economic development alongside a full range of named environmental impacts.

  West coast lochs, Scotland Rules, reserves and fish farms

• To control sea lice without relying only on chemicals, Scottish salmon farms have placed cleaner fish (wrasse and lumpfish) in cages since the 1990s to eat lice off the salmon, but large numbers of cleaner fish die in the cages, demand has pressured wild wrasse stocks, and in 2025 one of the biggest producers stopped using lumpfish.

  Shows a named management technique (biological control) that reduces one impact while creating new problems, allowing evaluation.

  Scotland Rules, reserves and fish farms

4.4.1Water pollution has multiple sources and has major impacts on marine and freshwater systems.

• The 2022 River Oder fish kill on the Poland–Germany border: salty waste water from upstream mines and industry, combined with low water levels and high summer temperatures, triggered a bloom of toxic golden alga (Prymnesium parvum) whose toxins attacked fish gills, killing around 250 tonnes of fish over hundreds of kilometres; the two nations ran a joint investigation, restricted salty discharges and set new salinity limits.

  Demonstrates how a diffuse industrial pollution source interacts with climate conditions to cause a major freshwater impact and how transboundary management responds.

  River Oder, Poland/Germany border Water Pollution and Water Quality

• The Lonza chemical plant at Visp, Valais discharged around 50 tonnes of mercury into a canal on the river Rhône between 1930 and 1976; the contamination was only discovered by chance in 2010 and clean-up continues today.

  Shows how an identifiable point-source industrial effluent can persist in sediments and soil for decades, making pollution long-lasting and hard to remediate.

  Visp, Valais, Switzerland Water Pollution and Water Quality

4.4.2Plastic debris is accumulating in marine environments. Management is needed to remove plastics from the supply chain and to clear up existing pollution.

• EPFL researchers towed a manta trawl across Lake Geneva and found microplastics in every beach sample, dominated by tiny polystyrene beads, at levels comparable to parts of the ocean, with the river Rhône carrying the plastic onward towards the Mediterranean.

  Shows that plastic accumulation is not just marine: ~80% of ocean plastic originates on land, so lakes and rivers are the source of the problem.

  Lake Geneva, Switzerland Water Pollution and Water Quality

• The Great Pacific Garbage Patch in the North Pacific is the best-known accumulation of floating plastic trapped in the calm centre of one of the five great ocean gyres, where wind- and Earth-spin-driven currents concentrate debris that persists as plastic for decades.

  Demonstrates how ocean gyre circulation concentrates persistent plastic debris, the mechanism behind marine plastic accumulation.

  Great Pacific Garbage Patch, North Pacific Ocean Water Pollution and Water Quality

4.4.5Eutrophication occurs when lakes, estuaries and coastal waters receive inputs of mineral nutrients, especially nitrates and phosphates, often causing excessive growth of phytoplankton.

• Lake Geneva's decline from the 1950s was driven by phosphate from washing powders, sewage and farm run-off, with total phosphorus peaking near 90 µg/L in 1979 (far above the healthy 10–15 µg/L target) and triggering thick blooms, because phosphate is the limiting nutrient in most fresh water.

  Shows that adding the limiting nutrient (phosphate) from detergents, sewage and fertilisers causes the excessive phytoplankton growth described in the point.

  Lake Geneva The lake that turned green

4.4.6Eutrophication leads to a sequence of impacts and changes to the aquatic system.

• Brittany's green tides show the eutrophication sequence on a coast: living *Ulva* seaweed is harmless, but when piled a metre thick on beaches it decomposes under a dry crust in airless conditions, and this rotting step releases poisonous hydrogen sulphide.

  Shows the decomposition step of the impact sequence is where harm arises, not the growth of algae itself.

  Brittany, France The lake that turned green

4.4.7Eutrophication can substantially impact ecosystem services.

• Brittany's *marées vertes* (green tides): rivers carry nitrate (over 90% from agriculture, according to the French Court of Auditors) into shallow sandy Atlantic bays where *Ulva* sea lettuce grows a metre thick, then rots to release toxic hydrogen sulphide that has killed animals including a horse and been linked to human deaths, closing beaches and harming fisheries.

  Quantifies how coastal eutrophication simultaneously damages fisheries, recreation, aesthetics and health services.

  Brittany, France The lake that turned green

4.4.8Eutrophication can be addressed at three different levels of management.

• Lake Geneva (Lac Léman), source of drinking water for over 600,000 people, was badly eutrophic after 1950s phosphate inputs pushed total phosphorus to a peak of ~90 µg/L in 1979; a Franco-Swiss commission (CIPEL, 1963) drove recovery via Switzerland's 1986 laundry-detergent phosphate ban, upgraded phosphorus-stripping sewage works and reduced farm phosphate, bringing total phosphorus down to 15.6 µg/L by the 2025 report with far fewer blooms.

  Demonstrates all three management levels (change activity, reduce release, clean up) working together to reverse eutrophication, quantified by the fall from 90 to 15.6 µg/L.

  Lake Geneva The lake that turned green