Physical Resources / 91half is frozen in the polar icecaps and glaciers and about 0.5 per cent is so far below ground as to be
beyond our reach. Atmospheric water vapour, falling rain and snow, and flowing rivers contain no
more than about 0.005 per cent of the planet’s water (KUPCHELLA AND HYLAND, 1986, pp.
222–223). Stated like this, the amount available to us sounds alarmingly small, but it is so only as a
proportion of the total. The quantity available to us, including that in lakes and inland seas, is in the
region of 15×10^18 litres.
Water can exist as either gas or liquid at temperatures commonly encountered near the surface and
consequently it is constantly evaporating and condensing again. Each year, some 336×10^15 litres
evaporates from the oceans and 64×10^15 litres from the land surface (including water transpired by
plants). About 300×10^15 litres falls as precipitation over the oceans and 100×10^15 litres over land, and
36×10^15 litres flows from the land back to the sea (HARVEY, 1976, p. 22). This movement of water
between oceans, air, and land constitutes the hydrologic cycle, and by dividing the quantity of water
at each stage of the cycle by the amount entering or leaving, it is possible to discover approximately
the average time a water molecule remains in each: its residence time. This reveals that a molecule
spends about 4000 years in the ocean, 400 years on or close to the land surface, and 10 days as
vapour in the atmosphere.
Most of the water falling on land evaporates again almost immediately or is taken up by plant roots
and returned to the atmosphere by transpiration. Some flows directly over the surface, down slopes
and into lower ground where it may enter lakes, rivers, or marshes. What remains drains downward
through the soil until it encounters a layer of impermeable clay or rock, then flows laterally, very
slowly, through the soil. Were it not to flow, but simply accumulate, the ground would soon be
waterlogged and water would lie at the surface. Above the impermeable material a layer of soil is
saturated with water. This is ground water and its upper limit, above which the soil is not saturated,
is the water table. Permeable material through which ground water flows is called an ‘aquifer’ and it
may lie deep beneath the surface. Aquifers are permeable because the particles of which they are
composed, such as gravel or sand, are not packed together so tightly as to leave no spaces between
them. They are said to be ‘unconsolidated’ and allow water to flow through them. Other aquifers are
made from material, such as chalk or sandstone, which are consolidated (solid) but nevertheless have
fissures, or pore spaces within their granular structure, through which water can flow.
It is obviously most convenient to obtain our supplies of fresh water from the nearest river or
lake, but this may be too distant or insufficient. In that case it may be possible to obtain water
from an aquifer, by sinking a borehole into it and pumping out the water. Figure 3.1 illustrates
this and also shows what happens: abstraction lowers the water table around the borehole,Figure 3.1 Water abstraction