Natural Water Cycle
Hydrology is the science concerned with the distribution of water
on the Earth, its physical and chemical reactions with other
naturally occurring substances, and its relation to life on Earth;
the continuous movement of water between the Earth and the
atmosphere is known as the hydrological cycle. Under several
influences, of which heat is predominant, water is evaporated from
both water and land surfaces and is transpired from living cells.
This vapour circulates through the atmosphere and is precipitated
in the form of rain or snow. On striking the
surface of the Earth, the water follows two paths.
In
amounts determined by the intensity of the rain and the porosity,
permeability, thickness, and previous moisture content of the
soil, one part of the water, termed surface run-off, flows
directly into rills and streams and thence into oceans or
landlocked bodies of water; the remainder infiltrates into the
soil. A part of the infiltrated water becomes soil moisture, which
may be evaporated directly or may move upwards through the roots
of vegetation to be transpired from leaves. The portion of the
water that overcomes the forces of cohesion and adhesion in the
soil profile percolates downwards, accumulating in the so-called
zone of saturation to form the groundwater reservoir, the surface
of which is known as the water table. Under natural conditions,
the water table rises intermittently in response to replenishment,
or recharge, and then declines as a result of continuous drainage
into natural outlets such as springs.