2
Hydrologic cycle
Earth's water resource is constant, though it can change forms i.e. from water
to ice and to moisture. Oceans contain saline water and is not usable by human
excepting in areas like cooling etc. Fresh water from the ocean surface is
evaporated consuming a huge quantity of solar energy leaving the salt in the
ocean. Evaporation is thus a very important physical process which balances
the hydrologic cycle. Transpiration is a process which allows water to be lost in
the atmosphere by absorbing solar energy. The combined process is known as
evapotranspiration. Precipitation that occurs on the land surface is very
important for living organism, parts of which is lost by evapotranspiration, while
the balance is subjected to surface run off that sustains the streams and rivers
which ultimately joins the ocean. A part of surface run off seeps underground to
form the groundwater reservoir. It is worthwhile to note that the total energy
used for the combination of evapotransportation requires an enormous amount
of energy, equivalent to roughly 4000 times the rate at which humankind utilizes
energy resources.
Figure 1.1 is a simplified diagram of 'Hydrologic Cycle" illustrating the natural
process of precipitation, evaporation, evapotranspiration, run-off.
Fig. 1.1 : Hydrologic cycle
Thus 'Hydrologic Cycle' has the following components (1) Evaporation and
Transpiration (E), (2) Precipitation, and (3) Run-off.
