Readings of the WRM Course

RDV Core Training Program FY98 Activity 2.2

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Comprehensive Assessment of the Freshwater Resources of the World --- an UN document, 1997

This is the latest "awareness" document, which is essentially a status report on the world water resources situation. It was prepared for the UN Committee for Sustainable Development by the Stockholm Environment Institute with the assistance of the UN ACC Sub-Committee for Freshwater and was published in February 1997. The Water Resources Management course uses this important UN document as a required reading material.


27. A growing number of regions face increasing water stresses because more people are both polluting and demanding more water for all uses from a renewable but finite resource. They are thus suffering from scarcities caused by failure to adapt to the amount of water that is regularly made available by rain and snowfall.

28. Concern over the global implications of water problems was voiced as far back as the United Nations Conference on the Human Environment in Stockholm in 1972. It has been the focus of a number of meetings, including the United Nations Water Conference in Mar del Plata, Argentina, in 1977, the Global Consultation on Safe Water and Sanitation for the 1990s in New Delhi, India, 1990, the International Conference on Water and the Environment: Development Issues for the 21st Century, in Dublin, Ireland, and the United Nations Conference on Environment and Development in Rio de Janeiro, Brazil, both in 1992. Since then, the Interministerial Conference on Drinking Water Supply and Environmental Sanitation, in Noordwijk, The Netherlands, in 1994, reinforced these concerns. Most recently, the Committee on Natural Resources of the Economic and Social Council þnoted with alarm that some 80 countries, comprising 40 per cent of the worldþs population, are already suffering from serious water shortages and that, in many cases, the scarcity of water resources has become the limiting factor to economic and social developmentþ. It further noted that þever- increasing water pollution has become a major problem throughout the world, including coastal zonesþ. The UN Commission on Sustainable Development, at its second session in 1994, noted that in many countries a rapid deterioration of water quality, serious water shortages and reduced availability of fresh water were severely affecting human health, ecosystems and economic development.

29. The Commission requested this Comprehensive Assessment of the Freshwater Resources of the World, to be submitted at its fifth session, and to the Special Session of the General Assembly in 1997. This assessment was prepared by a number of UN organizations, the Department of Policy Coordination and Sustainable Development, Department of Development Support and Management Services, Food and Agriculture Organization, United Nations Development Programme, United Nations Environment Programme, UNESCO, UNIDO, World Bank, World Health Organization and World Meteorological Organization, working in collaboration with the Stockholm Environment Institute, and with the advice of experts on a wide range of subjects. The support given to this project by the Governments of Sweden, Norway, Denmark, the Netherlands and Canada is acknowledged with sincere appreciation.

30. The recommendations in this report were guided by reports from previous conferences, particularly the report of the Dublin water conference and Chapter 18 of Agenda 21. More recent information has also been evaluated, particularly on water availability and use.

31. This assessment provides an overview of major water quantity and quality problems with the aim of helping people understand the urgent need to deal with these issues before they become even more serious. In spite of its limitations, the available information provides the basis for a broad understanding of the problems facing various regions of the world, and of the nature and magnitude of the global implications of not dealing with these problems.

Figure 1. Water plays many complex roles in human activities and natural systems. A comprehensive approach must thus relate to water use from many different aspects. The assessment describes the human interaction within the economic, social and environmental framework. It seeks to point out how the systems are interacting through different global linkages such as cultural influences, environmental impacts, global governance and trade, showing that the socio-ecological system is complex with connections within and between the different subsystems.


32. Fresh water is one of the most essential elements that supports human life and economic growth and development. It is irreplaceable for drinking, hygiene, food production, fisheries, industry, hydro power generation, navigation, recreation and many other activities. Water is equally critical for the healthy functioning of nature, upon which human society is built.

A. Water availability

Figure 2. Naturally dry zones of the world mean that there are limitations to the pattern of development that may be available on the basis of water resources availability, particularly for agriculture.

33. Many people have an image of the world as a blue planet, for 70 per cent of it is covered with water. The reality is that 97.5 per cent of all water on earth is salt water, leaving only 2.5 per cent as fresh water. Nearly 70 per cent of that fresh water is frozen in the icecaps of Antarctica and Greenland, and most of the remainder is present as soil moisture, or lies in deep underground aquifers as groundwater not accessible to human use. As a result, less than one per cent of the world's fresh water, or about 0.007 per cent of all water on earth, is readily accessible for direct human uses. This is the water found in lakes, rivers, reservoirs and those underground sources that are shallow enough to be tapped at an affordable cost. Only this amount is regularly renewed by rain and snowfall, and is therefore available on a sustainable basis.

34. Much of the approximately 110,000 cubic kilometres of precipitation that falls on the continents each year evaporates back into the atmosphere, or is absorbed by plants. About 42,700 cubic kilometres of water that falls on earth flows through the world's rivers. (This is roughly the amount of water now stored in some of the world's largest lake systems combined: Lake Baikal in Russia, and Lake Tanganyika and Lake Victoria in Africa combined.) When the world's total river flow is divided by the world population of 1995, that amounts to an average of 7,300 cubic metres of water per person each year. Due to the growing world population, that is a drop of 37 per cent per person since 1970.

Figure 3. Average Annual Runoff. The amount of freshwater varies sharply among continents. The size of the population determines how much water is potentially available per person. While Asia has the world's greatest river flow, it has billions of people, so the per capita availability is the lowest of all the continents. The high per capita runoff in Australia/Oceania shows that despite the fact much of Australia is very dry, the population density is quite low, and there is very heavy rainfall in parts of the country, and on the Pacific islands.

35. Fresh water resources are very unevenly distributed, ranging from the deserts, where almost no rain falls, to the most humid regions, which can receive several metres of rainfall a year. Most of the flow is in a limited number of rivers: the Amazon carries 16 per cent of global runoff, while the Congo-Zaire River basin carries one-third of the river flow in all of Africa. The arid and semi-arid zones of the world, which constitute 40 per cent of the land mass, have only 2 per cent of global runoff.

36. Even in parts of the world with large river flows, there can be a great amount of variability in when and where the water is available. Most of the annual water flow may come as floods following snow-melt or heavy rains, and unless captured by reservoirs, it flows to the seas, sometimes causing seasonal flooding. Later in the year, the same areas may suffer droughts. Another major factor in the availability of water is the rate of evapotranspiration, the loss of water from land to atmosphere by evaporation from the soil and water surfaces, and transpiration from plants. For example, Sweden and Botswana receive about the same amount of precipitation each year, yet the climate in Sweden is humid, while that of Botswana is semiarid, because so much of its water is drawn up by the heat of the sun. One more important factor is that much of the world's accessible runoff occurs in areas far from human settlements, and water is very expensive to transport over long distances.

Figure 4. Present water withdrawal and consumption by sector.

37. Experts have estimated the amount of the fresh water that is readily accessible for human use at about 9,000 cubic kilometres a year. They add another 3,500 cubic kilometres of water that is captured and stored by dams and reservoirs. Harnessing the remaining water resource for human needs becomes increasingly costly, because of topography, distance and environmental impacts. Currently, humans are using about half the 12,500 cubic kilometres of water that is readily available. Given an expected population increase of about 50% in the next 50 years, coupled with expected increases in demand as a result of economic growth and life-style changes, this does not leave a great room for increased consumption. Water needs to be left in rivers to maintain healthy ecosystems, including fisheries. Recreation, navigation, and hydro power generation all require the preservation of adequate amount of water. When the global water picture is examined at a country level, some countries still have large amounts of water per capita, but others, however, are already facing serious difficulties. Future increases in demand due to population growth and increased economic activities will inevitably impinge further on the available water resources.

B. Water uses

38. A number of human actions are changing the flow of water in parts of the world, including the building of dams and canals, the drainage of wetlands, and removal of forests and other plant cover. Trees and other plants modify the flow of water that falls on the land, consume water, and release some into the atmosphere, where it may result in more rain.

Figure 5. In the hydrological cycle, the sun constantly evaporates water into the atmosphere, part of which is returned on land as rain and snow. Part of that precipitation is rapidly evaporated back into the atmosphere. Some drains into lakes and rivers to commence a journey back to the sea. Part infiltrates into the soil to become soil moisture or groundwater. Under natural conditions, the groundwater gradually works its way back into surface waters and makes up the main source of dependable river flow. Plants incorporate some of the soil moisture and groundwater into their tissues, and release some into the atmosphere in the process of transpiration.

39. Humans interact with the hydrological cycle at many levels. We use surface water and groundwater. Pollution contaminates not only water on and beneath the ground, but it also changes the chemical composition of water in the atmosphere. Waste discharges from a wide range of sources, including motor vehicles, homes, offices and industries, as well as chemicals and animal wastes from agricultural production, create contaminated runoff, some of which seeps into groundwater. Changes in the landscape affect the run-off and quality of both surface water and groundwater.

40. Part of the water withdrawn from rivers, steams, lakes, reservoirs or groundwater is consumed and not later returned as available water. Such consumption includes water that plants use to build plant tissue or release into the air during evapotranspiration. It also includes water that evaporates from land or reservoirs, and water that is not returned to water sources from industrial production or community use. The major forms of water withdrawal and consumption are for agriculture, industry and domestic use. Most of the water withdrawn by industries and municipalities is used then returned to lakes and rivers or other watercourses, often degraded in quality. Water withdrawn for irrigation use is partly consumed in the process of crop production, and partly required to flush salts out of the soil. However, most irrigation is inefficient and about 60 percent of the withdrawn water returns to the river basin and to groundwater.

41. In addition to domestic and municipal water supply, irrigation of crops, production of energy, and industrial uses. Navigation and recreation also require that adequate flows be available in rivers, and that water levels be maintained in lakes and reservoirs. Reserving water for the healthy functioning of ecosystems is sometimes seen as far less important than providing all the water that humans demand. In recent years, however, an understanding has emerged on the need to maintain ecosystem health not just for ethical reasons, but because of the very practical benefits, sometimes called ecosystem services, that is provided to humans. These include the production of food, reduction of flood risk and filtering of harmful pollutants. Great strides have been made in understanding the freshwater needs of aquatic ecosystems. These needs are being accepted as legitimate calls on water, leading an increasing number of decision makers to give these "environmental" flows priority along with water use for economic activities.

C. Water scarcity

Water scarcity occurs when the amount of water withdrawn from lakes, rivers or groundwater is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, bringing about increased competition among potential demands. Scarcities are likely to happen sooner in regions where the per capita availability of water is low to start with and with high population growth. They become more serious if demand per capita is growing due to changes in consumption pattern.

42. Global withdrawals of water to satisfy demands have grown dramatically in this century. Between 1900 and 1995, water withdrawals increased by over six times, more than double the rate of population growth. This rapid growth in water demand is due to the increasing reliance on irrigation to achieve food security, the growth of industrial uses, and the increasing use per capita for domestic purposes.

Figure 6. Global water withdrawals by sector, 1940-2000.

43. The increased demands are causing water stress in many areas of the world, even in some humid areas where rising demand or pollution have caused over utilization of the local resource. Already, about 460 million people, more than 8 per cent of the world's population, live in countries using so much of their water resources that they can be considered to be highly water stressed. A further one-quarter of the world's population lives in countries where the use of water is so high that they are likely to move into situations of serious water stress.

D. Human induced stresses

1. Quantity

44. Irrigated agriculture takes about 70 per cent of water withdrawals, and the figure rises to 90 per cent in the dry tropics. Agriculture is by far the biggest consumptive use of water, representing 87 per cent of the total. Traditionally, most food has been grown on rainfed lands, relying on soil moisture supplied by rainfall, but as food demand rises, this is increasingly supplemented by irrigation, using water drawn from lakes, rivers and underground aquifers. Irrigated agriculture contributes nearly 40 per cent of world food production from just 17 per cent of cultivated land. Much of the dramatic increase in food production of recent decades, including the Green Revolution, requires high-yield plant varieties, combined with fertilizers and pest control, and depends on irrigation to ensure adequate and timely water for high growth. Water withdrawals for irrigation have increased by over 60 per cent since 1960.

45. Until the late 1970s, the growth in the amount of land being irrigated exceeded the rate of population growth. Since then, the amount of irrigated land has increased more slowly than population, due to a limited amount of additional land suitable for irrigation, increasing water scarcities and the loss of some irrigated areas to soil degradation including soil salinization. However, total agricultural output has continued to outstrip population growth, owing to productivity increases. Currently, the world can produce enough food for everyone, but an estimated 840 million people are lack access to sufficient food for their nourishment, and are hampered in carrying on productive, working lives because they cannot afford to buy enough food. As the number of people to feed increases, it will be ever more of a challenge to produce enough food at prices people can afford. In many regions, in particular arid and semi-arid regions, the amount of water available for irrigation will become increasingly limited and costly.

Figure 7. Amount of irrigated land in the world, and water consumption for irrigation. Dark-coloured bars depict the amount of water consumption while light-coloured bars show the amount of land that is irrigated.

2. Impacts of demand for water

46. In some areas, the withdrawals are so high that the flow of rivers decreases as they move downstream, and some lakes are shrinking.

47. Groundwater supplies one-third of the world's population, and is the main or only source of water for rural dwellers in many parts of the world and is also increasingly the main source for irrigation. Underground sources are being heavily overused in a number of regions, with water being pumped out faster than nature can replenish the supply. The excessive use of groundwater is likely to increase over the next 30 years. Over pumping groundwater has dropped water levels by tens of metres in places, making it increasingly difficult and expensive for people to have continued access to the water. In a number of regions, depletion has forced people to turn to lower quality groundwater sources, some of which contain natural contaminants. The overuse of groundwater can have a serious effect on the base flow of rivers, especially during dry periods, so vital for aquatic ecosystems.

48. Many groundwater aquifers are recharged on a regular basis by rain and melting snows. However, some groundwater reservoirs that were filled under different climatic conditions, often thousands of years ago, are known as fossil aquifers, and if used, they will not be recharged by nature for a very long time, if ever.

49. In some cases, groundwater depletion results in the land above aquifers sinking. Land subsidence caused by high water withdrawals has been recorded in many countries, including Mexico, the United States, Japan, China and Thailand, with the land sinking from 1 to 10 metres.

50. Over utilization of aquifers near seacoasts leads to intrusion from the ocean, which contaminates the fresh water with salt. Small islands fall in a special category because for many of them fresh water is a fragile resource. If the fresh water is overdrawn, this leads to salt water intrusion. People on some small islands have been forced to turn to expensive alternatives, including desalination and importing water by tanker.

3. Water pollution issues

51. For millennia, people have used water as a convenient sink into which to dump wastes. The pollution comes from many sources, including untreated sewage, chemical discharges, petroleum leaks and spills, dumping in old mines and pits, and agricultural chemicals that are washed off or seep downward from farm fields. In one area after another, the amounts and types of waste discharged have outstripped nature's ability to break them down into less harmful elements. Pollution spoils large quantities of water which then cannot be used, or at best can be used for restricted purposes only.

52. The impairment of water quality near major urban centres is recognized as a major problem. In parts of the world, water quality has been so degraded that it is unfit even for industrial purposes. Even when the levels of some pollutants seem to be low, they can pose a threat by accumulating in the aquatic food chain, affecting the health of these creatures, and threatening the health of humans who eat contaminated wildlife. Groundwaters, once contaminated, are very difficult to clean up because the rate of flow is usually slow.

Click for a complete copy of the UN document  Comprehensive assessment of the freshwater resources of the world

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