Water (Spring, first half)

Topic 13: The global hydrological cycle and its importance

Key ideas

• At the global scale, the hydrological cycle is a closed system. Water is not gained or lost from Earth, only moved between stores.
• The cycle is driven by solar energy, which drives evaporation and atmospheric circulation, and by gravitational potential energy, which drives precipitation, runoff and groundwater flow.
• Stores (approximate shares of all water on Earth, from USGS figures):

Store	% of all water	Notes
Oceans	~96.5%	Saline
Cryosphere (ice caps, glaciers, permanent snow)	~1.7%	The largest freshwater store (about two-thirds of fresh water)
Groundwater	~1.7%	About half of it saline; fresh groundwater is about 30% of fresh water
Surface water (lakes, rivers, swamps)	<0.02%	The most accessible store
Atmosphere	~0.001%	But a very fast flux
Biosphere	Tiny	Water held in living organisms

• Only about 2.5% of Earth's water is fresh, and most of that is locked in ice or deep groundwater. So the global water budget limits water available for human use.
• Residence times vary enormously: about 9 days in the atmosphere, weeks to months in rivers, decades to centuries in lakes and groundwater, thousands of years in the oceans and up to hundreds of thousands of years in the Antarctic ice sheet.
• Fossil water is ancient groundwater that is not being recharged today, e.g. the Nubian Sandstone Aquifer under Egypt and Libya, which Libya tapped through its Great Man-Made River. Fossil water and cryosphere losses are effectively non-renewable.
• Annual fluxes:
  • Evaporation from the oceans exceeds precipitation over the oceans.
  • The surplus is carried over land by advection (moisture transport) and falls as precipitation.
  • It returns to the sea as runoff, which balances the cycle.

Glossary

Term	Definition
Closed system	A system with energy inputs and outputs but no transfer of matter in or out.
Store	A reservoir where water is held (e.g. ocean, ice, groundwater).
Flux / flow	The rate at which water moves between stores.
Cryosphere	The frozen parts of Earth (ice sheets, glaciers, sea ice, permafrost).
Residence time	The average time water spends in a store.
Fossil water	Ancient groundwater not being recharged under the present climate.
Global water budget	An account of all water stores and flows on Earth.

Self-test

Q1: Why is the global hydrological cycle a closed system? A: Water is transferred between stores but the total amount is effectively fixed. Only energy (solar) enters and leaves.

Q2: What percentage of Earth's water is fresh, and where is most of it? A: About 2.5%. Most of it is in ice caps and glaciers (about two-thirds), and most of the rest is groundwater.

Q3: Why is fossil water a non-renewable resource? A: It accumulated under past, wetter climates and is not being recharged today, so abstraction permanently depletes it.

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Topic 14: The drainage basin as an open system

Key ideas

• A drainage basin is the area drained by a river and its tributaries, bounded by a watershed.
• It is an open system: water enters as precipitation and leaves as evapotranspiration, river discharge to the sea and groundwater flow out of the basin.
• Inputs: precipitation. Its type, amount, intensity, seasonality and variability all matter.
• Stores:
  • Interception (vegetation).
  • Surface storage (puddles, lakes).
  • Soil moisture.
  • Groundwater (aquifers).
  • Channel storage.
• Flows:
  • Interception and then stemflow/throughfall.
  • Infiltration into the soil. The infiltration capacity is the maximum rate (mm/hr) at which water can infiltrate.
  • Overland flow:
    • Infiltration-excess (Hortonian) overland flow: rainfall intensity exceeds infiltration capacity.
    • Saturated overland flow: the soil is full, so even gentle rain runs off.
  • Throughflow: lateral flow through the soil.
  • Percolation: downward flow into rock.
  • Groundwater flow (baseflow): slow.
• Outputs:
  • Evaporation.
  • Transpiration (together these are evapotranspiration).
  • Channel flow (discharge).
• Physical factors control the balance of inputs, flows and outputs:
  • Climate: temperature affects evaporation; intensity affects runoff.
  • Soils: sandy soils have high infiltration; clay soils have low infiltration.
  • Vegetation: dense forest gives high interception and transpiration.
  • Geology: permeable chalk gives high percolation; impermeable granite or clay gives fast runoff.
  • Relief: steep slopes give rapid runoff.
• Human disruption:
  • Deforestation reduces interception and transpiration, increases runoff and soil erosion, and reduces local rainfall recycling.
  • Urbanisation creates impermeable surfaces and drains.
  • Agriculture: compaction, drainage and irrigation.
  • Reservoirs create new stores.
  • Abstraction from rivers and aquifers.

Case study: Amazonia (deforestation and the water cycle)

• The Amazon rainforest recycles a large share of its own rainfall through evapotranspiration, forming "flying rivers" of moisture carried to southern Brazil and the La Plata basin.
• Deforestation for cattle ranching, soya and roads reduces interception and transpiration. Runoff and soil erosion increase.
• Scientists warn of a possible tipping point: if enough forest is lost, parts of the Amazon could shift towards drier savanna. Thresholds of roughly 20–25% deforestation have been suggested (Lovejoy and Nobre, 2018), but the figure is uncertain.
• Severe droughts struck in 2005, 2010 and 2023–24.

Glossary

Term	Definition
Drainage basin	The area of land drained by a river and its tributaries.
Watershed	The boundary (ridge of high land) between drainage basins.
Interception	Precipitation caught by vegetation before it reaches the ground.
Infiltration capacity	The maximum rate at which water can enter the soil.
Throughflow	Lateral movement of water through the soil.
Percolation	Downward movement of water from the soil into rock.
Baseflow	Groundwater-fed river flow that sustains discharge between storms.
Evapotranspiration	The combined loss of water by evaporation and plant transpiration.
Aquifer	A permeable rock that stores and transmits groundwater.

Self-test

Q1: Distinguish between Hortonian and saturated overland flow. A: Hortonian (infiltration-excess) flow occurs when rainfall intensity exceeds the soil's infiltration capacity, even if the soil is unsaturated. Saturated overland flow occurs when the soil is already full of water, so any further rain runs off.

Q2: How does deforestation alter the drainage basin cycle? A: It reduces interception and transpiration, which increases overland flow, flood peaks and soil erosion, and reduces moisture recycling and therefore regional rainfall.

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Topic 15: Water budgets and river systems at the local scale

Key ideas

• Water budget: precipitation (P) = evapotranspiration (E) + runoff/discharge (Q) ± change in storage (S).
• Annual water-budget graphs show monthly P against potential evapotranspiration (PET). They reveal four phases:
  • Soil moisture surplus: P > PET; the soil is full and the excess becomes runoff.
  • Soil moisture utilisation: PET > P; plants draw on stored soil water.
  • Soil moisture deficit: the store is exhausted, so there is a water shortage for plants and crops.
  • Soil moisture recharge: P > PET again, refilling the soil store.
• Water budgets by climate type:
  • Tropical rainforest: surplus most of the year.
  • Tropical savanna / monsoon: wet-season surplus, then a long dry-season deficit.
  • Temperate maritime (e.g. the UK): winter surplus and summer utilisation or deficit, especially in south-east England.
  • Polar: low P, but also low E; water is locked as ice for much of the year.
• River regimes show the annual variation in discharge. They reflect climate (rainfall and snowmelt), geology and soils (groundwater baseflow), and human regulation.
  • Yukon (Canada/Alaska): low winter flow (frozen), then a large peak in late spring and summer from snowmelt and glacier melt.
  • Amazon: a very high and relatively sustained discharge. It peaks around May–June because of seasonal rainfall over a huge basin with tributaries in both hemispheres.
  • Indus: fed by Himalayan and Karakoram snow and glacier melt plus the summer monsoon, so it peaks strongly in July–August and has low winter flow.
• Storm hydrographs show discharge after a rainfall event:
  • Components: rising limb, peak discharge, lag time (from peak rainfall to peak discharge), falling (recession) limb, baseflow and quickflow.
  • A flashy hydrograph has a short lag time and a high peak. A subdued hydrograph has a long lag time and a low peak.
• Physical factors affecting hydrograph shape:
  • Basin size and shape: circular basins have a shorter lag time than elongated ones.
  • Drainage density: high density gives a flashy response.
  • Rock type: impermeable rock gives a flashy response.
  • Soil: clay soils give a flashy response.
  • Relief: steep slopes give a flashy response.
  • Vegetation: dense vegetation subdues the response.
  • Antecedent conditions: wet ground gives a flashy response.
• Human factors affecting hydrograph shape:
  • Urbanisation: impermeable surfaces and drains give a flashy response.
  • Land use, e.g. deforestation or ploughing.
  • Planners can reduce flood risk with SuDS (sustainable drainage systems), green roofs and floodplain zoning (a P theme).

Glossary

Term	Definition
Water budget	The balance between precipitation, evapotranspiration, runoff and storage change.
Potential evapotranspiration (PET)	The water loss that would occur if there were unlimited water available.
Soil moisture deficit	The amount by which soil water is below field capacity.
River regime	The annual pattern of variation in a river's discharge.
Discharge	The volume of water passing a point per second (m³/s, cumecs).
Storm hydrograph	A graph of discharge over time following a storm.
Lag time	The time between peak rainfall and peak discharge.
Drainage density	Total stream length divided by basin area.
Antecedent moisture	The wetness of the basin before a storm.
SuDS	Sustainable Drainage Systems that slow and store urban runoff.

Exam technique

• A 6-mark resource question here might be "Explain the differences between the two hydrographs shown."
• Quote the lag times and peaks from the graph, using data with units.
• Then explain two or three causes, e.g. urban compared with rural land use, or basin shape.

Self-test

Q1: Describe the four phases of a UK water budget. A: In winter there is a surplus (P > PET, runoff). In spring and summer there is utilisation as plants use soil water. In late summer there can be a deficit when the store is used up. In autumn there is recharge as P > PET again.

Q2: Why does the Yukon peak in late spring/summer? A: Precipitation is stored as snow and ice in winter, giving low flow. Spring and summer warming melts the snowpack and glaciers, releasing the stored water.

Q3: Give three physical factors that make a hydrograph flashy. A: Any three of: impermeable rock, clay soils, steep slopes, high drainage density, a small circular basin, saturated (antecedent) conditions, sparse vegetation.

Q4: How does urbanisation change a storm hydrograph? A: Impermeable surfaces and drains reduce infiltration and speed up runoff, giving a shorter lag time, a higher peak and a steeper rising limb.

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Topic 16: Climate change impacts on the hydrological cycle, globally and locally

Key ideas

• Inputs and outputs:
  • A warmer atmosphere holds about 7% more water vapour per °C of warming (the Clausius–Clapeyron relationship).
  • This leads to more intense precipitation in many regions and higher evaporation.
  • Broadly, the pattern is "wet gets wetter, dry gets drier", with more extreme events, but regional patterns are complex.
• Stores and flows:
  • Shrinking glaciers and snowpack change the timing of river flow (earlier peaks) and threaten long-term supply, e.g. Himalayan rivers that are "water towers" for South Asia.
  • Permafrost thaw alters drainage and releases carbon.
  • Soil moisture declines in drying regions.
  • Reservoir and lake levels fall in drought, e.g. Lake Mead on the Colorado.
  • Runoff and streamflow are altered.
• Oscillations add uncertainty:
  • ENSO (El Niño and La Niña) alters rainfall worldwide. El Niño brings drought to Australia and Indonesia and floods to Peru.
  • Combined with global warming, this increases uncertainty, which concerns water managers (an F theme: projections of future drought and flood risk).
• Locally:
  • The UK may see wetter winters and drier summers, with more intense rain (higher flood risk) and more frequent summer drought, especially in the south-east.

Self-test

Q1: Why does global warming intensify rainfall? A: Warmer air can hold about 7% more water vapour per °C, so when it rises and condenses it can release more precipitation in intense events.

Q2: How might glacier retreat affect rivers such as the Indus in the short and long term? A: In the short term, increased melt raises flows and flood risk (including glacial lake outburst floods). In the long term, the shrinking ice store reduces dry-season flow and threatens supply.

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Topic 17: Physical and human causes of water insecurity

Key ideas

• Water security means reliable access to enough safe water for health, livelihoods and production, at acceptable risk.
• Falkenmark thresholds (renewable water per person per year):
  • Water stress: below 1,700 m³.
  • Water scarcity: below 1,000 m³.
  • Absolute scarcity: below 500 m³.
• Physical causes of water insecurity:
  • Climate variability, including drought and ENSO.
  • Saltwater encroachment into coastal aquifers, caused by over-pumping and sea-level rise, e.g. Bangladesh and coastal areas of the Mediterranean.
• Human causes of water insecurity:
  • Over-abstraction from rivers, lakes and aquifers, e.g. the Aral Sea (shrunk dramatically after Soviet irrigation diversions from the Amu Darya and Syr Darya from the 1960s) and the falling water table in northern India and the North China Plain.
  • Contamination from agriculture: nitrates and pesticides cause eutrophication.
  • Industrial pollution, e.g. heavy metals and untreated effluent.
• Rising demand on a finite resource:
  • Population growth.
  • Rising living standards: more meat in diets and more household appliances.
  • Industrialisation and energy production (cooling water).
  • Agriculture, which accounts for roughly 70% of global freshwater withdrawals.
• Future projections: the risk of water insecurity is increasing in places such as the Middle East and North Africa (MENA), South Asia and the western USA.

Glossary

Term	Definition
Water security	Reliable access to sufficient safe water.
Water stress	Less than 1,700 m³ of renewable water per person per year.
Water scarcity	Less than 1,000 m³ per person per year.
Over-abstraction	Removing water faster than it is replenished.
Saltwater encroachment	Seawater moving into freshwater aquifers.
Eutrophication	Nutrient enrichment that causes algal blooms and oxygen depletion.

Self-test

Q1: What are the thresholds for water stress and water scarcity? A: Stress: below 1,700 m³ per person per year. Scarcity: below 1,000 m³ per person per year.

Q2: Give one physical and two human causes of water insecurity. A: Physical: climate variability/drought or saltwater encroachment. Human: over-abstraction (e.g. the Aral Sea) and pollution from agriculture or industry.

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Topic 18: Consequences and risks of water insecurity

Key ideas

• Physical water scarcity: there is not enough water to meet demand, even with investment. This is typical of arid regions, e.g. MENA and parts of Australia.
• Economic water scarcity: water exists, but there is a lack of investment, infrastructure or institutions to supply it. This is typical of much of sub-Saharan Africa.
• Why the price of water varies globally:
  • The cost of supply: desalination, pumping, treatment.
  • Subsidies.
  • Private compared with public provision.
  • The poor in informal settlements often pay more per litre to water vendors than richer households with piped supplies.
• Water for economic development:
  • Agriculture: irrigation.
  • Industry: processing and cooling.
  • Energy: hydro-electric power (HEP) and thermal power-station cooling.
• Water for human wellbeing:
  • Drinking water.
  • Sanitation: water-borne diseases such as cholera and diarrhoeal disease.
  • Food preparation.
  • Time lost (often by women and girls) collecting water.
• Water conflict can occur at every scale:
  • Between users within a country: farmers, cities, industry and the environment.
  • Internationally, over trans-boundary rivers and aquifers.

Case study: the Nile

• Shared by 11 countries. Egypt depends on the Nile for about 90% of its fresh water.
• Colonial-era and 1959 agreements allocated most of the flow to Egypt and Sudan. Upstream states (e.g. Ethiopia) reject them.
• The Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile has a reservoir capacity of about 74 billion m³. Reservoir filling began in 2020.
  • Ethiopia sees it as vital for electricity and development.
  • Egypt and Sudan fear reduced flows during filling and droughts.
  • Negotiations (including the Nile Basin Initiative, founded 1999, and US- and AU-brokered talks) have not produced a binding agreement on filling and drought rules.

Case study: the Mekong (alternative)

• The river flows through China, Myanmar, Laos, Thailand, Cambodia and Vietnam.
• Upstream Chinese dams and Lao dams (e.g. Xayaburi) alter flow, sediment and fish migration.
• These changes affect the livelihoods of millions who depend on fisheries and on the rice-growing Mekong Delta.
• The Mekong River Commission (1995) includes only the four lower-basin states, which limits its power.

Exam technique

• A 12-mark question here might be "Assess the extent to which water conflicts are caused by physical rather than human factors."
• Work through the factors: physical (drought, river flows) compared with human (dams, population, politics, treaties).
• Give a judgement, e.g. "Physical scarcity sets the stage, but human decisions turn scarcity into conflict."

Self-test

Q1: Distinguish between physical and economic water scarcity. A: Physical scarcity is a lack of water relative to demand (arid areas). Economic scarcity is when water is available but people lack the investment or infrastructure to access it.

Q2: Why are Egypt and Ethiopia in dispute over the GERD? A: Ethiopia's dam on the Blue Nile (about 74 bn m³ reservoir) is for HEP and development. Egypt relies on the Nile for about 90% of its fresh water and fears reduced flow, especially during filling and droughts. There is no binding agreement on operating rules.

Assessment checkpoint (first Spring half-term): 3, 6 and 8-mark questions (self-assessed) and a teacher-marked 12-marker.

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