Much of this available water is located rather from human populations thus further complicating issues of water use.
505,000 km, or a layer 1.4 metres thick, evaporates from the oceans annually. Another 72 000 km. evaporates from the land. About 80 percent of all precipitation, or about 458 000 km/year, falls on the oceans and the remaining 119,000 km/year falls on land. The difference between precipitation on land surfaces and evaporation from those surfaces (119 000 km3minus 72 000 km3annually) is run-off and groundwater recharge - approximately 47 000 km. annually (Gleick 1993). More than one-half of all run-off occurs in Asia and South America, and a large fraction occurs in a single river, the Amazon, which carries more than 6,000 km of water a year (Shiklomanov 1999).
Water development projects during the 20th century have had significant impacts on freshwater ecosystems by eliminating marshes and wetlands, removing water for other uses, altering flows, and contaminating water with industrial and human wastes. In many rivers and lakes, ecosystem functions have been lost or impaired. In some areas, growing water demand has led to reductions in the volume of large rivers, affecting riverine and adjacent coastal areas (CSD 1997a).
was held in 1977 in Mar del Plata, Argentina. ...
was held in 1977 in Mar del Plata, Argentina. The focus on human needs led to the International Drinking Water Supply and Sanitation Decade(1981-90) and the efforts of the United Nations and other international organizations to provide basic water services (UN 2000). The concept of meeting basic water needs was reaffirmed during the 1992 Earth Summit in Rio de Janeiro and expanded to include ecological water needs. A United Nations report (UN 1999) recognized that all people require access to adequate amounts of safe water, for drinking, sanitation and hygiene. Indeed, the Second World Water Forum and Ministerial Conference in The Hague in 2000 produced a strong statement from more than 100 ministers in support of re-emphasizing basic human needs as a priority for nations, international organizations and donors.
Providing urban dwellers with safe water and sanitation services has remained a particular challenge. Some 170 million developing country urban dwellers were provided with safe water and 70 million with appropriate sanitation during the first half of the 1990s but this had limited impact because about 300 million more urban residents still lacked access to safe water supply, while nearly 600 million lacked adequate sanitation by the end of 1994 (CSD 1997b). However, a major area of success in many developing countries is related to investments in wastewater treatment over the past 30 years which have 'halted the decline in - or actually improved - the quality of surfacewater' (World Water Council 2000b).
Water quality problems can often be as severe as those of water availability but less attention has been paid to them, particularly in developing regions. Sources of pollution include untreated