Global water resources: patterns, human water cycle, trends in consumption

Water Resources and Management

Learning Objectives

• Describe the global distribution of freshwater and explain the concept of water security (access, reliability, resilience).
• Identify the physical and human drivers that shape water supply and demand at regional and national scales.
• Explain the human water cycle as an open‑system (inputs, outputs, stores, transfers) and discuss groundwater sustainability.
• Analyse recent trends in water consumption and link them to the main drivers (population, economy, lifestyle).
• Evaluate management strategies (supply‑side, demand‑side, IWRM) and discuss their advantages, limitations and trade‑offs using a real‑world case study.

5.1 Global Water Resources

5.1.1 Distribution of Freshwater

Only a minute fraction of Earth’s total water is available for direct human use. The table below summarises the main categories.

Thus, less than 0.01 % of Earth’s water is readily available as surface water, highlighting the need for careful management of both surface and groundwater resources.

5.1.2 The Human Water Cycle – An Open‑System View

The human water cycle is a subset of the natural hydrological cycle, treated as an open system in which water moves between defined stores and transfers under the influence of external inputs and outputs.

• Stores: surface water (rivers, lakes, reservoirs), groundwater (aquifers), artificial storage (tanks, dams).
• Transfers:
  • Infiltration & recharge – precipitation → groundwater.
  • Abstraction – withdrawal from stores for domestic, agricultural, industrial, or environmental use.
  • Conveyance – canals, pipelines, irrigation networks.
  • Return flow – treated or untreated wastewater returning to surface water or re‑charging aquifers.
  • Evaporation & transpiration – from natural surfaces and from reservoirs, irrigation canals, cooling towers, etc.
• Inputs (I): precipitation, imported water (inter‑basin transfers, desalinated water), artificial recharge.
• Outputs (O): water extracted for use, evaporation, water exported to downstream users, permanent loss to the atmosphere.

The water‑balance equation used in the Cambridge syllabus is:

Groundwater‑specific considerations (with evaluation)

• Recharge vs. abstraction: In many arid and semi‑arid regions recharge rates (< 1 mm day⁻¹) are far lower than abstraction rates, causing falling water tables.
• Sustainable yield: The maximum abstraction rate that does not exceed long‑term recharge. Evaluation – While sustainable yield protects the aquifer, determining accurate recharge rates is difficult; over‑reliance on short‑term monitoring can lead to hidden depletion.
• Consequences of over‑extraction:
  • Land subsidence (e.g., Mexico City, the Gulf Coast of the USA).
  • Salt‑water intrusion in coastal aquifers (e.g., North Africa, parts of California).
  • Reduced base‑flow to rivers, affecting ecosystems.
  Evaluation – These impacts are often irreversible or expensive to remediate, underscoring the need for strict abstraction permits and managed aquifer recharge programmes.

5.1.3 Trends in Water Consumption – Causes and Effects

Global water use has risen sharply since the 1970s, driven by three inter‑linked drivers: population growth, economic development, and changing lifestyles.

Overall global per‑capita water use ≈ 1 200 m³ person⁻¹ yr⁻¹, but regional variation is large.

Cause‑and‑Effect Mini‑Chart

5.1.4 Water Security – Global Pattern

Water security is the ability of a population to obtain sufficient, safe water of acceptable quality for its needs, now and in the future. It comprises three inter‑related dimensions:

• Access – physical availability and affordability.
• Reliability – consistency of supply over time (e.g., seasonal variability, drought resilience).
• Resilience – capacity to recover from shocks such as extreme events or contamination.

Two principal forms of scarcity are recognised (UN‑WWDR 2023):

• Physical scarcity – renewable water resources are insufficient to meet demand (e.g., Middle East, parts of South‑West Asia).
• Economic scarcity – water is physically available but not accessible because of inadequate infrastructure, investment, or governance (e.g., large parts of Sub‑Saharan Africa).

Approximately 2 billion people live in water‑insecure regions, with the highest concentrations shown below.

5.2 Factors Influencing Supply and Demand

5.2.1 Physical Drivers of Supply

• Climate – precipitation amount and seasonality, temperature, evapotranspiration.
• Geology & hydrogeology – aquifer storage capacity, permeability, presence of confining layers.
• Topography & land cover – slope, vegetation, snow‑melt timing, which affect runoff and infiltration.

5.2.2 Human Drivers of Demand

• Population size and growth rate.
• Economic structure – proportion of agriculture, industry, services.
• Agricultural practices – irrigation methods, crop choices, livestock density.
• Urbanisation and lifestyle – higher per‑capita domestic use, water‑intensive appliances, cooling demand.
• Policy, pricing and governance – water tariffs, allocation rules, subsidies, water‑rights regimes.

5.2.3 Water Security in Context

The interaction between the physical supply drivers and human demand drivers determines a region’s water‑security status. For example, the Western Cape of South Africa experiences physical scarcity because of low rainfall, but the severity of the 2015‑2018 crisis was amplified by rapid population growth and limited investment in alternative supplies – an illustration of the combined physical + economic dimensions of water security.

5.3 Management of Water Resources

5.3.1 Integrated Water Resources Management (IWRM)

Co‑ordinated planning across sectors and stakeholder groups, based on basin‑wide governance.

• Pros: promotes equity, reduces conflicts, aligns with sustainability pillars.
• Cons: complex to implement; requires strong institutions and data sharing.

5.3.2 Demand‑Side Measures

• Water‑saving appliances & fixtures – low‑flow taps, efficient toilets.
  • Pros: relatively low cost, quick behavioural impact.
  • Cons: effectiveness depends on public uptake and maintenance.
• Tiered pricing & water‑use restrictions.
  • Pros: economic incentive to reduce waste; can be targeted during droughts.
  • Cons: may disproportionately affect low‑income households if not carefully designed.
• Public‑awareness campaigns (e.g., “Save Every Drop” in Cape Town).
  • Pros: fosters community ownership.
  • Cons: behavioural change can be slow and hard to quantify.

5.3.3 Supply‑Side Solutions

• Rainwater harvesting & small‑scale storage.
  • Pros: decentralised, reduces pressure on central reservoirs.
  • Cons: limited by rainfall variability; requires maintenance.
• Desalination (large‑scale).
  • Pros: provides a reliable, climate‑independent source.
  • Cons: high energy demand, brine disposal issues, expensive capital costs.
• Artificial recharge of aquifers.
  • Pros: enhances groundwater sustainability, can store excess flood water.
  • Cons: requires suitable hydrogeology; risk of contaminant transport.
• Improved storage (dams, reservoirs).
  • Pros: buffers seasonal variability.
  • Cons: ecological impacts (habitat loss, altered flow regimes) and high construction costs.

5.3.4 Pollution Control & Water‑Quality Management

• Stricter effluent standards and enforcement.
• Catchment protection (reforestation, buffer strips).
• Nutrient‑management plans for agriculture.
• Waste‑water recycling for irrigation or industrial use.

5.3.5 Groundwater Sustainability

• Monitoring extraction and establishing sustainable‑yield limits.
• Managed aquifer recharge (e.g., spreading basins, injection wells).
• Regulating abstraction permits and pricing groundwater use.
• Evaluation – These measures protect long‑term supply but require robust data collection, institutional capacity, and often face resistance from users reliant on free‑access groundwater.

5.3.6 Trade‑offs Between Supply‑Side and Demand‑Side Measures

Supply‑side options (e.g., desalination) can quickly augment water availability but are usually capital‑intensive and may have environmental side‑effects. Demand‑side measures are generally cheaper and promote long‑term conservation, yet they rely on behavioural change and may be less effective in the short term during acute droughts. Effective water‑resource management therefore combines both approaches, tailoring the mix to local economic, social and environmental contexts.

5.3.7 Case Study – Cape Town, South Africa (2015‑2018)

Key Take‑aways

• Only a tiny proportion of Earth’s water is directly usable; both surface and groundwater must be managed sustainably.
• Water security depends on physical availability **and** on the capacity of societies to deliver reliable, affordable water.
• The human water cycle provides a useful framework for quantifying inputs, outputs, stores and transfers, and for applying the water‑balance equation.
• Trends in consumption are driven by population, economic development and lifestyle changes; climate change adds a further layer of uncertainty.
• Effective management blends integrated planning (IWRM) with a balanced mix of supply‑side and demand‑side measures, while explicitly evaluating their benefits, limitations and trade‑offs.