Water (Spring, second half)

Topic 19: Deficits within the hydrological cycle – drought and its impacts

Key ideas

• Types of drought:
  • Meteorological drought: a period of below-average precipitation (short-term deficits).
  • Hydrological drought: reduced river flow, groundwater and reservoir levels. It lags behind meteorological drought.
  • Agricultural drought: a soil moisture deficit that harms crops.
  • Socio-economic drought: demand exceeds supply.
• Physical causes:
  • Short-term precipitation deficits, e.g. blocking anticyclones in the UK.
  • Longer-term trends, e.g. shifts in the ITCZ affecting the Sahel.
  • ENSO cycles: El Niño causes drought in Australia, Indonesia and southern Africa; La Niña causes drought in the south-west USA.
• Human contributions to drought risk:
  • Over-abstraction of surface water and aquifers.
  • Deforestation and desertification, e.g. overgrazing in the Sahel reduces vegetation and albedo feedbacks.
• Impacts on ecosystems:
  • Wetlands shrink: water levels fall, salinity rises and bird and fish habitats are lost.
  • Forest stress: trees die, fire risk rises and pests take hold.
  • Resilience varies: some ecosystems recover quickly when rain returns, but repeated droughts push them past thresholds.

Case study: Australia's Millennium Drought (about 1997–2009) and the Murray–Darling Basin

• Causes:
  • Several El Niño years and a positive Indian Ocean Dipole.
  • Long-term drying in the south-east.
  • Over-allocation of irrigation water in the Murray–Darling Basin.
• Impacts:
  • Inflows to the Murray fell to record lows (2006–09).
  • Irrigation allocations were cut drastically: in South Australia they fell to as low as 2% in some years.
  • Wetlands were disconnected.
  • The Lower Lakes and Coorong suffered rising salinity and acidification.
  • Cities had water restrictions.
  • Rice and cotton output fell.
  • Rural hardship increased.
• Responses:
  • Desalination plants were built in Perth, Sydney, Melbourne, Brisbane/Gold Coast and Adelaide, costing over A$10 billion in total. Some (e.g. Sydney's) were mothballed after the rains returned.
  • The Murray–Darling Basin Plan (2012) returns water to the environment.
  • Water trading.

Case study: the Sahel (alternative)

• Prolonged droughts in the 1970s–80s, linked to Atlantic sea-surface temperature patterns and the southward position of the ITCZ.
• Overgrazing and population pressure worsened land degradation and famine (e.g. 1983–85 in Ethiopia and Sudan).

Self-test

Q1: Why does hydrological drought lag behind meteorological drought? A: Soil, groundwater and reservoirs buffer the system. River flows and aquifer levels fall only after the rainfall deficit has persisted long enough to deplete these stores.

Q2: How does El Niño affect eastern Australia? A: Warm water and convection shift east across the Pacific, so eastern Australia gets less rainfall, which raises the risk of drought (e.g. during the Millennium Drought).

Q3: Give two ecosystem impacts of drought. A: Wetland loss (falling water levels, rising salinity, e.g. the Coorong) and forest stress (tree die-off, increased fire risk).

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Topic 20: Surpluses within the hydrological cycle – flooding and its impacts

Key ideas

• Meteorological causes of flooding:
  • Intense convective storms cause flash floods, e.g. Boscastle, Cornwall, on 16 August 2004.
  • Prolonged heavy rainfall from a series of depressions, e.g. winter 2013–14 in the UK.
  • Extreme monsoon rainfall, e.g. Pakistan in 2010 and 2022.
  • Snowmelt, sometimes with rain-on-snow events.
• Human actions that make flood risk worse:
  • Land-use change in the catchment: urbanisation, deforestation, field drainage, overgrazing and compaction.
  • Mismanagement through hard engineering: channelisation speeds water downstream, embankments transfer flooding, and building on floodplains increases exposure.
• Impacts:
  • Environmental: soil erosion or deposition of silt, contamination (sewage, chemicals), damage to ecosystems.
  • Floods also bring benefits: they recharge wetlands and add fertile silt to floodplains.
  • Socio-economic: deaths, homes, businesses, infrastructure (power, water supply, transport), insurance costs and mental health.

Case study: UK summer 2007 floods

• Weather: the wettest May–July on record in England and Wales up to that time. A southerly jet stream steered depressions over the UK. There were intense storms on 25 June (Yorkshire, e.g. Hull and Sheffield) and 20 July (Gloucestershire and the Thames Valley).
• Impacts:
  • 13 deaths.
  • About 55,000 properties flooded.
  • The Mythe water treatment works near Tewkesbury flooded, leaving about 350,000 people without piped water for up to about 17 days.
  • Nearly half a million people lost water or power.
  • The total economic cost was estimated at about £3.2 billion (Environment Agency, 2010).
• Response: the Pitt Review (2008) made 92 recommendations, e.g. protecting critical infrastructure and making the Environment Agency responsible for strategic overview. These led to the Flood and Water Management Act 2010.

Case study: Somerset Levels floods, winter 2013–14

• Causes:
  • A series of Atlantic storms brought exceptional rainfall (January 2014 was exceptionally wet in southern England).
  • Low-lying, flat land drains slowly into the rivers Parrett and Tone, and high tides can stop the rivers discharging.
  • Locals argued that a lack of dredging had reduced river capacity.
• Impacts:
  • At its peak about 65 km² was flooded.
  • About 165 properties were flooded internally.
  • Villages such as Muchelney and Moorland were cut off for weeks.
  • Farmland was lost and livestock evacuated.
  • The economic impact on Somerset was estimated at about £82–147 million (central estimate about £118 million).
• Responses:
  • The Environment Agency dredged about 8 km of the Parrett and Tone (completed in 2014).
  • The Somerset Levels and Moors 20-year Flood Action Plan was drawn up, and the Somerset Rivers Authority was set up.
  • There was political controversy over Environment Agency management.

Self-test

Q1: Give two meteorological causes of flooding with UK examples. A: An intense convective storm causing a flash flood (Boscastle, 2004) and prolonged heavy rainfall from successive depressions (Somerset Levels, 2013–14; summer 2007).

Q2: State three impacts of the 2007 floods. A: 13 deaths; about 55,000 properties flooded; about 350,000 people without tap water after the Mythe works flooded. Total cost about £3.2 billion.

Q3: How can hard engineering make flooding worse? A: Channelisation and embankments speed water downstream and transfer the flood peak to other communities. They also encourage development on floodplains (increasing exposure), and defences can be overtopped catastrophically.

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Topic 21: Different approaches to managing water supply and their sustainability

Key ideas

• Hard engineering ("techno-fix") – large-scale supply-side solutions:
  • Water transfers: inter-basin pipelines and canals.
  • Mega-dams: storage and HEP.
  • Desalination: removing salt from seawater, mainly by reverse osmosis. It is energy-intensive and produces brine waste.
  • These schemes are expensive and have social and environmental costs, but they can produce large volumes.
• More sustainable schemes – demand-side approaches and restoration:
  • Water conservation: metering, leak reduction and efficient appliances.
  • Smart irrigation: drip irrigation and soil-moisture sensors.
  • Water recycling (grey water and treated wastewater).
  • Restoration of wetlands and catchments.
  • Rainwater harvesting.
• Integrated drainage basin management (IDBM) plans for the whole basin across sectors and borders.
• Treaties and frameworks reduce the risk of conflict (a P theme):
  • The UNECE Water Convention (Helsinki, 1992) became open to all UN members in 2016.
  • The EU Water Framework Directive (2000) requires river-basin management plans.
  • The Berlin Rules (2004) on water resources.

Case study: China's South–North Water Transfer Project (hard engineering)

• Purpose: to move water from the wetter Yangtze basin to the dry, industrial north (Beijing, Tianjin and the North China Plain, where groundwater is over-pumped).
• Design: three routes (eastern, central and western). The planned transfer is 44.8 billion m³ per year when complete. The eastern and central routes were operating by 2013–14; the western route has not yet been built.
• Cost and disadvantages:
  • Estimated cost of about US$62 billion (2008 estimate); some estimates are higher.
  • Raising the Danjiangkou Dam displaced about 345,000 people.
  • Pollution problems on the eastern route.
  • Ecological impacts downstream on the Han River.
  • Very high energy use for pumping.
• Advantages: reduces water stress in Beijing and northern cities and eases groundwater depletion.

Case study: Singapore (sustainable "Four National Taps")

• Singapore has scarce land and natural water, and has historically relied on imports from Malaysia (the Johor agreement runs to 2061).
• The Four National Taps:
  1. Local catchment water: about two-thirds of the land area is catchment, e.g. Marina Reservoir behind the Marina Barrage.
  2. Imported water from Malaysia.
  3. NEWater: high-grade recycled wastewater that can meet up to about 40% of current demand. PUB plans to expand both NEWater and desalination as demand grows.
  4. Desalination: can meet up to about 25% of current demand.
• Demand management: water pricing (including a water conservation tax), efficiency labelling and public campaigns.
• Evaluation:
  • A highly integrated, resilient system with diverse sources and less dependence on Malaysia.
  • However, NEWater and desalination are energy-intensive and costly. Singapore is a wealthy city-state, so the model is not easily transferable.

Case study: the Colorado River (IDBM and treaties)

• The 1922 Colorado River Compact allocated 7.5 million acre-feet a year each to the Upper and Lower Basins, plus 1 million acre-feet a year more to the Lower Basin (16.5 MAF in total, including Mexico's 1.5 MAF under a 1944 treaty).
• Over-allocation: the allocation was based on unusually wet years. Average natural flow is closer to 14–15 MAF, so the river is over-allocated.
• Effects:
  • Lake Mead and Lake Powell fell to record lows in 2021–22.
  • The first-ever federal shortage declaration was made in 2021, which cut Arizona's supply.
  • The river rarely reaches the Gulf of California.
• Responses:
  • Drought Contingency Plans (2019).
  • Negotiations on new operating rules after 2026.

Exam technique: the 20-mark "Evaluate" question

Example question: "Evaluate the view that hard-engineering schemes are the best way to manage water insecurity" (20 marks).

Introduction: define water insecurity and hard and soft approaches, and set out your criteria (effectiveness, cost, environmental and social sustainability).

Paragraph 1: Hard engineering works at scale. Use the SNWTP's 44.8 bn m³ per year and the desalination plants of Australia, Singapore and the Gulf.

Paragraph 2: But it has costs. Use the SNWTP's 345,000 people displaced and pollution, the energy and brine from desalination, and conflict over dams (GERD).

Paragraph 3: Sustainable alternatives. Use Singapore's NEWater and demand management, drip irrigation, and the Murray–Darling environmental flows.

Paragraph 4: Governance. Use the Colorado Compact over-allocation, IDBM and the UNECE convention. Show that institutions matter as much as technology.

Conclusion: judge, e.g. "Hard engineering is necessary for rapidly growing urban demand but is only sustainable when combined with demand management and fair governance; the 'best' approach depends on wealth, scale and context."

Glossary

Term	Definition
Water transfer	Moving water from a surplus basin to a deficit basin by canal or pipeline.
Desalination	Removing salt from seawater or brackish water (e.g. by reverse osmosis).
Techno-fix	A large technological, engineering solution to an environmental problem.
IDBM	Integrated Drainage Basin Management: whole-basin, multi-stakeholder planning.
NEWater	Singapore's brand of ultra-purified recycled wastewater.
Water Framework Directive	EU law (2000) requiring river-basin management to achieve good water status.

Self-test

Q1: Give two advantages and two disadvantages of desalination. A: Advantages: a reliable, climate-independent supply, and unlimited source water for coastal cities. Disadvantages: high energy use and cost (and therefore carbon emissions), and brine discharge that harms marine ecosystems.

Q2: What are Singapore's Four National Taps? A: Local catchment water, imported water (from Malaysia), NEWater (recycled water) and desalinated water.

Q3: Why is the Colorado River over-allocated? A: The 1922 Compact divided about 16.5 MAF a year (including Mexico's share) using unusually wet early-20th-century flows. The long-term natural flow is lower (about 14–15 MAF) and is falling with climate change.

Assessment checkpoint: 20-mark Water question (after half-term). Water complete by Easter.

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