Like all other natural resources, global fresh water resources, also, have their own limitations. There is a final limit up to which mankind can draw water available in various deposits on earth’s crust, without damaging the natural resource base or without causing any adverse changes in the environment around. What is this limit? Up to what extent will, the withdrawal of fresh water by humanity be ecologically sustainable?
We have huge deposits of fresh water on earth’s surface as well as in its subsurface layers. Water in these deposits is in a state of perpetual movement from one compartment to another. However, inputs in each compartment are balanced by an equal output, so that a state of dynamic equilibrium is maintained. If withdrawal from any of these exceeds the input, the pool size diminishes.
Total annual precipitation on land surface has been estimated to be 1, 13,500 cubic kms and loss via evapo-transpiration about 72,500 cubic kms. Therefore, there is a net annual gain of about 41,000 cubic kms on land surface, which trickles out, drains down and flows back to the sea. This is the extra amount which can be safely used by mankind without causing any detrimental effect on ecology or environment because its use shall not disturb the existing deposits on earth’s surface.
Any over-draft beyond this quantity either from surface deposits or ground water shall diminish the natural resource base which in turn could bring about adverse changes in environment and ecology. Greenery disappears, the flora and fauna undergo a drastic change and desertification follows.
Of this 41,000 cubic kms of surplus water about 27,000 cubic kms consist of flood flow which rush down to sea, too quickly to be of any use to mankind. Of the remaining 14,000 cubic kms about 5,000 cubic kms happen to be placed in inhabitable or very sparsely populated area and are of little use to the mankind. It is only about 9,000 cubic kms which can serve the mankind.
The ill effects of withdrawal of more water than the total annual input may be drastic. In United States, six states namely Colorado, Kansas, Nebraska, New Mexico and Oklahoma, relied heavily on the underground fresh-water aquifer called Ogallala for the supply of fresh water. Its depletion due to huge over-drafts in these states caused the total agricultural area to decline by more than 15% (Slogett and Dickason, 1986).
In Tamil Nadu, India, heavy pumping has dropped the water table by 25 metres or so (Sandra Postel, 1985). In Beijing-Tienjin area of China, in Phoenix and Tucson, Arizona, a combination of agricultural and urban needs has been lowering the ground water table by several metres a year (Brown, 1987). In absence of rains it is the water table below, which sustains natural plant growth and keeps earth’s surface moist and humid. With receding water table agriculture suffered and the process of gradual desertification was started.