Global pattern of water resources

Global Water Resources

Objective

To understand the global pattern of water resources, the physical and human factors that create spatial variation, how these patterns have changed over time, and how they are managed.

1. The Global Water Cycle

1.1 Natural water‑cycle

• Evaporation & transpiration (combined as evapotranspiration)
• Condensation → cloud formation
• Precipitation (rain, snow, hail)
• Runoff, infiltration and percolation
• Storage in ice & snow, groundwater, lakes, rivers and wetlands

1.2 Human water‑cycle (Cambridge 5.1.2)

1. Capture – abstraction for irrigation, domestic, industrial and power generation.
2. Use – water applied to crops, cooling, drinking, sanitation, etc.
3. Disposal – return flow to rivers, discharge to seas, or infiltration to aquifers.
4. Reuse – recycling of wastewater, rain‑water harvesting, reclaimed water for non‑potable uses.

Pros / Cons of each stage (with real‑world examples)

Stage	Advantages (Pros)	Disadvantages (Cons)	Illustrative case
Capture	Provides water where natural availability is low. Supports high‑value agriculture and industry.	Over‑abstraction can lower river flows and groundwater tables. Often energy‑intensive (e.g., pumped irrigation).	Israel’s drip‑irrigation system – high efficiency but requires sophisticated infrastructure.
Use	Directly meets food, energy and domestic needs. Can be targeted (e.g., precision irrigation).	High consumptive use reduces downstream availability. Water‑use conflicts arise in trans‑boundary basins.	California’s Central Valley – intensive agriculture consumes >70 % of state withdrawals.
Disposal	Returns water to the environment, maintaining flow regimes. Can recharge aquifers if managed well.	Poorly treated discharge pollutes rivers and coastal waters. Thermal and nutrient loading can alter ecosystems.	Industrial effluent discharge into the Ganges – severe water‑quality issues.
Reuse	Reduces pressure on fresh supplies. Supports circular‑economy approaches.	Public acceptance can be low for “recycled” water. Requires advanced treatment and monitoring.	Singapore’s NEWater – high‑grade reclaimed water used for industry and indirect potable reuse.

2. Global Distribution of Freshwater

Only 2.5 % of Earth’s water is fresh; the remaining 97.5 % is saline ocean water.

Freshwater Source	≈ % of Total Freshwater	Key Regions (examples)
Ice and snow (glaciers, ice caps)	68 %	Antarctica, Greenland, Himalayas, Andes, Rocky Mountains
Groundwater	30 %	All continents – Nubian Sandstone (Sahara), Great Plains (USA), Indo‑Gangetic Basin, Great Artesian Basin (Australia)
Surface water (lakes, rivers, wetlands)	2 %	Amazon Basin (low‑income, high rainfall), Congo Basin (low‑income), Great Lakes (high‑income, temperate), Scandinavian river systems (high‑income)

2.1 Water security – a global gradient

Water security is the capacity of a population to obtain sufficient, safe water for its needs while maintaining the ecological functions of water systems. Globally, security follows a clear income‑related gradient:

• High‑income, temperate regions (e.g., Europe, North America, Japan) – abundant renewable resources, well‑developed infrastructure; challenges are mainly seasonal droughts and ageing networks.
• Upper‑middle‑income, tropical/sub‑tropical regions (e.g., Brazil, South Africa) – plentiful water in some basins but uneven spatial distribution; water‑security gaps arise from seasonal variability and limited storage.
• Lower‑middle‑income, monsoonal regions (e.g., India, China, parts of Southeast Asia) – high total availability but strong intra‑annual swings, rapid population growth and inadequate infrastructure create mixed physical‑economic scarcity.
• Low‑income, arid or semi‑arid regions (e.g., sub‑Saharan Africa, Sahel, parts of the Middle East) – both physical scarcity (low renewable resources) and economic scarcity (poor infrastructure) combine to produce chronic water‑insecurity.

This gradient underpins the Cambridge requirement to distinguish between “water‑secure” and “water‑insecure” parts of the world.

2.2 Change over time (5.1.1)

• Glacier melt: global glacier volume has declined by ~27 % since the 1960s (World Glacier Monitoring Service, 2023).
• Groundwater depletion: annual draw‑down of global aquifers ≈ 30 km³ yr⁻¹ (UN‑FAO, 2022); most acute in India, China, the USA and the Middle East.
• Surface‑water trends: Amazon river discharge fell ~15 % over the past three decades, linked to reduced rainfall and upstream abstraction.

3. Latitudinal Pattern of Precipitation

1. Polar regions – < 200 mm yr⁻¹
2. Temperate zones – 400–800 mm yr⁻¹
3. Tropics – 800–4 000 mm yr⁻¹
4. Equatorial rainforests – > 2 000 mm yr⁻¹

Drivers (5.2 – supply factors)

• Solar heating & Hadley‑cell circulation (rising air at the equator, sinking air in the subtropics).
• Inter‑tropical Convergence Zone (ITCZ) – seasonal migration creates monsoons.
• Mid‑latitude westerlies – generate frontal rainfall in temperate zones.
• Orographic uplift – windward slopes receive heavy rain; leeward sides develop rain‑shadows.

4. Factors Influencing Water Resources (Supply & Demand)

4.1 Supply factors (physical)

• Climate – temperature, total rainfall and its seasonality.
• Topography – altitude, slope, mountain barriers.
• Geology – rock permeability, aquifer storage capacity.
• Land‑use – deforestation reduces transpiration; afforestation can increase interception and infiltration.

4.2 Demand factors (human)

• Population size and spatial distribution.
• Agricultural practices – irrigation intensity, crop water‑footprint.
• Industrial activities – cooling, processing, mineral extraction.
• Domestic consumption – per‑capita use, sanitation standards.
• Economic development – higher incomes raise per‑capita demand but often improve water‑use efficiency.

4.3 Scale matrix – how factors operate at different spatial levels

Scale	Supply factors (examples)	Demand factors (examples)
Local (watershed / catchment)	Soil permeability, land‑use change, micro‑climate	Household water use, small‑scale irrigation, local industry
Regional (country or river basin)	Regional climate patterns, mountain ranges, aquifer extent	National agricultural policies, urban‑area demand, inter‑city water transfers
Global (continents, world)	Planetary circulation (Hadley cell, jet streams), ice‑sheet volume	World population growth, global trade in water‑intensive commodities, trans‑boundary river treaties

5. Water Stress, Scarcity and Trends (5.1.3)

5.1 Key definitions

• Water Stress Index (WSI): \[ \text{WSI} = \frac{\text{Total annual water withdrawal}}{\text{Total renewable water resources}} \]
  WSI > 0.5 = high stress; 0.2–0.5 = moderate; < 0.2 = low.
• Physical water scarcity – renewable supply is insufficient to meet all legitimate demands (e.g., arid basins).
• Economic water scarcity – adequate water exists but lack of infrastructure, investment or governance prevents access (e.g., many parts of sub‑Saharan Africa).

5.2 Global trends (1990‑2020)

• Per‑capita water withdrawal rose from ~1 800 m³ person⁻¹ yr⁻¹ (1990) to ~2 200 m³ person⁻¹ yr⁻¹ (2020) – a 22 % increase.
• Renewable freshwater resources declined slightly (≈ 3 %) due to glacier loss and altered precipitation.
• Countries with WSI > 0.5 increased from 23 (1990) to 33 (2020).

5.3 Illustrative WSI table – contrasting economies (5.1.1)

Country (Income group)	WSI (2022)	Type of scarcity	Key drivers
Egypt (Upper‑middle)	0.69	Physical	Reliance on the Nile, arid climate, high irrigation demand
India (Lower‑middle)	0.43	Mixed – physical & economic	Population pressure, monsoon variability, uneven infrastructure
United Kingdom (High)	0.12	Economic	Well‑developed supply networks; seasonal droughts stress reservoirs
Brazil (Upper‑middle)	0.18	Physical (regional)	Contrast between wet Amazon and semi‑arid Northeast

5.4 WSI calculation example

Country X withdraws 200 billion m³ yr⁻¹ from renewable resources of 400 billion m³ yr⁻¹.

WSI = 200 / 400 = 0.50 → moderate to high water stress.

6. Human Modifications to the Water Cycle (5.1.2)

• Irrigation – diverts surface or groundwater to crops; raises evapotranspiration and reduces downstream flow.
• Dams & reservoirs – store water, alter seasonal flow, trap sediment, modify thermal regimes.
• Urbanisation – increases impervious surfaces → higher runoff, lower infiltration; creates urban heat‑island effects on local evaporation.
• Desalination – produces freshwater from seawater; high energy demand and brine discharge can affect marine ecosystems.
• Rain‑water harvesting – captures precipitation at source, reducing pressure on centralized supplies.
• Water recycling & reuse – treats wastewater for agricultural, industrial or indirect potable reuse, closing the human‑water loop.

7. Climate‑Change Impacts on Global Water Patterns

• More intense and frequent extreme precipitation in mid‑latitudes → higher flood risk.
• Lengthening of dry spells in subtropical and arid zones (Sahel, Southwest USA, Mediterranean) → greater drought stress.
• Accelerated melting of polar ice and mountain glaciers → short‑term rise in river discharge, long‑term loss of stored freshwater.
• Shifted river‑flow regimes – earlier spring peaks in snow‑fed basins, reduced summer flows.
• Sea‑level rise → saline intrusion into coastal aquifers, reducing usable groundwater.

8. Management of Water Resources (5.3)

8.1 Supply‑side strategies

• Construction of dams and reservoirs (e.g., Three Gorges Dam, China).
• Desalination plants (e.g., Saudi Arabia, Australia’s Perth plant).
• Rain‑water harvesting and small‑scale storage (e.g., rooftop systems in Kenya).
• Managed aquifer recharge (e.g., Israel’s recharge basins).

8.2 Demand‑side strategies

• Water pricing and tiered tariffs to encourage conservation.
• Regulatory water‑use caps for agriculture and industry.
• Technological improvements – drip irrigation, water‑efficient appliances.
• Public‑awareness campaigns and community‑based water‑saving programmes.

8.3 Integrated case study – Cape Town, South Africa (Day Zero, 2018)

Context: A combination of prolonged drought, growing population and limited storage capacity reduced reservoir levels to <10 % of capacity in early 2018.

Supply‑side response:

• Accelerated construction of new desalination capacity (≈ 100 ML day⁻¹).
• Emergency groundwater abstraction from peripheral aquifers.

Demand‑side response:

• Tiered water‑use restrictions – “Level 5” ban on non‑essential outdoor use (e.g., watering lawns, car washing).
• Public‑information campaign (“Save Water Now”) that achieved a 50 % reduction in per‑capita consumption within weeks.
• Incentives for installing water‑efficient fixtures and for rain‑water harvesting in households.

Evaluation:

• Successes – The demand‑reduction measures averted the projected “Day Zero” and demonstrated that rapid behavioural change is possible when clear risk communication is paired with enforceable limits.
• Limitations – Reliance on emergency desalination is costly and energy‑intensive; long‑term resilience requires diversified supply (e.g., expanded aquifer recharge) and improved governance of water‑rights.
• Lesson for the syllabus – Effective water‑resource management often combines supply augmentation with strong demand‑management policies, and the evaluation of outcomes must consider economic, social and environmental dimensions.

9. Assessment Questions

1. Explain how the Hadley cell creates the characteristic latitudinal pattern of rainfall.
2. Using the WSI formula, calculate the water stress for a country that withdraws 200 billion m³ of water per year from renewable resources of 400 billion m³.
3. Discuss two ways in which dam construction can alter the natural water cycle in a river basin.
4. Predict how a 2 °C rise in global temperature might affect water availability in the Mediterranean region, citing both supply and demand factors.