Water resources

More than 99% of Earth’s water in its natural state is unavailable or unsuitable for human use. Thus, the amount of freshwater for which all the people, ani­mals and plants on earth compete is even less than 1%.

This water which is available as fresh water comes from inland surface water sources (such as riv­ers, streams, lakes & reservoirs) and ground water sources.

Surface Water Sources:

Most surface water originates directly from precipitation in the form of rainfall or snow. Ground water from springs and seeps also contribute to flow of most of the streams.

The various surface water sources along with their characteristics are described as under:

(a) Sea Water:

Though the oceans contain about 97% of the total water in the world, but as ocean waters contain high concentration of salts (approx. 3.5%) in solution, it becomes uneconomical to make this water potable. Still in places, where sea water is the only source available potable water is ob­tained from sea water by carrying out desalting or demineralizing.

(b) Rivers and Streams:

Precipitation that does not evaporate or infiltrate runs off the surface towards the sea, in the form of streams and rivers. Rivers and streams are important sources of water supply even though the water from these sources is generally more variable in quality as well as less satisfactory than the water from lakes and impounded reservoirs.

(c) Natural Lakes and Ponds:

Lakes are inland depressions that hold stand­ing fresh water throughout the year. Ponds are generally small, temporary or permanent shallow water bodies. As their water is much more accessible than groundwater or glaciers, they are considered as an important (though minor) source of fresh water supply. Water from these sources is more uni­form in quality than water from flowing rivers and streams.

(d) Artificial Impounding Reservoirs:

These are formed by constructing hydraulic structures (like dams) across river valleys. The deeper and nar­rower the valley is, the easier it is to construct the dam. The water quality is similar to that of natural lakes and ponds.

Inland surface water is the major source of fresh water for agricultural, domes­tic and industrial use throughout the world. The major environmental issue regarding inland surface water sources is the degradation of these sources by the disposal of sewage and industrial effluents without treatment.

Ground Water Sources:

After glaciers, ice caps and snowfields, ground water is the next largest fresh water reservoir. Precipitation that does not evaporate back into the air or run off over the surface percolates through the soil and either accumulates in an underground basin or flows underground in sub-surface streams.

The quality of ground water is generally uniform. As a result, ground water is the major source of fresh water for agricultural and domestic use in many areas of the world; particularly areas having insufficient surface water resources.

Over use of ground water sources can cause several kinds of problems if groundwater is being with­drawn from aquifers faster than natural recharge can replace it, such as:

(a) Excessive pumping of ground water causes porous formations to collapse, resulting in subsidence or settling of the above surface.

(b) Heavy pumping can lower the local water table as a result of which shal­lower wells go dry or can deplete a whole aquifer.

(c) Overuse of freshwater reservoirs along coastlines often allows salt water to intrude into aquifers used for domestic and agricultural purposes.