Distinguish physical and economic water scarcity, explain droughts, eutrophication, salinisation and pressures on lakes and aquifers, and examine stakeholder power in water quality and the Nile dispute over the GERD.
Each is in the Geography glossary, with every other term in the subject.
A shortage of soil moisture during the growing season that reduces crop yields and pasture, caused by low rainfall, high evaporation or poorly timed rain.
A rapid increase in algae or cyanobacteria in a body of water, usually caused by excess nutrients, which discolours the water and can release toxins.
The upward movement of water through narrow spaces between soil particles, against gravity, which can carry dissolved salts towards the surface.
A resource, such as an aquifer or fishery, from which it is difficult to exclude users and where each user's consumption reduces what is left for others.
The funnel-shaped lowering of the water table around a well that pumps groundwater faster than it can flow in from the surrounding aquifer.
An area of a lake or sea where oxygen levels are so low, usually because of eutrophication, that most aquatic life cannot survive.
Pollution that enters water from many dispersed sources across a wide area, such as fertiliser runoff from fields, so that no single source can be identified.
A method of irrigation that delivers water slowly through pipes and emitters directly to the roots of each plant, reducing losses to evaporation and seepage.
Distinguish between physical and economic water scarcity, and between water quantity and water quality.
Explain how physical and human factors control the availability of freshwater in different places.
Describe the four types of drought and explain their causes and their impacts in places at different levels of development.
Explain how agricultural activity reduces water quality through eutrophication and salinisation.
Examine the roles and power of different stakeholders in agricultural water pollution, using detailed examples.
Explain why human pressures on lakes and aquifers are growing, including economic growth and migration.
Examine why shared water resources cause conflict, and evaluate attempts to resolve one such conflict.
Construct and interpret maps, graphs and calculations of water per person and percentage change.
Usable freshwater is unevenly spread between places and seasons, and growing populations, irrigation and industry take more of it each year. Where demand outruns supply, or where water is available but unsafe to drink, people go without.
Who controls a river, a reservoir or a well decides who receives water, so the key concept of power runs through the topic: farmers and cities compete for the same aquifer, and an upstream state can hold back water that a downstream state depends on. Place matters too, because the same dry spell becomes a crisis in one country and an inconvenience in another, depending on wealth, infrastructure and governance.
78 exam-style questions on this subtopic, written in the style of the IB papers. The first has its markscheme open. Counted on
Examine the relative importance of physical and human factors in causing water scarcity in different places.
10 marks
Marks should be allocated according to the markbands.
Water scarcity exists when the demand for freshwater cannot be met by the available supply. Physical water scarcity occurs where there is not enough water to meet demand (often taken as more than 75% of river flows being withdrawn), while economic water scarcity occurs where water is available but a lack of investment, infrastructure or institutional capacity prevents people from using it. Accept both types. Physical factors include climate (aridity, rainfall variability, drought), geology and evaporation; human factors include population growth, agricultural and industrial demand, over-abstraction, pollution, poverty and governance. Accept a broad interpretation of “different places”, including contrasting regions, countries or cities.
Possible applied themes (AO2) demonstrating knowledge and understanding (AO1):
physical causes: low and unreliable rainfall and high evaporation in arid and semi-arid regions (eg the Middle East, where several countries have less than 500 m³ of renewable water per person per year); drought linked to ENSO, blocking anticyclones or shifts in the ITCZ (eg the Horn of Africa drought of 2020–2023 after five failed rainy seasons)
geology: impermeable rocks limit groundwater storage; permeable limestone areas may lack surface water
human causes of physical scarcity: over-abstraction for irrigation, which accounts for about 70% of global freshwater withdrawals (eg the Aral Sea basin, the Colorado River, which in most years no longer reaches the sea)
population growth and urbanization concentrating demand (eg Cape Town’s “Day Zero” crisis in 2018, when demand from a fast-growing city met a 2015–2017 drought)
economic scarcity: lack of investment in storage, pipes and treatment in much of sub-Saharan Africa, where rainfall may be adequate but many people walk to unsafe sources
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A shortage caused by a lack of investment, infrastructure or institutional capacity rather than of water, so that people cannot access water that is physically available.
The enrichment of a body of water with nutrients, especially nitrates and phosphates, causing excessive growth of algae and plants and leading to oxygen depletion.
A measure of water availability as renewable freshwater per person per year, with thresholds of 1,700 m³ for water stress, 1,000 m³ for scarcity and 500 m³ for absolute scarcity.
Groundwater that accumulated long ago, often in a wetter past climate, and receives little or no recharge today, so it is effectively non-renewable.
A long-term fall in the volume of groundwater stored in an aquifer, caused when abstraction exceeds recharge.
A period in which river flows, lake and reservoir levels and water tables fall well below normal, usually lagging months behind a meteorological drought.
A condition of very low dissolved oxygen in water, usually below about 2 mg per litre, that most aquatic animals cannot survive; anoxia is the complete absence of oxygen.
The artificial application of water to land to help crops grow, using canals, flooding, sprinklers, drip systems or pumped groundwater.
The gradual sinking of the ground surface, often caused by the compaction of clay and silt layers when groundwater is pumped out.
The downward removal of soluble substances, such as nitrates or salts, from the soil by water percolating through it.
A period in which precipitation is well below the long-term average for a place, usually over several months; the first stage of a drought.
A shortage in which natural water supplies cannot meet all demands, including environmental flows; usually defined as more than about 75% of river flows being withdrawn.
Pollution that enters water from a single identifiable place, such as a sewage outfall, factory pipe or slurry tank, so it can be measured and regulated.
The downward movement of water from the surface into an aquifer, replenishing groundwater; it may come from rainfall, rivers or irrigation.
A country through which a river flows or which borders a river or lake, and which therefore has a stake in how its water is used.
The accumulation of soluble salts in soil, often caused by irrigation in hot, dry climates, which reduces fertility and can make land unusable for crops.
The movement of seawater into a coastal freshwater aquifer, usually because over-pumping has lowered the water table and the pressure of fresh groundwater.
A river, lake or aquifer whose basin or extent crosses the borders of two or more countries, so that its use affects all of them.
The unequal power between states sharing a river that results from their positions on it, since upstream states can control the quantity, timing and quality of water downstream.
A dispute between countries, regions or groups over the allocation, quality or management of a shared water resource, ranging from diplomatic tension to violence.
The chemical, physical and biological characteristics of water that determine whether it is safe and suitable for a particular use, such as drinking, irrigation or supporting ecosystems.
The amount of water available in a place over a period of time, measured as a volume or a flow.
A situation in which the available supply of freshwater of acceptable quality is not enough to meet all demands in an area, including the needs of ecosystems.
A condition in which demand for water approaches or exceeds the renewable supply, causing periodic shortages; often defined as less than 1,700 m³ per person per year.
A formal agreement between two or more states that sets out how a shared water resource will be allocated, used or managed, and how disputes will be settled.
The saturation of soil with water, usually because the water table has risen close to the surface, which starves plant roots of air.
Scope: The topic covers physical and economic water scarcity and the factors behind them, including droughts; the difference between water quantity and water quality; the effects of farming on water quality through eutrophication and salinisation, with detailed examples of the stakeholders involved; growing pressures on lakes and aquifers; and conflict over internationally shared water, with one case study of a shared resource (here the Nile and the Grand Ethiopian Renaissance Dam). Freshwater is one of seven options: SL students study two options and HL students three.
Water scarcity exists when the demand for freshwater in an area cannot be met by the water available to its people. Demand includes drinking, farming and industry, and the needs of rivers, lakes and wetlands. Scarcity therefore depends on the balance between supply and demand as well as on rainfall. A desert town with few people may have enough water, while a humid region with a large, thirsty population may not.
In 2007 the International Water Management Institute (IWMI) divided scarcity into two types, which need very different responses. Physical water scarcity occurs where there is not enough water to meet all demands, including environmental flows; IWMI classed a basin as physically scarce when more than about three-quarters of its river flow is withdrawn. North Africa, the Middle East, Central Asia and northern China fall into this group, as do basins where water has been over-committed to irrigation.
Economic water scarcity occurs where water is physically available but people cannot obtain it, because there is too little investment in wells, pipes and treatment works, or too little institutional capacity to run them. Much of sub-Saharan Africa is in this category: rivers and aquifers may be large, yet many rural households walk to unprotected springs or buy water from vendors.
Water stress describes the pressure on supplies before they run out: needs are met most of the time, but shortages occur in dry years and there is little margin for growth. It is measured either as water per person (the Falkenmark indicator) or as the share of renewable freshwater withdrawn each year, the measure used for Sustainable Development Goal (SDG) target 6.4.
The simplest measure divides a country’s renewable freshwater by its population. In 1989 the Swedish hydrologist Malin Falkenmark proposed thresholds that are now standard: the Falkenmark indicator treats a country as water-stressed below 1,700 m³ per person per year, as facing scarcity below 1,000 m³, and as facing absolute scarcity below 500 m³.
The chart applies the indicator to thirteen countries using renewable internal freshwater, which counts only the water generated within a country, not rivers flowing in from upstream. That choice explains the most striking result: Egypt has only about 9 m³ of internal water per person, because almost all of its water arrives in the Nile from other countries.
The indicator is quick to calculate, but a national average hides regional differences: China’s dry north has far less water per person than its humid south. It ignores who can reach the water, so it cannot detect economic scarcity. It also ignores trade in food, which carries embedded water from wet countries to dry ones; Jordan, with about 61 m³ per person, survives partly by importing cereals grown with other countries’ rain.
The IWMI world map of water scarcity shades each basin by category. It shows a band of physical scarcity across the dry subtropics, from Morocco through the Middle East to north-west India and northern China, with outliers in the south-western USA and south-eastern Australia. Economic scarcity covers almost all of sub-Saharan Africa outside the south, with parts of South and South-east Asia and the Andes.
The map uses five categories. Basins with little or no water scarcity withdraw less than a quarter of their river flow. Physical water scarcity means more than about 75% is withdrawn, and approaching physical scarcity means more than about 60%, so that physical scarcity is likely in the near future. Economic water scarcity marks basins where less than a quarter of the flow is withdrawn but malnutrition and poor access to water persist, because the human and financial capacity to use the water is lacking. Areas without enough data are shown as not estimated.
Availability depends on how much water enters, how much is stored and how much people take out. Most real shortages combine physical and human factors.
Climate is the main control on supply. The great deserts lie under the subtropical high-pressure belts at about 20–30° north and south, where sinking air brings little rain and high evaporation. Rain shadow areas on the leeward side of mountains are dry for a similar reason. Seasonality matters as much as the annual total: in monsoon climates most rain falls in three or four months, so water must be stored for the rest of the year. Variability from year to year is usually greatest where rainfall is lowest, which makes supplies hard to plan.
Geology controls storage. Permeable sandstone, limestone and chalk can hold large aquifers that keep wells and rivers supplied through dry seasons, while granite or clay stores little. Geology can also spoil water: arsenic in the river sediments of the Bengal basin dissolves into the groundwater that millions of people drink.
Human factor | Effect on availability | Example |
|---|---|---|
Population growth | The same supply is shared by more people | Ethiopia: 1,083 m³ per person (2018) to 973 m³ (2022) |
Urbanisation and migration | Demand is concentrated and drawn from distant sources | Chennai grew from about 500,000 to more than 10 million in a century |
Rising wealth | More direct use; more meat and dairy, which need much more water to produce | Growing middle classes in China and India |
Agriculture | Irrigation dominates withdrawals | About 71% of global freshwater withdrawals (2022) |
Pollution | Contaminated water cannot be used without costly treatment | Toxic algal blooms in Lake Erie |
Governance, pricing, corruption | Leaking pipes, very low prices that encourage waste, diverted funds | High losses from city networks |
Conflict | Damage to pumps, pipes and treatment works; displacement into camps | Yemen, Gaza |
Sources: FAO AQUASTAT via World Bank WDI (2022); Wikipedia, "2019 Chennai water crisis".
Human factors explain why scarcity is growing faster than the physical supply is changing. In Egypt and Pakistan irrigation uses most of the water withdrawn, and as incomes rise demand shifts towards water-intensive foods and goods, so the water footprint of a typical household grows. These pressures link to water security and to the water–food–energy nexus, because water used for crops or for cooling power stations cannot also be drunk.
A drought is a period of unusually low precipitation that leads to a shortage of water. It is defined against the normal climate of a place, so a dry season in the Sahel is not a drought, but a failed rainy season is. Droughts develop slowly and have no clear start or end, which makes them harder to recognise and respond to than floods or storms. Four types usually follow one another as a dry spell continues.
Meteorological drought is defined purely by rainfall and can begin and end within a season. If it lasts long enough to dry the soil, it becomes an agricultural drought.
Hydrological drought follows when rivers, lakes, reservoirs and aquifers fall well below normal. It lags behind the rainfall deficit by months and can persist after the rains return, because stores take time to refill. A socio-economic drought occurs when shortages begin to affect the supply of food, electricity, jobs and incomes.
Persistent anticyclones (blocking highs). A stalled high-pressure system brings sinking, dry air for weeks and blocks rain-bearing depressions, as in the European droughts of 2018 and 2022.
El Niño–Southern Oscillation teleconnections. El Niño and La Niña shift areas of rising and sinking air across the tropics, bringing drought to some regions and floods to others. The 2020–2023 Horn of Africa drought followed five failed rainy seasons, linked to a run of La Niña years from 2020 and, in 2022, a negative Indian Ocean Dipole, both of which weaken the short rains of East Africa.
Shifts in the rain belts. Sahel rainfall depends on how far north the Intertropical Convergence Zone (ITCZ) moves in summer; when it stays further south, as in the 1970s and 1980s, the rains fail.
Climate change. Higher temperatures increase evaporation. Global climate change is projected to make droughts more frequent in the subtropics and Mediterranean regions.
Over-abstraction. Taking more water from rivers and aquifers than is replaced turns a meteorological drought into a hydrological one sooner.
Crops fail and livestock die, so food prices rise. Low river levels cut hydroelectric output and stop river transport, and wildfires become more frequent. Health suffers as people use unsafe water and malnutrition weakens resistance to disease; in the worst cases families sell assets, children leave school and people migrate.
Impact | High-income country | Low-income country |
|---|---|---|
On people | Restrictions and higher bills; rarely deaths | Malnutrition, disease from unsafe water, excess deaths |
On the economy | Large losses in farming, hydropower and shipping, often insured | Losses fall on households, whose savings are their crops and animals |
On settlement | Little permanent migration | Displacement to towns and camps; herder–farmer conflict |
Response capacity | Reservoirs, transfers, desalination, forecasts, insurance | Dependence on humanitarian aid and underfunded early warnings |
The difference in impacts reflects vulnerability more than rainfall. A household that depends on rain-fed crops and animals has few savings, no insurance and little access to stored water, so the same rainfall deficit does far more harm.
Cape Town, South Africa, relies almost entirely on six mountain reservoirs filled by winter rain. After three dry winters, with 2017 the driest year since records began in 1933, the dams held only 14–29% of capacity, and the city warned of a "Day Zero" when taps would be turned off. From February 2018 use was limited to 50 litres per person per day; daily demand halved to about 500 million litres, Day Zero never arrived, and the 2018 rains refilled the dams. The crisis shows a physical drought becoming a socio-economic one because demand had grown close to the limit of supply.
Sources: Wikipedia, "Cape Town water crisis" (City of Cape Town data).
In Somalia, Ethiopia and Kenya five rainy seasons failed in a row, and the 2022 rains were the driest in more than 40 years. About 9.5 million livestock died, about 4 million of them in Ethiopia, 2.5 million in Kenya and more than 3 million in Somalia. Because pastoralist families store their wealth in animals, the losses removed both food and savings. An estimated 43,000 people died in Somalia in 2022 alone, half of them children under five, and many families moved to camps around towns to reach food aid.
The drought shows each type in turn. The failed rains were a meteorological drought; dry pasture and failed harvests were an agricultural drought; dry wells, water pans and rivers were a hydrological drought; and the collapse of herds, incomes and food supplies was a socio-economic drought, made worse by high food prices in 2022 and by conflict in Somalia that limited aid.
Sources: Wikipedia, "2020–2023 Horn of Africa drought" (UN OCHA and WHO estimates).
The amount of water in a place and its fitness for use are separate questions, measured in different ways.
Quality is judged against the intended use, so water that is fine for irrigation may be unsafe to drink.
A place can have plenty of water quantity and poor water quality. In Bangladesh, despite large rivers, millions of tube wells were sunk from the 1970s to avoid polluted surface water, but many draw groundwater with natural arsenic above Bangladesh’s limit of 50 micrograms per litre, five times the WHO guideline of 10. In Flint, Michigan, a change of water source in April 2014 without corrosion control leached lead from old pipes and exposed about 100,000 residents.
The WHO/UNICEF Joint Monitoring Programme (JMP) measures access with a drinking water ladder: "safely managed" water (on the premises, available when needed and free from contamination), then basic (an improved source within a 30-minute round trip), limited, unimproved and surface water. In 2024, 2.1 billion people, one in four, still lacked safely managed drinking water, including 106 million who drank untreated surface water. SDG 6 aims at universal, safe and affordable drinking water by 2030, which the JMP now considers increasingly out of reach.
Sources: WHO/UNICEF JMP, Progress on household drinking water and sanitation 2000–2024 (2025); Wikipedia, "Arsenic contamination of groundwater" and "Flint water crisis".
Farming is the largest user of freshwater and one of its largest polluters. Fertilisers, manure, pesticides and soil washed from fields all reach rivers, lakes and the sea, and the most widespread effect is eutrophication.
Farmers spread nitrogen and phosphorus fertiliser and manure on fields, often more than crops can take up. Rain washes phosphate, which sticks to soil particles, into streams as surface runoff, while nitrate dissolves and moves down through the soil by leaching. In warm, still, sunlit water the extra nutrients cause an algal bloom; some blooms are of cyanobacteria such as Microcystis, which release toxins. The bloom shades the water, so plants on the bed die. When the algae die too, bacteria decompose them and use up the dissolved oxygen, and fish and invertebrates suffocate or leave.
Phosphate usually limits algal growth in lakes and nitrate in coastal seas, so the two nutrients need different controls.
Hypoxia is the condition at the heart of the process. Most fish need more than about 2 milligrams of dissolved oxygen per litre. In lakes and seas that are stratified in summer, warm surface water floats on cold, dense bottom water, so oxygen from the air cannot mix down to replace what bacteria consume.
When low oxygen persists over a large area, the result is a dead zone.
A 2008 study counted 405 dead zones worldwide. The largest in the USA forms each summer in the northern Gulf of Mexico, fed by nutrients the Mississippi carries from the farms of the US Midwest. In 2025 it covered about 11,400 km²; its five-year average of about 12,300 km² is more than twice the US Hypoxia Task Force’s target of about 4,900 km² by 2035. In the almost enclosed Baltic Sea, low-oxygen areas grew from about 5,000 km² to more than 60,000 km² over the twentieth century.
Sources: NOAA, Gulf hypoxic zone measurement, July 2025; Wikipedia, "Dead zone (ecology)" (Diaz and Rosenberg, 2008; Baltic Nest Institute).
A diffuse source is the opposite case.
Point-source pollution can be measured at the pipe, licensed and fined. Diffuse pollution cannot: nutrients from any one field are small and vary with the weather, so regulators cannot prove who caused a bloom, and each farmer gains little from cutting back when the benefit goes to people downstream. As sewage and industry were cleaned up from the 1970s, farming became the main source of nutrients in many rivers. Pesticides, eroded soil and slurry also pollute water.
Lake Erie, shared by the USA and Canada, is the shallowest and warmest of the Great Lakes. It recovered after phosphate controls under the 1972 Great Lakes Water Quality Agreement, but blooms returned in the 2000s. Much of the phosphorus now comes from fertiliser on maize and soybean fields in the Maumee River basin of Ohio, washed into the western basin by spring storms. In August 2014 toxins from a Microcystis bloom overwhelmed the treatment plant in Toledo, Ohio, and about 400,000 people were told not to drink their tap water for two days, from 2 to 4 August. The 2015 bloom was the largest recorded.
In 2016 the two governments adopted a target of a 40% cut in phosphorus entering the western and central basins, compared with 2008, by 2025. Loads exceeded the target in most years after 2012, and a bloom of about 1,300 km² formed near Toledo in 2019.
Stakeholder | Interest | Power and role |
|---|---|---|
Farmers in the Maumee basin | Yields and income | Control the main source, but act individually; most measures are voluntary |
Fertiliser and agribusiness firms | Sales | Shape farm advice; lobby against mandatory limits |
Toledo’s water utility and residents | Safe, affordable drinking water | Pay for extra treatment but cannot control farms upstream |
Ohio, Michigan and Ontario | Public health and the farm economy | Set rules on fertiliser and manure; balance urban and rural voters |
US and Canadian governments, scientists and NGOs | A healthy shared lake | Set the 40% target; NOAA forecasts blooms; NGOs press for regulation |
Sources: Wikipedia, "Lake Erie" and "Harmful algal bloom"; US EPA, Great Lakes Water Quality Agreement Annex 4 (2016).
The case shows how power is spread across a basin: those who bear the costs in Toledo have no control over the farms that cause them. In the European Union, by contrast, the Nitrates Directive (1991) requires nitrate vulnerable zones, where farmers must follow rules on when and how much fertiliser and manure they spread; Wales made its whole territory such a zone in 2021.
Irrigation allows farming in dry regions, two or three harvests a year and much higher yields; irrigated land produces about 40% of the world’s food from about a fifth of its farmland. In hot, dry climates it can also poison the land it waters.
All river water and groundwater contains dissolved salts. When irrigation water evaporates, the salts stay behind and build up until crops cannot grow. This is salinisation.
Water is added. Unlined canals and over-watering let much irrigation water seep into the ground.
The water table rises. Seepage raises the water table to within a metre or two of the surface, and fields become waterlogged.
Salts rise. Near the surface, capillary action draws water up through the tiny spaces in the soil, carrying dissolved salts.
Evaporation concentrates the salts in the root zone and as a white crust.
Crops fail, because salty soil makes it harder for roots to take up water and some salts are toxic.
Rivers become saltier downstream, as drainage from fields returns to them: the lower Murray in Australia, the lower Colorado and the Indus in Pakistan.
Waterlogging on its own adds no salt. Salinisation begins when the raised water table brings salty groundwater within reach of evaporation, usually within about 2 m of the surface.
Waterlogging and salinisation usually occur together, the "twin menace" of irrigated plains such as Pakistan’s Indus basin. Both are slow to develop and slow to reverse.
Method | How it works | Limitation |
|---|---|---|
Drainage | Lowers the water table; carries salty water away | Expensive; the salty water goes downstream |
Leaching with extra water | Flushes salts below the root zone | Needs spare water and good drainage |
Lining canals | Cuts seepage | High cost across thousands of kilometres |
Drip irrigation | Small amounts to each plant’s roots; little evaporation or seepage | High cost per hectare; suits fruit and vegetables more than grain |
Salt-tolerant crops or trees | Barley or date palms tolerate salt; deep-rooted trees lower the water table | Lower-value crops; trees take years |
Drip irrigation is costly for smallholders, and where farmers use saved water to irrigate more land, total use can rise.
The Aral Sea, on the border of Kazakhstan and Uzbekistan, was the world’s fourth-largest lake in 1960, with an area of 68,000 km². It is fed by the Amu Darya and Syr Darya, which rise in the mountains of Tajikistan and Kyrgyzstan. From the early 1960s Soviet planners diverted both rivers to irrigate the desert for cotton, and by 1988 Uzbekistan was the world’s largest cotton exporter. The canals were poorly built: in 2012 only 12% of Uzbekistan’s irrigation canals were lined.
By 2007 the sea had shrunk to about 10% of its original area and its salinity had killed the native fish. The port of Moynaq, which once employed about 30,000 people in fishing, now lies 30–90 km from the water. Winds blow salt and pesticide residues from the dry seabed onto fields already salinised by irrigation. In 2005 Kazakhstan, with World Bank support, completed the 13 km Kok-Aral dam across the North Aral Sea: by 2008 its level had risen 12 m, salinity fell from about 30 to 8 grams per litre and fishing resumed.
Stakeholder | Interest and role |
|---|---|
Soviet planners (to 1991); governments of Uzbekistan and Turkmenistan | Cotton exports and farm employment; ordered and maintain the diversions |
Cotton farmers and workers | Depend on irrigation; also suffer salinised soils and polluted water |
Fishers and residents of Moynaq and Aralsk | Lost livelihoods and health; little power over upstream decisions |
Government of Kazakhstan and the World Bank | Funded the Kok-Aral dam to restore the North Aral fishery |
Sources: Wikipedia, "Aral Sea" (NASA, UNEP and World Bank sources).
Compared with Lake Erie, the Aral Sea shows a different pattern of power: the damage was caused by a centrally planned state that valued cotton above the lake, not by thousands of independent farmers. The north recovered because one state could act on its own part of the sea; the south, which depends on decisions in several countries, has not.
YouTubeLakes and aquifers hold water for years or centuries, so they buffer supplies through droughts but recover slowly when overused.
Lakes shrink when feeder rivers are diverted or dammed and when evaporation rises; their quality falls as nutrients and waste flow in. Lake Chad, shared by Chad, Cameroon, Niger and Nigeria, shrank from about 25,000 km² of open water in 1960 to about 2,000 km² in 1980, as the Sahel droughts coincided with irrigation schemes on its rivers. More than 30 million people live in its basin.
Sources: Wikipedia, "Lake Chad" and "Aral Sea".
Aquifers are refilled by recharge from rain, rivers and irrigation. As long as pumping does not exceed recharge, groundwater is renewable; where it does, the water table falls year after year. This is groundwater depletion.
Recharge ranges from a fraction of a millimetre a year in deserts to hundreds of millimetres in humid regions with permeable rock.
Around a pumping well the water table is drawn down into a cone of depression. Where cones merge, shallow wells run dry and farmers must drill deeper; poorer farmers are the first to lose access.
Some aquifers receive almost no recharge today: their fossil water infiltrated thousands of years ago in a wetter climate. The Nubian Sandstone Aquifer System beneath Egypt, Libya, Sudan and Chad is the largest known, and Libya’s Great Man-Made River pipes its water to coastal cities. Using it is effectively mining a non-renewable resource.
Land subsidence. Pumped clay and silt layers compact and the ground sinks. Mexico City, built on the clay of a former lake, takes about 59.5 m³ s⁻¹ from its aquifer against a recharge of about 31.6 m³ s⁻¹; parts have sunk by up to 9 m in a century and still sink by 5–40 cm a year, breaking pipes and reversing the gradient of drainage canals.
Saltwater intrusion. As coastal water tables fall, denser seawater moves inland and contaminates wells, as in the Gaza Strip and parts of coastal Bangladesh.
Loss of base flow. Rivers, springs and wetlands fed by groundwater dry up.
Subsidence is largely irreversible: compacted clay does not expand again when the water table recovers.
The Ogallala Aquifer lies beneath about 27% of the USA’s irrigated land; since the late 1940s about 410 km³ more has been withdrawn than replaced, and recharge in parts of Texas is as low as 0.6 mm a year.
In north-west India, GRACE satellites, which detect changes in gravity caused by changing water storage, showed that groundwater beneath Rajasthan, Punjab and Haryana (including Delhi) fell by about 109 km³ between 2002 and 2008, about 4 cm of water a year, while rainfall was close to normal. About 89% of the groundwater India extracts is used for irrigation.
Sources: Wikipedia, "Ogallala Aquifer" (USGS); Rodell, Velicogna and Famiglietti, Nature (2009); Wikipedia, "Water scarcity in India" and "Water management in Greater Mexico City".
Economic growth raises demand as industry needs water for processing and cooling, farmers switch to irrigated cash crops, and urban households gain piped supplies and appliances. Migration concentrates demand: rapid urban growth places millions of new users on systems built for far fewer, and tourists can double demand in resort towns in the driest months. Chennai, a city of more than 11 million people, shows both. After two weak monsoons, all four of its main reservoirs ran dry and on 19 June 2019 officials declared "Day Zero"; water arrived by tanker lorry and train, while groundwater was being pumped at about twice the rate of recharge.
Sources: Wikipedia, "2019 Chennai water crisis".
Groundwater is prone to over-use because it is a common-pool resource. Anyone owning land above an aquifer can usually sink a well, gaining the full benefit of the water pumped while the cost, a falling water table, is shared by all: the tragedy of the commons. Licensing and metering wells, community management and charging for pumping energy can help, but enforcement among millions of small users is difficult.
Many rivers, lakes and aquifers cross international borders. In 2018 the register of international river basins identified 310 basins shared by about 150 countries (313 basins in 2024), covering about 47% of the Earth’s land and home to 52% of its people.
Each country that shares such a resource is a riparian state.
Transboundary water creates an upstream–downstream asymmetry: an upstream state can dam, divert or pollute water before it reaches its neighbours. Geography is not the only source of power, however. A downstream state may be richer or militarily stronger, and may claim "historic rights" from decades of use; the term hydro-hegemony describes a state that dominates a shared basin whatever its position on the river. Türkiye’s dams on the Tigris and Euphrates reduce flows to Syria and Iraq; China controls the headwaters of the Mekong and has not joined the Mekong River Commission; on the Nile, Egypt is downstream but has long been the strongest power in the basin.
A water conflict may range from diplomatic protest to violence, but most water conflicts between states are political: threats, propaganda and the withholding of data. Since 1945 international water events have involved far more cooperation than armed conflict.
Cooperation usually takes the form of a water treaty. The Indus Waters Treaty, signed on 19 September 1960 after negotiations led by the World Bank, gave India the three eastern rivers (Ravi, Beas, Sutlej) and Pakistan mainly the three western rivers (Indus, Jhelum, Chenab). It survived three wars, but on 23 April 2025, after an attack on tourists at Pahalgam in Indian-administered Kashmir, India announced it was holding the treaty in abeyance, which shows that treaties depend on wider political relations.
Sources: McCracken and Wolf, International Journal of Water Resources Development (2019); Oregon State University, Transboundary Freshwater Spatial Database (2024); Wikipedia, "Indus Waters Treaty".
International water law rests on two principles that pull in opposite directions. Equitable and reasonable use gives every riparian state a fair share, which upstream states cite to justify new dams; no significant harm obliges states not to damage their neighbours, which downstream states cite to defend existing flows. Both are in the UN Watercourses Convention, adopted on 21 May 1997, which entered into force only in August 2014 and has not been joined by many states with the most at stake, including those on the Nile.
Stakeholders include upstream and downstream governments, farmers, cities and industry, hydropower developers and investors, mediators (the World Bank, the African Union, the UN), NGOs, and riverine and Indigenous communities whose fisheries depend on natural flows.
Approach | How it helps | Why it can fail |
|---|---|---|
Treaty allocating fixed volumes | Clear rules | Ignores droughts, climate change and excluded states |
Joint basin commission | Shared data and technical trust | Often advisory; key states may not join |
Benefit sharing | Shares electricity, flood control or food rather than water | Needs long-term trust and trade links |
Third-party mediation | Neutral expertise and money (World Bank on the Indus) | Mediators may be seen as biased; no enforcement |
Agreements tend to succeed when they share benefits as well as water, include every riparian state, can adapt to changing flows and rest on shared data. They fail when one state can act alone without paying a price, or when an agreement excludes upstream states that later become strong enough to ignore it. The Nile basin combines several of these weaknesses.
The Nile is shared by eleven countries. The White Nile rises in the Lake Victoria basin and flows north through Uganda and South Sudan; the Blue Nile, the Abay in Ethiopia, rises at Lake Tana in the Ethiopian Highlands and joins it at Khartoum, and the Atbara joins further north. Because the highlands receive heavy summer monsoon rain, more than two-thirds of the water reaching Egypt originates in Ethiopia, most of it between July and October.
The three states most involved occupy very different positions. Ethiopia, upstream, generates most of the water, but in 2023 only 55.4% of its people had electricity. Sudan, in the middle, relies on the Nile for irrigation and for hydropower at Roseires and Sennar on the Blue Nile and Merowe on the main Nile. Egypt, downstream in the Sahara, depends on the Nile for about 90% of its water and had more than 116 million people in 2024, almost all living along the river or its delta.
Egypt and Sudan face physical scarcity and need every cubic metre they receive; upstream Ethiopia faces economic scarcity and sees the river as its best resource for development, and this difference in needs underlies the conflict.
A 1929 agreement between Egypt and Britain, then the colonial power in Sudan and East Africa, gave Egypt 48 km³ and Sudan 4 km³ a year and a veto over upstream projects. In 1959 Egypt and Sudan raised their shares to 55.5 km³ and 18.5 km³, almost all of the river’s average flow at Aswan. Ethiopia was party to neither. Egypt regards them as establishing historic rights; Ethiopia regards them as colonial-era arrangements that bind no one else.
In February 1999 nine riparian states launched the Nile Basin Initiative to share data and plan joint projects. Its negotiations produced the Cooperative Framework Agreement (CFA), based on equitable and reasonable use and signed in 2010 by Ethiopia, Kenya, Uganda, Rwanda and Tanzania (Burundi in 2011). Egypt and Sudan rejected it as a breach of the 1959 agreement. The CFA came into force on 13 October 2024, but without the two downstream states it cannot govern the whole basin.
Ethiopia laid the foundation stone of the Grand Ethiopian Renaissance Dam (GERD) on 2 April 2011, on the Blue Nile in the Benishangul-Gumuz Region close to the Sudanese border. With 5,150 MW of capacity it is Africa’s largest hydroelectric power station, and its 74 km³ reservoir holds more than the Blue Nile’s average annual inflow of about 49 km³. It cost about US$5 billion. Ethiopia paid for most of it from bonds sold to citizens and the diaspora and from donations, as Egypt lobbied against international loans; a loan of about US$1 billion from the Export-Import Bank of China paid for the turbines.
Date | Event |
|---|---|
May 2010 | Five upstream states sign the CFA; Egypt and Sudan reject it |
2 April 2011 | GERD foundation stone laid |
March 2015 | Egypt, Sudan and Ethiopia sign a Declaration of Principles on the dam |
Nov 2019 – Feb 2020 | Talks facilitated by the US Treasury and World Bank; Ethiopia misses the final round |
July 2020 | First filling to 540 m; African Union-led talks begin |
8 July 2021 | UN Security Council meets on the dispute; second filling to 575 m |
20 February 2022 | First electricity (375 MW); third filling to 600 m in August |
2023 and 2024 | Fourth (625 m) and fifth, final filling (640 m) |
9 September 2025 | GERD formally inaugurated |
Ethiopia’s government presents the dam as a national project to end energy poverty and export electricity to Sudan, Kenya and Djibouti. It argues that the dam releases the water it uses for power, regulates floods, and that storing water in the cool highlands wastes less to evaporation: about 1.5 km³ a year, compared with 10–16 km³ lost from Egypt’s Lake Nasser.
Egypt’s government fears that filling the reservoir will cut its supply, especially in a drought, and that Ethiopia will later use the water for irrigation. It has demanded a legally binding agreement on filling and on operation in dry years; in the US-facilitated talks Ethiopia proposed releasing 35 km³ a year during filling and Egypt asked for 40 km³. Egyptian leaders have said that "all options are open", and Egypt took its complaints to the UN Security Council.
Sudan’s government at first welcomed the dam, which should reduce Blue Nile floods, trap sediment that fills its reservoirs and supply cheap electricity. After the first filling in 2020 was announced without shared data, it grew concerned about the safe operation of the Roseires Dam a short distance downstream and joined Egypt in calling for a binding agreement.
About two million Egyptian farmers could be affected by reduced flows during filling but have no seat at the negotiations, while Ethiopian bondholders have a personal stake in the dam. Mediators have had limited influence: Ethiopians accused the USA and the World Bank of siding with Egypt, especially after the US suspended some aid in September 2020.
YouTubeAttempt | Achievement | Limitation |
|---|---|---|
Nile Basin Initiative (1999) and CFA (2010; in force 2024) | A forum for all riparian states; a framework accepted by most upstream states | Rejected by Egypt and Sudan, dividing the basin into two camps |
Declaration of Principles (2015) | Accepted equitable use, no significant harm and cooperation on filling | High-level and non-binding; interpreted differently by each side |
US and World Bank facilitation (2019–2020) | Draft rules for filling and drought years | Ethiopia saw the mediators as biased and did not sign |
African Union talks (from 2020) and UN Security Council (2021) | Kept the dispute diplomatic | No binding agreement; the dam was filled and completed without one |
By 2025 the dam was finished and full, so whether it should be built had been settled by Ethiopia acting alone. The remaining questions concern operation: releases during a multi-year drought, any future irrigation or dams upstream, and coordination with Sudan’s dams and the Aswan High Dam. Coordinated operation could benefit all three states, by storing water where evaporation is lower and generating more power; this is the logic of benefit sharing. Without trust and shared data, each state sees the others’ control of the same system as a threat.
Key concept | How the case shows it |
|---|---|
Place | The same river is a development opportunity in highland Ethiopia, a lifeline in desert Egypt, and both a flood risk and a resource in Sudan. |
Power | Geographical power upstream (Ethiopia) against historic, economic and military power downstream (Egypt); mediators weakened by perceived bias. |
Possibility | Futures range from coordinated operation and benefit sharing to a crisis if a severe drought coincides with disputed releases. |
Sources: Wikipedia, "Grand Ethiopian Renaissance Dam", "Nile", "Nile Basin Initiative" and "Water politics in the Nile Basin"; FAO AQUASTAT, UN Population Division and World Bank via World Bank WDI (2022–2024).
To what extent has Ethiopia’s upstream position given it more power over the Nile than Egypt’s economic and military strength?
Why might a fixed allocation such as the 1959 agreement become harder to sustain as populations grow and the climate changes?
Suggest how benefit sharing could form the basis of an agreement acceptable to all three states.
Paper 1 sets, for each option, a 10-mark structured question on a stimulus and a choice of two 10-mark extended questions.
Precise terminology earns credit in both question types.
Table: freshwater resources in five Nile basin states, 2022.
Country | Renewable internal freshwater per person (m³ per year) | Water stress: withdrawal as % of available freshwater |
|---|---|---|
Egypt | 9 | 141.2 |
Sudan | 81 | 118.7 |
Ethiopia | 973 | 32.3 |
Uganda | 824 | 5.8 |
South Sudan | 2,359 | 4.2 |
Sources: FAO AQUASTAT via World Bank, World Development Indicators (2022 data).
Part (b) moves from reading the stimulus to explaining it.
Part (c) asks for two developed explanations.
Extended questions on scarcity and water quality usually ask for an examination of stakeholders’ roles, an evaluation of attempts to resolve a water conflict, or a judgment on physical and human causes of scarcity.
A second common essay asks for a judgment on causes rather than on management.
Physical water scarcity means not enough water for all demands; economic water scarcity means investment and institutions are lacking.
The Falkenmark indicator sets stress at 1,700, scarcity at 1,000 and absolute scarcity at 500 m³ per person per year, but national averages hide regional and social differences.
Climate and geology control supply; population growth, urbanisation, consumption, irrigation, pollution and governance control demand and usable supply.
Meteorological drought can lead to agricultural, hydrological and socio-economic drought; its impacts depend on vulnerability.
Water quantity and water quality differ: 2.1 billion people lacked safely managed drinking water in 2024.
Diffuse farm pollution causes eutrophication, hypoxia and dead zones; irrigation in hot, dry climates causes waterlogging and salinisation.
Over-pumping of aquifers causes cones of depression, land subsidence and saltwater intrusion, driven by economic growth and migration.
Shared rivers create upstream–downstream asymmetry; the GERD dispute shows why mediation has not produced a binding Nile agreement.
I can distinguish between physical and economic water scarcity, and between water quantity and water quality, with an example of each.
I can explain how climate, geology and human factors control water availability in named places.
I can describe the four types of drought, explain their causes and compare their impacts in an HIC and an LIC.
I can explain eutrophication and salinisation step by step and draw a diagram of each.
I can examine the roles of stakeholders in agricultural water pollution using Lake Erie and the Aral Sea basin.
I can explain why pressures on lakes and aquifers are growing, using examples of depletion, subsidence and saltwater intrusion.
I can examine why shared rivers cause conflict and evaluate attempts to resolve the dispute over the Nile and the GERD.
I can determine water available per person and percentage change, and interpret a map of water scarcity.
pollution reducing usable supply (eg nitrate or arsenic contamination of groundwater in parts of Bangladesh and India), so scarcity can result from poor water quality as well as low quantity
climate change, itself partly human-caused, altering rainfall patterns and glacier meltwater supply
Good answers may be well structured (AO4) and may additionally offer a critical evaluation (AO3) which recognizes that physical and human factors interact: drought is a physical trigger, but whether it becomes a crisis depends on demand, management and wealth (places and power). Another approach might be to argue that the relative importance varies with the type of scarcity, with physical factors more important where scarcity is physical and human factors dominant where it is economic. Another approach might be to consider scale and time, noting that climate change blurs the distinction between physical and human causes in the long term (possibilities).
For 5–6 marks, expect some weakly-evidenced outlining of both physical and human factors.
For 7–8 marks, expect a structured account which includes:
either well-evidenced explanation of physical and human factors in at least two different places
or a discursive conclusion (or ongoing evaluation) grounded in geographical concepts and/or perspectives about which factors are more important.
For 9–10 marks, expect both of these traits.
NB. Do not expect perfect balance between physical and human factors.
Marks | Level descriptor |
|---|---|
0 | The response does not reach a standard described by the descriptors below. |
1–2 | Too brief, lists unconnected information, not focused on the question and lacks structure. Knowledge is very general, with large gaps or errors; examples or case studies are missing or only listed. No analysis; terminology is missing or used incorrectly. |
3–4 | Too general, lacks detail, not focused on the question and largely unstructured. Outlines examples, statistics and facts, both relevant and irrelevant, with links to the question only listed. Basic terminology is used, with errors or inconsistently. No critical evaluation of evidence; any conclusion is irrelevant. |
5–6 | Partially addresses the question, with a narrow argument, an unsubstantiated conclusion and limited evaluation. Describes relevant supporting evidence (examples, case studies) and outlines links to the question. The argument repeats one point or covers only part of the question. Other perspectives, or strengths and weaknesses of evidence, are listed. |
7–8 | Addresses the whole question; the analysis is evaluated and the conclusion is relevant but lacks balance. Relevant evidence is described correctly and covers all the main points. The argument is clear but one-sided or unbalanced. Complex terminology is used correctly but not consistently. Other perspectives, or strengths and weaknesses of evidence, are described. |
9–10 | In-depth and specific to the question (topic and command term). Correct, relevant examples, statistics and detail are integrated and their link to the question explained. The argument is balanced, explaining complexity, exceptions and comparisons. Evaluation of evidence and perspectives is well developed, and the conclusion is balanced and aligned with the evidence. Logically structured and easy to follow. |
To what extent is economic water scarcity a more serious problem than physical water scarcity?
10 marks
Examine why the impacts of drought can vary between places at contrasting levels of economic development.
10 marks