Capture, management, use, disposal, reuse of water

The Human Water Cycle – Cambridge International AS & A Level Geography (9696)

1. Introduction

The human water cycle describes how societies capture, manage, use, dispose of and reuse water. It operates alongside the natural hydrological cycle and is shaped by technology, policy, economics and environmental constraints.

2. Global Water Resources

Understanding the size, distribution and trends of the world’s water resources provides the context for all later discussion.

2.1 Types of Water Resources (syllabus requirement 5.1)

• Rivers and streams
• Lakes and reservoirs
• Oceans (source for desalination)
• Underground water (ground‑water aquifers)
• Ice sheets and glaciers
• Precipitation (rainfall, snowfall)
• Recycled water (treated wastewater, storm‑water reuse)

2.2 Major Surface‑Water Basins (≈ 70 % of global runoff)

• Amazon (South America) – ~6 % of global runoff
• Nile (Africa) – ~1 % of global runoff
• Ganges‑Brahmaputra (South Asia) – ~2 % of global runoff
• Mississippi (North America) – ~2 % of global runoff
• Yangtze (East Asia) – ~2 % of global runoff
• Great Lakes (North America) – largest freshwater lake system by volume

2.3 Major Trans‑boundary Aquifers

Examples include the Guarani Aquifer (South America) and the Nubian Sandstone Aquifer (North Africa).

2.4 Spatial Variation

• Humid tropical regions – abundant surface water, high recharge rates.
• Arid & semi‑arid regions – low runoff, high reliance on groundwater and imported water.
• Cold regions – large stores as ice and permafrost; seasonal melt contributes to runoff.

2.5 Trends (syllabus requirement 5.1)

• Population growth and urbanisation increase per‑capita demand.
• Climate change alters precipitation patterns, intensifies droughts and floods.
• Land‑use change (deforestation, agriculture) affects infiltration and runoff.
• Renewable vs. non‑renewable water resources:
  • Renewable – water that is replenished on a human time‑scale (e.g., river flow, annual rainfall, sustainably pumped groundwater).
  • Non‑renewable – water stored over geological time‑scales (e.g., fossil aquifers, ancient glacial ice) that is depleted faster than it can be recharged.

3. The Human Water Cycle – Five Components

3.1 Capture of Water

• Surface‑water abstraction: dams, reservoirs, weirs, intake structures on rivers, lakes and canals.
• Ground‑water extraction: boreholes, wells, pumped aquifers.
• Rainwater harvesting: rooftop gutters, catch‑basins, underground tanks.
• Desalination: reverse‑osmosis or thermal processes turning seawater into potable water.

3.2 Management of Water Resources

1. Assessment of availability – hydro‑geological surveys, climate modelling.
2. Allocation – water licences, permits, priority‑use hierarchies.
3. Regulatory frameworks – national Water Acts, EU Water Framework Directive, UN‑WRM guidelines.
4. Monitoring & enforcement – flow gauging, water‑quality testing, remote sensing.
5. Future‑demand planning – population forecasts, industrial expansion, climate‑adaptation scenarios.

3.3 Use of Water

Sector	Typical % of total use	Key uses	Demand drivers
Agriculture	≈ 70 %	Irrigation, livestock watering, aquaculture	Population growth, dietary shift to water‑intensive foods, intensification, seasonal stress
Domestic	≈ 15 %	Drinking, cooking, sanitation, garden watering	Urbanisation, per‑capita consumption, living‑standard expectations, water‑saving appliances
Industrial	≈ 10 %	Cooling, processing, cleaning, product formulation	Industrial expansion, technology choice (water‑intensive vs. water‑efficient processes)
Other (mining, energy generation, fire‑fighting reserves)	≈ 5 %	Ore processing, hydro‑electric power, emergency reserves	Resource extraction rates, electricity demand, climate‑related fire risk

3.4 Disposal of Water

• Waste‑water treatment
  • Primary – sedimentation.
  • Secondary – biological oxidation (activated sludge, trickling filters).
  • Tertiary – nutrient removal, disinfection (chlorination, UV).
• Effluent discharge – release to rivers, seas or groundwater under regulatory limits.
• Solid‑waste (sludge) management – land application, drying, incineration, safe disposal.

3.5 Reuse and Recycling

• Non‑potable reuse – landscape irrigation, industrial cooling, toilet flushing.
• Potable reuse (direct or indirect) – advanced treatment (membrane filtration, UV, advanced oxidation) to meet drinking‑water standards.
• Closed‑loop systems – industries (e.g., semiconductor manufacturing) recycle water internally, minimising discharge.

4. Factors Influencing Water Resources (syllabus requirement 5.2)

4.1 Supply‑side Drivers

Driver	Effect on Water Availability	Typical Scale
Climate (precipitation, temperature, evapotranspiration)	Controls the amount of renewable water entering the system.	Global to regional
Catchment geology (permeability, aquifer storage)	Determines infiltration, groundwater recharge and storage capacity.	Local to basin
Land‑use change (deforestation, urbanisation, agriculture)	Alters runoff, infiltration and sediment loads.	Local to regional
Upstream abstraction (dams, diversions)	Reduces downstream flow and alters timing of water delivery.	River‑basin
Inter‑basin transfers (pipelines, canals)	Redistributes water from water‑rich to water‑poor areas.	Regional to national

4.2 Demand‑side Drivers

Driver	Effect on Water Demand	Typical Scale
Population growth & urbanisation	Increases total volume required for domestic and industrial uses.	Local to global
Economic development (industrialisation, service sector)	Raises industrial water use and per‑capita consumption.	National to regional
Agricultural intensification (high‑yield crops, livestock numbers)	Boosts irrigation and livestock‑watering demand.	Local to basin
Per‑capita consumption trends (lifestyle, appliance efficiency)	Either raises or lowers domestic water use.	Household to national
Seasonal & climatic variability (droughts, floods)	Creates short‑term spikes in demand for irrigation and emergency supplies.	Regional

5. Water Security & Water Stress (syllabus requirement 5.3)

Water security is the ability of a population to secure sufficient, safe water for livelihoods, health and ecosystem functions.

5.1 Types of Scarcity

• Physical water scarcity – renewable water resources are insufficient to meet demand (e.g., Cape Town “Day Zero” 2018).
• Economic water scarcity – adequate resources exist but lack of infrastructure, investment or governance prevents access (e.g., many sub‑Saharan African regions).

5.2 Water‑Security Indicators (syllabus language)

• Falkenmark (or “water‑stress”) indicator: per‑capita renewable freshwater availability. < 1 000 m³ person⁻¹ yr⁻¹ = water‑stress; < 500 m³ person⁻¹ yr⁻¹ = scarcity.
• Water Stress Index (WSI): ratio of total withdrawals to renewable availability (expressed as a %).
• Human Development Index (HDI) combined with water‑availability data – used in UN‑WRM reporting.

6. Management Strategies (syllabus requirement 5.4)

6.1 Supply‑side Strategies – Pros & Cons

• Dams and reservoirs
  • Pros: store seasonal runoff, provide hydro‑electric power, regulate flows.
  • Cons: displacement of communities, loss of biodiversity, sediment trapping, high capital cost.
• Inter‑basin transfers
  • Pros: rapid relief for water‑short regions, supports economic growth.
  • Cons: ecological impacts on donor basin, high energy use, political disputes.
• Desalination
  • Pros: virtually unlimited seawater source, reliable supply.
  • Cons: high energy consumption, brine disposal issues, expensive for low‑income users.
• Rainwater harvesting & artificial recharge
  • Pros: low cost, reduces pressure on centralized supply, improves groundwater levels.
  • Cons: limited by climate, requires storage infrastructure, variable yields.
• Groundwater management (sustainable yield limits, recharge schemes)
  • Pros: taps a large, often under‑used resource; can be locally controlled.
  • Cons: risk of over‑extraction, slow recharge, water‑quality degradation (e.g., salinisation).

6.2 Demand‑side Strategies – Pros & Cons

• Pricing reforms (volumetric tariffs, tiered rates)
  • Pros: economic incentive to conserve, generates revenue for infrastructure.
  • Cons: can be regressive, may be politically unpopular.
• Metering & leakage reduction
  • Pros: identifies losses, encourages efficient use, can cut network losses to < 10 %.
  • Cons: high installation/maintenance costs, requires skilled staff.
• Water‑saving technologies (low‑flow fixtures, drip irrigation, recirculating cooling)
  • Pros: measurable water savings, often payback within a few years.
  • Cons: upfront capital, need for farmer/industry training.
• Public awareness & education campaigns
  • Pros: low cost, builds long‑term behavioural change.
  • Cons: impact difficult to quantify, may need sustained effort.
• Regulatory restrictions (seasonal bans, allocation caps)
  • Pros: immediate reduction in use during crises.
  • Cons: can hurt livelihoods if not paired with alternatives.

6.3 Overall Management Approaches

• Integrated Water Resources Management (IWRM) – coordinated development of water, land and related resources to maximise economic and social welfare without compromising ecosystems.
• Stakeholder participation – governments, local communities, private sector, NGOs.
• Cross‑sectoral planning – linking agriculture, industry, urban supply and environmental flows.
• Adaptive management – iterative decision‑making based on monitoring data and climate forecasts.
• Transboundary water governance – basin‑wide institutions (e.g., Nile Basin Initiative, Mekong River Commission) negotiate allocation, data sharing and joint projects.

7. Detailed Case Study – Murray‑Darling Basin (Australia)

Background – The basin supplies water to ~ 12 % of Australia’s population and 70 % of its agricultural output. Over‑extraction, recurrent droughts and water‑quality decline prompted major reforms after 2000.

7.1 Management Actions (post‑2000)

1. Water‑resource plan (2012) – introduced a “cap‑and‑trade” water‑rights market, setting Sustainable Diversion Limits (SDLs) for each catchment.
2. Environmental water holdings – 2 % of total allocations reserved for ecosystem flows; released to improve river health.
3. Infrastructure upgrades – modernised irrigation (drip, sprinkler) and installed on‑farm water‑use monitoring.
4. Community engagement – water‑user groups participate in allocation decisions and monitoring.

7.2 Evaluation (Social | Economic | Environmental | Governance)

Dimension	Positive outcomes	Challenges / Limitations
Social	Improved river health benefits downstream Indigenous communities; water‑user groups have a voice.	Perceived inequity: large agribusinesses secure more rights than smallholders; some communities face reduced allocations.
Economic	More efficient irrigation lowered water use per hectare while maintaining yields; market‑based water rights encourage water‑saving investments.	Transition costs for new technology are high; some farmers experience income loss due to reduced allocations.
Environmental	Increased environmental flows have boosted native fish populations and reduced salinity in downstream wetlands.	Recovery is slow; severe droughts still limit flow volumes despite caps.
Governance	Transparent water‑rights market and stakeholder participation have reduced conflicts.	Complex administrative procedures and ongoing legal disputes over water‑right definitions.

8. Integrated Water Resources Management (IWRM) – Recap

• Holistic, cross‑sectoral approach.
• Key pillars: stakeholder participation, coordinated planning, adaptive management.
• Links to all five components of the human water cycle and to both supply‑ and demand‑side strategies.

9. Key Equation – Water Balance for the Human Water Cycle

Quantitative analysis often uses a modified water‑balance equation that incorporates human activities:

$$ \Delta S = P + I - E - D + R $$

• \(\Delta S\) – Change in stored water (reservoirs, aquifers).
• \(P\) – Direct precipitation captured (including rainwater harvesting).
• \(I\) – Imported water (inter‑basin transfers, bulk purchases).
• \(E\) – Evapotranspiration from open water bodies and irrigated land.
• \(D\) – Total water withdrawn for all uses (agriculture, domestic, industrial, other).
• \(R\) – Return flow after treatment (reused water, discharged effluent).

Simple Calculation Example

Consider a small catchment with the following annual data:

• Precipitation captured (P) = 150 M m³
• Imported water (I) = 20 M m³
• Evapotranspiration (E) = 80 M m³
• Withdrawals (D) = 100 M m³
• Return flow after treatment (R) = 30 M m³

Applying the equation:

$$ \Delta S = 150 + 20 - 80 - 100 + 30 = 20\;\text{M m³} $$

The catchment gains 20 million cubic metres of water storage in the year, indicating a net increase in reservoir or aquifer storage.

10. Suggested Diagram (text description)

Insert a flow diagram that shows the five components of the human water cycle – Capture → Management → Use → Disposal → Reuse – with arrows linking each stage. Overlay the diagram with a background map of major basins and trans‑boundary aquifers, and add a feedback loop labelled “IWRM & Adaptive Management” that connects back to the Management stage.