3
1.
Evaporation and Transpiration (E) :
Evaporation is the amount of water that evaporates from the surface of oceans,
rivers, streams, lakes and all other water bodies like reservoirs of dams etc.
and also from atmosphere. Transpiration is the process by which water is lost
from the leaves of plants. The combined process is known as
Evapotranspiration (E) and is a component of Water Budget.
2.
Precipitation (P) :
Precipitation is the amount of moisture that fall on the surface of the earth and
can be classified as :
(a) Liquid precipitation : Rainfall
(b) Frozen precipitation : i) Snow
ii) Hail
iii) Sleet
iv) Freezing rain
3.
Run-off (R) :
Part of the precipitation that falls on the earth's surface is lost due to
evaporation from the surface, soil and vegetation and by transpiration. The
balance part of precipitation is the 'run-off' which travels to the rivers, streams,
lakes and ultimately join the oceans. Part of run off also flows as sub-surface
run off which also joins the rivers or oceans depending upon the geology of the
area. Part of the sub-surface flow infiltrates further down and joins the
groundwater which also flows towards rivers and ultimately oceans.
The hydrologic cycle, thus, may be expressed as :
Precipitation (P) = Evaporation (E) + Run-off (R).
This is the fundamental equation of water budget.
