1.3 - Water demand, quality and supply

Demand, surplus and shortage

People use water for drinking, washing and sanitation (human hygiene), crop irrigation and livestock (agriculture), manufacturing and cooling (industry), and activities such as boating, fishing and swimming (leisure). These uses can compete. Water retained for a habitat or recreation may also be wanted for irrigation or urban supply.

Population growth increases the number of users. Higher incomes can increase appliance use, meat consumption and demand for manufactured goods, raising both direct and indirect water demand. Hot, dry weather can increase irrigation and garden demand while reducing river flows. Demand therefore depends on lifestyles, efficiency, prices and production as well as population.

A water surplus exists where usable supply exceeds demand over a stated period; water shortage occurs where demand exceeds available supply. Compare the same area and timescale. A basin may have an annual surplus but a dry-season shortage. Abundant floodwater is not automatically a safe supply: polluted water needs treatment, and water arriving in a brief peak needs storage if it is to be used later.

This simulated two-season balance uses million cubic metres over each season, not a daily flow rate:

The annual surplus of 20 conceals a dry-season shortage of 20. Transferring at least 20 from the wet season could close that simplified gap, provided storage, treatment and distribution work and losses are allowed for. The table isolates timing; it is not measured London or Dhaka demand.

Why water quality varies

Water quality depends on what enters a river, how it is treated and how much flow is available to dilute or disperse it. Untreated sewage can introduce disease-causing organisms and organic matter. Microorganisms decomposing that matter consume oxygen, harming aquatic organisms. A river can look clear and still contain pathogens, so appearance alone is an unreliable safety test.

Industrial waste can contain toxic chemicals, metals, dyes or heated water. Effects depend on the substance and concentration; not every factory produces the same pollution. In agriculture, fertiliser nutrients can be washed into rivers. Excess nutrients encourage algal growth; when the algae die, decomposition can reduce dissolved oxygen. Pesticides, animal waste and soil erosion create further pressures.

Low flows may concentrate pollution, while heavy rain can wash pollutants from land or overload drainage and sewer systems. Upstream discharge can affect downstream users across administrative boundaries. Preventing a pollutant entering the river can be more effective than expecting every downstream household to remove it.

Storage, treatment and distribution

A dam is a barrier; a reservoir is the water store behind a dam or within an engineered basin. Storage can move wet-season water into a dry season and buffer short interruptions. Costs include inundated land, displaced people, evaporation and altered sediment or downstream flow. A reservoir's stored water still needs protection and, for drinking, appropriate treatment.

Pipelines transfer raw or treated water between sources, works and users. They connect a surplus to demand but require construction, energy where pumping is needed, and maintenance to limit leakage. A transfer can reduce the donor basin's flow, so its environmental and social costs must be assessed as well as the receiving city's benefits.

At water-treatment works, processes remove suspended material and address microbial or chemical risks. Screening, settling, filtration and disinfection have different functions: disinfection does not remove every dissolved chemical, while a filter does not necessarily kill pathogens. Sewage-treatment works treat wastewater before discharge; they are not interchangeable with drinking-water works.

Supply investment can be combined with leak repair, efficient irrigation, water-saving appliances and demand management. A scheme is more sustainable when it meets human needs while maintaining ecological flows and remaining affordable and maintainable.

The Thames: managing a large urban demand

The UK Thames region is the developed-country case. The specification classifies very-high-HDI countries as developed; UNDP's 2025 report places the UK in that category using 2023 data. This category provides context, not proof that every household or river is secure.

London concentrates domestic, commercial and leisure demand. Thames Water's water-resources planning documents describe abstraction from the Thames and Lee into reservoirs, followed by treatment and distribution; the Thames Water Ring Main helps move treated water across London. This combination uses storage to reduce the mismatch between source availability and urban demand.

The Environment Agency's London abstraction licensing strategy, updated in May 2025, uses flow-related restrictions to protect watercourses. Some new abstraction opportunities are mainly in wetter winter conditions, making storage important. Taking more water during drought could damage habitats and reduce dilution, so demand reduction and leakage control can complement new infrastructure.

On a map, locate the Thames, Lee, reservoir groups, treatment sites and the urban demand area as separate layers. A pipeline's route shows a connection; it does not prove unlimited source capacity or safe water quality. Evaluate supply reliability alongside river health, costs and the distribution of benefits.

Bangladesh: abundant rivers and unequal access

Bangladesh, downstream in the Ganges–Brahmaputra–Meghna system, is the emerging-country contrast under Pearson's HDI definition: its 2023 HDI was in the medium category in UNDP's 2025 report. Seasonal monsoon flows, agricultural demand and dense urban populations create a different management setting from London. River abundance does not remove the need for clean-water infrastructure.

Dhaka's water supply has historically relied heavily on groundwater, alongside surface-water treatment such as Saidabad. Abstraction can lower groundwater levels, while polluted rivers make switching to surface supply more difficult and costly. Sewage collection, industrial treatment, protected intakes and reliable distribution must therefore be managed together.

In February 2026, the World Bank approved a US$370 million Metro Dhaka programme to improve sanitation, waste management, pollution monitoring and river restoration. It identified rapid urban and industrial growth outpacing wastewater services. Approval describes a financed response and intended benefits; it does not demonstrate that all those benefits have already occurred.

The contrast is not “rich country has no problem, poorer country has no solution”. Both cases need storage, treatment, regulation and demand management. Dhaka faces especially large gaps in wastewater services and rapid growth; London has extensive networks but still faces dry-weather pressure and infrastructure needs. Judge a response using measured quality, reliable access, affordability and effects downstream.