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Water in the 21st Century

Emerging Global Water Issues
The Looming Water Crisis

Earth is the blue planet with water one of the most plentiful natural substances in its environment. There is more than 1.4 billion cubic kilometers (km3) of the stuff-enough to give every man, woman, and child more than 230 million cubic meters (m3) each if we were to divide it evenly.

However, more than 98 percent of the world's water is salt water and we depend for our basic vital needs on freshwater. Most freshwater is locked in the polar ice caps. Less than 1 percent of the earth's freshwater is accessible in lakes, rivers, and groundwater aquifers. That vital 1 percent of available freshwater is constantly in motion, either flowing in rivers, evaporating and moving around the globe as water vapor, falling from the sky as rain or snow, or filtering slowly through the earth to emerge somewhere else. It is a renewable resource on which we all completely depend. It is the genesis and continuing source of all life on earth.

The most accessible water is that which flows in river channels or is stored in freshwater lakes and reservoirs. The major portion of the water diverted for human needs is taken from this renewable, readily accessible part of the world's freshwater resources. Although the total volume of water conveyed annually by the world's rivers is about 43,000 km3 (see figure), most of this occurs as floods-the low river flows (base flows) make up only about 19,000 km3.1 Of this, about 12,500 km3 can be accessed, and present levels of withdrawal account for about 4,000 km3. Withdrawals are anticipated to reach 5,500 km3 per year by 2025.

The demand for freshwater increased sixfold between 1900 and 1995, twice the rate of population growth.2 The 1997 United Nations (UN) Comprehensive Assessment of Freshwater Resources of the World concluded that one third of the world's population today already live in countries experiencing medium to high water stress. High water stress and unsustainable rates of withdrawal are already being experienced in Central and South Asia, where annual water withdrawals compared with available water resources are 50 percent or more. The northern People's Republic of China (PRC) and Mongolia have medium stress conditions with 25 percent water use. Although water stress (see box on page 8) computes at less than 10 percent in Southeast Asia (including southern PRC) and the Pacific and is therefore considered to be low, this measure is highly distorted by seasonally high river flows. In the dry season, water scarcity occurs throughout Asia and the Pacific, and increased rainfall variability as a result of global climate change will worsen this problem. Water scarcity will affect food security throughout Asia and the Pacific.

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