Factors influencing water resources: supply, demand, security

Water Resources and Management (Cambridge 9696 – Topic 5)

Learning objective

Analyse the factors that influence water resources – supply, demand and security – and evaluate the effectiveness of management approaches, using quantitative evidence and real‑world case studies.

5.1 Global water resources

5.1.1 Distribution of water on Earth

• Total water: ≈ 1 470 million km³
• Fresh water: ≈ 2.5 % of total (≈ 35 million km³)
• ‑ Renewable (precipitation, river flow, groundwater recharge): ≈ 13 000 km³ yr⁻¹
• ‑ Non‑renewable (fossil groundwater, glacier ice): ≈ 2 000 km³ yr⁻¹
• Geographical pattern: 70 % of renewable fresh water occurs in high‑latitude regions (e.g., Canada, Russia); only 10 % is in arid & semi‑arid zones where demand is highest.

5.1.2 The human water cycle

1. Capture – abstraction from rivers, lakes, reservoirs, aquifers or direct precipitation (rain‑water harvesting).
2. Use – domestic, industrial, agricultural and environmental (ecological‑flow) purposes.
3. Disposal – wastewater, effluent or return flow to the environment.
4. Reuse / recycling – treatment and re‑use (e.g., reclaimed water, desalinated water, Singapore’s NEWater).

5.1.3 Recent trends (1990‑2020)

Year	Total withdrawals (km³ yr⁻¹)	Per‑capita use (L day⁻¹)	Share of renewable supply (%)
1990	3 850	≈ 900	≈ 70 %
2000	4 300	≈ 950	≈ 68 %
2010	4 650	≈ 1 000	≈ 66 %
2020	5 000	≈ 1 050	≈ 64 %

Withdrawals are rising faster than the replenishment of renewable resources, increasing the risk of water‑stress and water‑scarcity.

5.1.4 Water‑stress vs. water‑scarcity

• Water‑stress: annual water use exceeds 25 % of renewable supply (e.g., many parts of India, Mexico).
• Water‑scarcity: annual water use exceeds 40 % of renewable supply, often leading to severe social and environmental impacts (e.g., parts of the Middle East).

Link to the trend table: when the “Share of renewable supply” column falls below 75 % the region moves toward water‑stress; below 60 % it approaches water‑scarcity.

5.1.5 Spatial mismatch (visual aid)

World‑map (see Cambridge textbook, Fig. 5.1) shows the concentration of renewable fresh water in the north‑west (Canada, Russia, Scandinavia) versus the concentration of population and agriculture in the south‑east (South Asia, Sub‑Saharan Africa). This geographic mismatch is a key concept in the syllabus.

5.2 Factors influencing water resources

5.2.1 Supply – physical and human drivers

• Climatic variables – amount and seasonality of precipitation, temperature, evapotranspiration.
• Hydrological characteristics – basin size, river discharge, groundwater recharge rates, natural storage (lakes, wetlands).
• Geology & soils – permeability, aquifer extent, soil texture influencing infiltration.
• Land‑use change – deforestation reduces infiltration; urbanisation increases runoff; intensive agriculture can both increase recharge (through irrigation return flow) and reduce it (soil sealing).
• Human interventions – dams & reservoirs, inter‑basin transfers, large‑scale abstraction, artificial recharge.

Contrasting examples

• Low‑income country (Ethiopia) – supply limited by highly seasonal rainfall, low groundwater productivity and extensive deforestation, leading to high runoff losses.
• High‑income country (Germany) – abundant precipitation, well‑developed storage (reservoirs, pumped‑storage), and extensive groundwater monitoring that stabilise supply despite industrial demand.

5.2.2 Demand – physical and human drivers

• Population size & distribution – higher density → higher domestic demand.
• Economic development – industrialisation and service sectors raise per‑capita water use (≈ 150 L day⁻¹ in high‑income vs ≈ 50 L day⁻¹ in low‑income economies).
• Agricultural practices – irrigation intensity, crop water‑footprint, livestock numbers (≈ 70 % of global withdrawals).
• Technological efficiency – drip irrigation, water‑saving appliances, wastewater recycling.
• Cultural & lifestyle factors – water‑intensive habits (e.g., car washing, garden irrigation).
• Policy & pricing – subsidies or volumetric tariffs directly affect consumption patterns.

Contrasting examples

• Low‑income country (Bangladesh) – rapid population growth, rice‑dominant agriculture with flood‑irradiated paddies, limited industrial demand, and low per‑capita domestic use (~70 L day⁻¹).
• High‑income country (Australia) – relatively low population density but high per‑capita domestic use (~250 L day⁻¹), water‑intensive horticulture in arid zones, and strong pricing policies that encourage efficiency.

5.2.3 Water security

Water security is the ability of a population to obtain sufficient, safe water of acceptable quality at an affordable price. It is assessed through three inter‑linked dimensions:

1. Availability – physical presence of water resources.
2. Accessibility – infrastructure, distribution networks and economic means.
3. Quality – suitability for drinking, agriculture and industry without health risk.

Key threats: over‑exploitation of aquifers, climate‑change‑induced droughts, pollution, and trans‑boundary conflicts.

5.2.4 Climate‑change impacts on supply & demand

• Shifted precipitation patterns → more intense floods and prolonged droughts.
• Rising temperatures increase evapotranspiration, reducing runoff and raising irrigation demand.
• Sea‑level rise contaminates coastal aquifers with saltwater intrusion.
• Increased frequency of extreme weather events stresses infrastructure and water‑treatment capacity.

5.3 Management of water resources

5.3.1 Supply‑side strategies

• Infrastructure development – dams, reservoirs, inter‑basin transfer schemes.
• Rainwater harvesting – rooftop or catch‑ment collection, especially in arid regions.
• Artificial groundwater recharge – infiltration basins, injection wells.
• Desalination – thermal or reverse‑osmosis plants; high energy cost but vital for coastal, water‑scarce nations.
• Water‑recycling & reuse – treatment of municipal/industrial effluent for irrigation or industrial processes.

5.3.2 Demand‑side strategies

• Economic instruments – volumetric pricing, water‑use taxes, subsidies for efficient technology.
• Regulatory measures – abstraction licences, minimum environmental‑flow standards.
• Technology & practice – drip irrigation, low‑flow fixtures, leak‑detection programmes.
• Public awareness & education – campaigns promoting water‑saving habits.
• Reuse & grey‑water systems – household or industrial recycling for non‑potable uses.

5.3.3 Overall management approaches

• Integrated Water Resources Management (IWRM) – coordinated development of water, land and related resources to maximise economic and social welfare without compromising ecosystem sustainability.
• Regulatory frameworks – national water acts, licensing regimes, pollution‑control standards.
• Trans‑boundary water governance – river‑basin organisations and treaties (e.g., Nile Basin Initiative, Mekong River Commission) that allocate water, resolve conflicts and promote joint monitoring.
• Risk‑management tools – drought early‑warning systems, flood‑plain zoning, climate‑resilient infrastructure.

5.3.4 Evaluation criteria for management options

Criterion	What to assess	Typical indicators
Cost‑effectiveness	Capital & operating costs vs. water saved/produced	$/m³ of water supplied, pay‑back period
Environmental impact	Effects on ecosystems, carbon footprint, water quality	Change in river‑flow regime, GHG emissions, biodiversity index
Social equity	Distribution of benefits & costs among different groups	Access for low‑income households, displacement, gender impacts
Reliability & resilience	Performance under climate variability or extreme events	Supply continuity during drought, flood‑protection rating
Institutional feasibility	Governance capacity, legal framework, stakeholder acceptance	Number of licences issued, compliance rates, public support

5.3.5 Case study – Singapore’s NEWater

• Context: Small, highly urbanised city‑state; local catchments supply ≈ 2 % of water needs.
• Challenges: Rapid population and economic growth, limited land for reservoirs, vulnerability to regional droughts.
• Strategies employed:
  1. Construction of the Marina Reservoir and historic imports from Malaysia.
  2. High‑technology water‑reclamation plants producing NEWater (advanced membrane filtration + reverse‑osmosis).
  3. Public‑education programme encouraging water‑saving behaviour.
  4. Dynamic pricing – higher tariffs for high consumption.
  5. Integrated management via the Public Utilities Board (PUB) under an IWRM framework.
• Quantitative outcomes (2023):
  • NEWater contributes ≈ 40 % of total water demand (≈ 55 % of industrial use, 30 % of domestic use).
  • Per‑capita consumption fell from 165 L day⁻¹ (1990) to 140 L day⁻¹ (2022).
  • Cost of NEWater ≈ US$0.50 per m³ – comparable to imported water when long‑term security benefits are accounted for.
• Evaluation (using the criteria above):
  • Cost‑effectiveness: high upfront capital but low operating cost; long‑term savings on import fees.
  • Environmental impact: minimal – reduces reliance on upstream river extraction and protects ecosystems.
  • Social equity: uniform pricing; strong public acceptance.
  • Reliability & resilience: supplies water even during regional droughts.
  • Institutional feasibility: strong centralised governance (PUB) and clear legal framework.
• Lesson for other regions: Advanced treatment and reuse can substantially augment supply where natural resources are scarce, provided there is strong institutional capacity and public buy‑in.

5.4 Quantitative summary tables

Water use by sector (global averages, 2020)

Sector	Percentage of total withdrawals	Typical per‑capita use (L day⁻¹)
Agriculture	70 %	≈ 300
Industry	20 %	≈ 150
Domestic	10 %	≈ 100

Per‑capita water use by income group (2020)

Income group	Per‑capita use (L day⁻¹)	Typical drivers
Low‑income	≈ 50‑80	Limited infrastructure, low‑intensity industry
Middle‑income	≈ 120‑180	Growing domestic demand, expanding industry
High‑income	≈ 250‑350	High domestic consumption, water‑intensive services

5.5 Water‑balance equation (catchment scale)

The water‑balance framework links supply and demand within a basin:

$$\Delta S = P - ET - Q$$

• \(\Delta S\) – change in storage (soil moisture, groundwater, reservoirs)
• \(P\) – precipitation (input)
• \(ET\) – evapotranspiration (output)
• \(Q\) – runoff (surface & subsurface flow out of the basin)

Quantifying each term helps planners identify where deficits arise and which management actions (e.g., increasing storage, reducing ET through shading, enhancing infiltration) will be most effective.