Freshwater resources and water quality

Abstract

A number of estimates suggest that of all the water around the globe 94 per cent is salt water in the oceans and 6 per cent is fresh. Of the latter, about 27 per cent is in glaciers and 72 per cent is underground. Less than 1 per cent of the world’s fresh water is therefore to be found in the atmosphere, in rivers and streams, and in lakes (1). This freshwater supply is continually replenished by precipitation as rain or snow. It has been estimated that the total annual run-off from continents is about 41 000 cubic kilometres (km³). Of these, 27 000 km³ return to the sea as flood run-off, and another 5000 km³ flow into the sea in uninhabited areas. This leaves 9000 km³ of water readily available for human exploitation worldwide (2,3). As both the world’s population and usable water are unevenly distributed, the local availability of water varies widely. Much of the Middle East and North Africa, parts of Central America and the western United States are already short of water. By the year 2000, water will be scarce in many countries due to increasing demand from agriculture, industry and domestic use.