It is a renewable resource on which we all completely depend upon. 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, most of this occurs as floods. The low river flows (base flows) make up only about 19,000 km3.
Of this, about 12,500 km3 can be accessed, and present levels of withdrawal accounts for 4000km3. This withdrawal is expected to reach 5000 Km3 per year by the 2025. The demand for freshwater increased six-fold between 1900 and 1995 nearly twice the rate of population growth. One third of the world's population today already live in countries experiencing medium to high water stress. Water Stress Water stress for a river basin is defined as the water resources available in that basin. The water stress for a country is the summation of water stress for all its river basins.
Water stress begins when the withdrawals of water of freshwater rises above 10 percent of renewable resources. Medium to high stress translates as water use that exceeds 20 percent of available water supply. Countries experience high water stress when the ratio of water use to supply exceeds 40 percent. At such levels, their patterns of use may not be sustainable, and water scarcity is likely to become the limiting factor to economic growth.
High water stress and unsustainable rates of withdrawal are already being experienced in Central and South Asia, where annual water ithdrawals compared with available water resources are 50 percent or more. 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. The global population will expand from today's 6 billion people to almost 8 billion in 2025. By then, more than 80 percent of the world's population will be living in developing countries.
The World Meteorological Organization estimates, assuming the renewable water resources will remain unchanged, that the number of countries facing water stress will increase from 29 today to 34 in 2025. How these countries manage their water resources, and whether they can produce sufficient food for their growing populations while catering to their water needs and preserving natural environments, have important implications. Nearly 70 percent of global freshwater withdrawals are directed toward agriculture, mainly for irrigation.
By some estimates (UN 1997), annual irrigation water use will have to increase about 30 percent above present use for annual crop production to double and meet global food requirements by 2025. The industry sector, which accounts for about 22 percent of current freshwater withdrawals globally, is likely to require an increasing share in all regions of the world. In developing countries, where 56 percent of the population will be living in urban areas by 2025, the share of water going toward domestic uses will also need to grow substantially. Asia and Water
Asia has the lowest per capita availability of freshwater resources among the world's continents. The contrasts within the region are stark. Annual freshwater resources (in m3 per capita) reach as high as 200,000 in Papua New Guinea and as low as 2,000 in parts of South Asia and the PRC, and are generally below 20,000 in Southeast Asia. The region's weather is largely governed by a monsoon climate, which creates large seasonal variations in addition to spatial variation.
The two most populous nations in the world, the PRC and India, will have 1. 5 billion and 1. billion people, respectively, by 2025, by which time the availability of freshwater will have dropped to 1,500 m3 per capita in India and 1,800 m3 in the PRC. Many of countries depend heavily on groundwater exploitation to supplement scarce surface water resources. In Bangladesh, groundwater abstraction already represents 35 percent of total annual water withdrawals; in India, 32 percent; in Pakistan, 30 percent; and in PRC, 11 percent. Groundwater overuse and aquifer depletion are becoming serious problems in the intensively farmed areas of northern PRC, India, and Pakistan.
In heavily populated cities land is subsiding as groundwater is withdrawn to serve the needs of their growing urban populations, and saltwater intrusion is rendering much of the groundwater unusable. War for Water International conflicts over water are becoming more frequent as competition for available freshwater resources increases. There are 215 international rivers as well as about 300 groundwater basins and aquifers that are shared by several countries. The 1996 treaty signed by Bangladesh and India for managing flows in the Ganges-Brahmaputra system represents a major breakthrough for rational approaches to shared water resources.
However, more than 70 water-related flash points have been identified, mainly in Africa, Middle East, and Latin America. Eight countries in Asia (Bangladesh, Cambodia, Kazakhstan, Pakistan, Tajikistan, Thailand, Uzbekistan, and Viet Nam) rely on international rivers to supply more than 30 percent of their annual water resources. Four of these (Bangladesh, Cambodia, Uzbekistan, and Viet Nam) rely on water from external sources for more than 65 percent of their annual water resources. Making better use of Asia's shared rivers is an unfinished agenda with potentially large benefits to millions of poor people in the region.
However, formulating agreements between sub-regions to enable equitable sharing of resources and better control of trans-boundary pollution has proven to be highly controversial and, in some cases, strongly divisive The reliability of water supplies in the face of such dependence is a key issue when seasonal variations, particularly droughts etc enter the equation. Unsustainable rates of groundwater extraction can only make matters worse. The impact of global climate change, which cannot be determined at this time, will be to increase the overall uncertainty within which water planners operate.
Floods and droughts Floods and droughts have always been features of life on earth and have produced some of the worst natural disasters in recorded history. Due to inappropriate land use and land management practices, uncoordinated and rapid growth of urban areas, and loss of natural flood storage wetlands, floods are becoming more frequent. Flooding is the hazard that affects more people than any other associated damage to property and is escalating. Destruction of forest cover has altered the hydrologic cycle and reduced water retention in forest soils.
Accompanying soil erosion has permanently stripped fertile topsoil from vast areas, leading to further degradation of river basins and threatening the basis for sustainable natural resource management. Global climate change will have unpredictable but potentially devastating consequences for the hydrologic cycle by changing the total amount of precipitation, its annual and seasonal distribution, the onset of snowmelt, the frequency and severity of floods and droughts, and the reliability of existing water supply reservoirs.
According to the Intergovernmental Panel on Climate Change, the frequency of droughts could rise by 50 percent in certain parts of the world by 2050. Water Pollution Emerging Asia, published by ADB in 1997, identified water pollution as the most serious environmental problem facing the region. Water pollution exacerbates the problem of water scarcity at local and regional levels by reducing the amount of water available for productive purposes.
Water pollution comes from many sources, including untreated sewage, chemical discharges, spillage of toxic materials, harmful products leached from land disposal sites, agricultural chemicals, salt from irrigation schemes, and atmospheric pollutants dissolved in rainwater. The direct disposal of domestic and industrial wastewater into watercourses is the major source of pollutants in developing countries. In Asia and the Pacific, faecal pollution is one of the most serious problems, affecting both surface water and groundwater bodies and leading to a tenacious persistence of such waterborne diseases as cholera, typhoid, and hepatitis.
Estimates of the increase in water pollution loads in high growth areas of Asia over the next decades are as high as 16 times for suspended solids, 17 times for total dissolved solids, and 18 times for biological pollution loading. The combined volume of water used and water needed to dilute and flush pollutants is almost equal to the volume of accessible freshwater in the world's river systems. The development of freshwater resources for human uses has compromised natural ecosystems that depend on these resources for their continued integrity.
Freshwater ecosystems, comprising lakes, rivers, and wetlands, have already lost a greater proportion of species and habitat than land or ocean ecosystems. Unrestricted development of surface water and groundwater has altered the hydrologic cycle and threatens the natural functions of deltas and wetlands. Wetlands have been converted to cropland, and rivers that channelled water to estuaries and deltas have dried up. Diminished productive potential, loss of vegetation, increased health risks, and irreversible desecration of aquatic biota are the sad legacy.
Water Management Traditionally seen as limitless bounty, water has only recently been recognized as a scarce resource, and only since the 1950s have policymakers begun to espouse the economic and environmental values of water. A consensus is growing among scientists, water planners, governments, and civil society that new policies and approaches will have to be adopted within the next two decades to avoid calamity, and that supply, use, and management of water resources will have to be integrated across sectors and between regions sharing the same source.
New projects for dams, water storage, irrigation, drainage, flood protection, and water supply will continue to be needed in many countries where the basic water requirements for people have not yet been met. Lack of effective water policies and institutional arrangements is a pressing issue. Sustainability criteria will predominate in decision making and particular emphasis will be given to environmental and social values.