Assessment of global water resources based on several different calculation methods have produced varied estimates:
Glaciers and icecaps cover about 10% of the world's landmass. These are concentrated in Greenland and Antarctica and contain ~70% of the world's freshwater. According to the United States Geological Survey (USGS), 96% of the world's frozen freshwater is at the South and North poles, with the remaining 4% spread over 550 000 sq.km of glaciers and mountainous icecaps measuring about 180 000 cub.km (UNEP).
Groundwater is by far the most abundant and readily available source of freshwater, followed by lakes, reservoirs, rivers and wetlands. About 1.5 billion people depend upon groundwater for their drinking water supply (WRI, UNEP, UNDP, World Bank). The amount of groundwater withdrawn annually is roughly estimated at ~600-700 cub.km, representing about 20% of global water withdrawals (WMO). Comprehensive picture of the quantity of groundwater withdrawn and consumed annually around the world does not exist.
Most freshwater lakes are located at high altitudes, with nearly 50% of the world's lakes in Canada alone. Many lakes, especially those in arid regions, become salty through evaporation, which concentrates the inflowing salts.
Reservoirs are artificial lakes, produced by constructing physical barriers across flowing rivers, which allow the water to pool and be used for various purposes. The volume of water stored in reservoirs worldwide is estimated at 4 286 cub.km.
Wetlands include swamps, bogs, marshes, mires, lagoons and floodplains. The total global area of wetlands is estimated at ~2 900 000 sq.km (Groombridge and Jenkins, 1998). Most wetlands range in depth from 0 to 2 meters. Estimating the average depth of permanent wetlands at about one meter, the global volume of wetlands could range between 2 300 cub.km and 2 900 cub.km.
Rivers form a hydrological mosaic, with an estimated 263 international river basins covering 45.3% (~231 059 898 sq.km) of the land surface area of the Earth, excluding Antarctica (UNEP, Oregon State University et al.). The total volume of water in the world's rivers is estimated at 2 115 cub.km.
Although freshwater ecosystems such as rivers, lakes and wetlands occupy less than 2% of the Earth's total land surface, they provide a wide range of habitats for a significant proportion of the world's plant and animal species. Although many are yet to be discovered, the number of freshwater species worldwide is estimated at between 9 000 and 25 000. This number is rapidly decreasing due to human interference. Physical alteration, habitat degradation, excessive water withdrawal and pollution have contributed directly or indirectly to the decline in freshwater species. Other factors that reduce freshwater biodiversity include the incursion of non-native species and the mismanagement of inland fisheries. Today, an estimated 20% of the world's freshwater fish are vulnerable, endangered or extinct.
Freshwater use by continents is partly based on several socio-economic development factors, including population, physiography, and climatic characteristics.
The agricultural sector is by far the biggest user of freshwater. In Africa and Asia, an estimated 85-90% of all the freshwater used is for agriculture. According to estimates for the year 2000, agriculture accounted for 67% of the world's total freshwater withdrawal, and 86% of its consumption (UNESCO). By the year 2000, an estimated 15% of the world's cultivated lands were irrigated for food crops, accounting for almost half of the value of global crop production.
In the industrial sector, the biggest share of freshwater is stored in reservoirs and dams for electrical power generation and irrigation. However, the volume of water evaporated from reservoirs is estimated to exceed the combined freshwater needs of industry and domestic consumption. This greatly contributes to water losses around the world, especially in the hot tropical regions (UNESCO). Industrial uses account for about 20% of global freshwater withdrawals. Of this, 57-69% is used for hydropower and nuclear power generation, 30-40% for industrial processes, and 0.5-3% for thermal power generation (Shiklomanov).
Domestic water use is related to the quantity of water available to populations in cities and towns. People in developed countries on average consume about 10 times more water daily than those in developing countries. It is estimated that the average person in developed countries uses 500-800 litres per day (300 cub.m per year), compared to 60-150 litres per day (20 cub.m per year) in developing countries (UNESCO). In large cities with a centralized water supply and an efficient canalization system, domestic consumption does not usually represent more than 5-10% of the total water withdrawal (intake).
It is estimated that two out of every three people will live in water-stressed areas by the year 2025. Today, 450 million people in 29 countries suffer from water shortages. Clean water supplies and sanitation remain major problems in many parts of the world, with 20% of the global population lacking access to safe drinking water. Water-borne diseases from faecal pollution of surface waters continue to be a major cause of illness in developing countries. Polluted water is estimated to affect the health of 1.2 billion people.
Over the past decade, efforts based on Agenda 21's freshwater management guidelines in Chapter 18, which address the protection of the quality and supply of freshwater and the application of integrated approaches for the development, management and use of water resources, have focused on the following areas:
Quotation from the UNEP’s “Vital Water
Graphics. An Overview of the State of
Last update 03/03/2003