Water Scarcity and Sustainable Water Supply
Water scarcity is a shortage of fresh water relative to demand, either because supplies are physically limited or because people lack the infrastructure and money to access them; a sustainable water supply meets present needs without depleting sources or harming ecosystems, through supply management, conservation and reuse. In brief: a country with less than about 1,700 m3 of renewable water per person per year is said to face water stress, below 1,000 m3 water scarcity, and below 500 m3 absolute scarcity (the Falkenmark indicator). Scarcity can be physical, economic or caused by pollution. Solutions include reducing leaks and waste, efficient irrigation, wastewater reuse, rainwater harvesting, desalination and fair management of shared rivers.
Billions of people live with water shortage for at least part of the year, and demand continues to grow. This guide explains how water is supplied and used, why scarcity occurs, the main conservation and supply options, compares them in a table, works through numerical examples, outlines the history of water management and corrects common mistakes.
How is water supplied and used?
Water supply is the process of collecting, treating and delivering water to users. The sources are surface water (rivers, lakes, reservoirs), groundwater (wells and boreholes), rainwater, and increasingly desalinated seawater and reused wastewater. Water is treated (filtered, disinfected) and distributed by pipes. Globally, agriculture, mostly irrigation, uses about 70 percent of the fresh water withdrawn by people, industry about 20 percent and households about 10 percent (approximate values that vary by region and year). The amount used per person varies widely, from fewer than 20 liters per day in some poor regions to several hundred liters per day in rich countries. “Virtual water” is the water used to produce goods: producing food and goods uses more water than direct household use, and traded goods carry this water across borders.
Why does water scarcity occur?
- Physical scarcity: there is not enough water to meet demand, even with efficient use; typical of arid regions such as the Middle East and North Africa, northern China and parts of India and the southwestern United States.
- Economic scarcity: water exists, but there is a lack of investment in infrastructure, so people cannot get it; this is common in parts of sub-Saharan Africa.
- Pollution: contaminated sewage, industrial waste, fertilizers and salt make water unsafe to use and reduce the available supply.
- Rising demand: population growth, urbanization, irrigated agriculture and higher living standards.
- Over-extraction: pumping groundwater faster than it recharges (the Ogallala Aquifer, the North China Plain, northwest India) and diverting rivers (the Aral Sea).
- Climate change and variability: shifts in rainfall, shrinking glaciers and snowpacks, longer droughts and higher evaporation.
- Conflict and unequal access: shared rivers such as the Nile, the Indus and the Mekong raise questions of fair sharing between countries.
What are the solutions?
Demand management: repairing leaks (in some cities, a large share of the water is lost from pipes), metering and pricing water, efficient appliances and gardens, and more efficient irrigation (drip irrigation uses less water than flooding fields), growing crops that need less water. Supply increases: reservoirs and dams, groundwater management (including artificial recharge), rainwater harvesting, water transfers between basins, treated wastewater reuse and desalination (reverse osmosis uses roughly 3–4 kWh of energy per m3 and produces brine; it is widely used in the Gulf states, Israel and Australia). Institutions: integrated water resources management by river basin, international agreements, education and community management of local supplies. A sustainable approach needs a balance between these, as well as the water that the environment itself needs (environmental flows).
Options compared
| Option | Type | Advantage | Limitation |
|---|---|---|---|
| Leak repair | Conservation | Cheap saving of treated water | Needs investment in old pipes |
| Drip irrigation | Conservation | Less water used per crop | Equipment costs |
| Rainwater harvesting | Supply | Local, low cost | Depends on rainfall |
| Wastewater reuse | Supply | Reliable, local | Treatment and public acceptance |
| Desalination | Supply | Unlimited seawater | High energy, brine disposal |
| Dams and transfers | Supply | Stores water for dry seasons | Environmental and social costs |
Worked example: stress, leaks and desalination
- Per person water (hypothetical country): renewable water resources of 40 km3 per year (40 × 109 m3) and a population of 30 million give 40 × 109 ÷ 30 × 106 = 1,333 m3 per person per year, which is below 1,700, so the country is water-stressed.
- Population growth: when the population grows to 45 million, the figure becomes 40 × 109 ÷ 45 × 106 = 889 m3 per person, below 1,000, which is water scarcity, although the water supply has not changed.
- Household use: 150 liters a day per person is 150 × 365 = 54,750 liters = 54.8 m3 a year. That is only 54.8 ÷ 889 = 6 percent of the 889 m3 per person above, so the largest share of the water goes to agriculture and industry.
- Leak repair: a city supplies 500 million liters per day (500 ML/d) and loses 20 percent through leaks, which is 100 ML/d. Fixing half of the leaks saves 50 ML/d, enough for 50,000,000 ÷ 150 = 333,333 people at 150 liters a day.
- Desalination energy: a plant produces 100,000 m3 a day using 3.5 kWh/m3, and so uses 100,000 × 3.5 = 350,000 kWh a day, or 350,000 × 365 ÷ 1,000 = 127,750 MWh a year. The water is produced for 350,000 ÷ 100,000 = 3.5 kWh per 1,000 liters.
How has water management developed?
Ancient societies built irrigation canals (Mesopotamia and Egypt), qanats (underground channels for groundwater in Persia), cisterns and aqueducts (Rome). In the 19th century, piped water supplies and sewage treatment reduced cholera and typhoid; John Snow’s work on the Broad Street pump in London in 1854 showed the link between contaminated water and disease. The 20th century saw giant dams and irrigation schemes, along with the “Green Revolution”. In 2010 the United Nations General Assembly recognized the human right to water and sanitation, and the Sustainable Development Goal 6 (2015) aims at universal access to clean water by 2030.
Which mistakes are common?
Mistake 1: Thinking scarcity is only a problem of deserts
It also occurs in wet regions because of pollution, poor infrastructure or demand that exceeds supply.
Mistake 2: Considering only household water use
Agriculture and industry use most of the water, so saving there is very important.
Mistake 3: Treating desalination as a simple solution
It needs a lot of energy and creates brine, and it only serves coastal areas economically.
Mistake 4: Confusing water stress with water scarcity
Stress is below about 1,700 m3 per person per year; scarcity is below about 1,000 m3, and absolute scarcity below about 500 m3.