Planet Earth is often called water planet
or blue planet, because of abundanceof
water on its surface. The water bodies
cover 71 per cent of the earth�s surface. 60.7
per cent of the total area of the northern
hemisphere and 80.9 per cent of the southern
hemisphere are covered with water. If we take
into account only the water surface of the
earth, then 43 per cent lies in the northern
hemisphere and 57 per cent in the southern
hemisphere.
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A D V E R T I S E M E N T
The earth receives essentially pure water
in droplets condensing from the atmosphere.
Different kinds of water are found in different
geological environments. Over 97 per cent of
all the water on the surface of the earth is in
the oceans, and most of the remainder is in
glaciers.
The largest reservoir of water is the ocean.
Of the total expanse of water bodies, about 93
per cent is covered by four oceans: Pacific,
Atlantic, Indian and Arctic
HYDROLOGICAL CYCLE
Water from oceans is evaporated and lifted into
the atmosphere. It is eventually condensed
and is returned back to the earth�s surface in
the form of rain, hail, dew, snow or sleet.
Some of the precipitation, after wetting the
foliage and ground, runs off over the surface
to the streams. It is the water that sometimes
causes erosion and is the main contributor to
floods. Of the precipitation that soaks into the
ground, some is available for growing plants
and for evaporation. Some reaches the deeper
zones and percolates through springs and
seeps to maintain the streams during dry
period. The streams, in turn, eventually, return
the water back to the oceans where it
originated. It is because of this never ending
circulation that the process has come to be
known as hydrological cycle.
The hydrological cycle is sometimes
expressed mathematically as:
RF = RO + ET
Where RF includes all types of
precipitation, RO is runoff and ET is
evapotranspiration.
Runoff occurs when precipitation, that does
not have an opportunity to infiltrate into soil,
flows across the land surface. However, most
of it enters the stream channel untimately,
which carries it to the oceans. A part of the
precipitation that infiltrates the soil percolates
downward to the water table through springs.
Broadly speaking, runoff is composed of water
from both surface flow and seepage flow. It is
an extremely important segment of
hydrological cycle. Rainwater that reaches the
soil surface is wholly or partly absorbed by
the soil in the process of infiltration. The amount
of rainfall entering the soil depends upon the
rate of rainfall and the infiltration rate of the
soil.
About 4,23,000 cubic kilometres of water
is evaporated each year from the oceans.
About 73,000 cubic kilometres is evaporated
from lakes and land surfaces of the
continents. Most of the water precipitates
back onto the ocean but excess falls on the
land. Because, more snow and rain falls every
year than can be evaporated, about 37,000
cubic kilometres of water drips, seeps and
flows from the land to the sea annually. The
water evaporated from the land is not only
from exposed surfaces of lakes and
streams but also from plants and animals.
Some evaporation and absorption of
water is carried by plant roots which is
transpirated through leaves, termed as
evapotranspiration. The total evaporation is
equalled by total precipitation, of which
about1,10,000 cubic kilometres of precipitation
falls on the land surface annually,
which if distributed evenly on the entire earth
is 117 cm thick (Table 14.1).
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AKHAND PRATAP SINGH
