163
climates , which pass into the arctic without interruption. The endorheic regions tend to lie between
the two arheic and the three exorheic regions.
The Caspian Sea, into which the Volga drains,
adds greatly to the area of the endorheic regions
of the north temperate zone. Conversely, the
exorheic regions contain the main lake districts
of the world.
7.1.3 Water Balance of the
Hydrosphere and of
Continental Surfaces
Kalle ( 1945 ) and others had made several
attempts to estimate the total precipitation on and
evaporation from the earth’s surface. Later,
Sverdrup ( 1952 ) had reviewed the question of
evaporation from the ocean. Jacobs ( 1951 ) had
also considered both evaporation and precipitation. Further, a number of independent estimates
of total runoff had been made (Henkel 1912 ) by
attempting to sum up the rates of discharge of the
rivers of the world.
It may be noted here that most of the
groundwater entering the ocean might be moving as hidden rivers in the alluvial filling of
river valleys (Halbfass 1934 ). However, the
details of the movement of water from the
ocean to the land surfaces are not well known.
It would seem that about 1/5 of the rain, which
falls on land surfaces, may be expected to
appear in the lakes and rivers. About 90 % of
the rainfall in the Mississippi comes from
maritime air. It appears that very little of this
rain can represent water which has evaporated
from the surface of the basin and has then been
re-precipitated therein. It further appears that
90 % of the rain falling within the basin is of
external origin, and 86–88 % is directly
derived from the ocean.
7.1.4 The Water Balance of Lakes
and Variations in Lake Level
The water balance of a lake is expressed by an
equation indicating that the rate of change of
volume of a lake is equal to the rate of infl ow
from all sources, less the rate of water loss.
The sources of income are:
(a) Precipitation falling on the lake surface
(b) Water in surface infl uents
(c) Goundwater seepage through the fl oor of the
lake
(d) Groundwater entering by discrete springs
It is probable that, in lakes, nearly all the water
enters in one of the above ways. It had been
reported that c 76 % of the water entering Lake
Victoria is from precipitation on the lake surface.
In the case of the Dead Sea, the proportion would
be practically zero, and, in most of the large lakes
of central Europe, only a few per cent.
Birge and Juday ( 1934 ) emphasised the distinction between drainage lakes (with an outlet) and seepage lakes (into which, groundwater
enters and from which, water leaves by seeping
through the wall of the lake basin). Further,
many lakes in semi-arid regions lie in basins
without any kind of effl uent (outlet), loosing
water only by evaporation. Such lakes may be
called ‘ closed ’, in contrast to ‘ open lakes ’ having an effl uent. All seepage lakes are almost
certainly open in this sense.
Many lakes in karstic landscapes fi ll and
empty mainly by sub-lacustrine channels. They
receive nearly all their contents from sublacustrine springs at the time of high water. Many
other lakes are spring-fed. Further, a few crenogenic meromictic lakes show adequate chemical
evidence of sub-lacustrine springs of suffi cient
magnitude to be potential water sources. Forel
( 1898 ) had opined that delayed freezing in some
of the Swiss lakes could be due to disturbance by
the waterfowls. Further, it is argued that some
kind of correlation might exist between the
ground water and the lake water. It is, probably,
determined by the irregular distribution of calcareous materials.
Concomitant to above, the modes of loss of
water from the lakes are mainly discharge at the
effl uent and evaporation. Discharge generally
occurs from a single effl uent. However, in a few
very young lakes in the Canadian Arctic and
Labrador, discharge occurs by two (Cabot 1946 )
or even by fi ve channels (Watson 1897 ).
7.1 The Hydrological Cycle and the Water Balance of Lakes
climates , which pass into the arctic without interruption. The endorheic regions tend to lie between
the two arheic and the three exorheic regions.
The Caspian Sea, into which the Volga drains,
adds greatly to the area of the endorheic regions
of the north temperate zone. Conversely, the
exorheic regions contain the main lake districts
of the world.
7.1.3 Water Balance of the
Hydrosphere and of
Continental Surfaces
Kalle ( 1945 ) and others had made several
attempts to estimate the total precipitation on and
evaporation from the earth’s surface. Later,
Sverdrup ( 1952 ) had reviewed the question of
evaporation from the ocean. Jacobs ( 1951 ) had
also considered both evaporation and precipitation. Further, a number of independent estimates
of total runoff had been made (Henkel 1912 ) by
attempting to sum up the rates of discharge of the
rivers of the world.
It may be noted here that most of the
groundwater entering the ocean might be moving as hidden rivers in the alluvial filling of
river valleys (Halbfass 1934 ). However, the
details of the movement of water from the
ocean to the land surfaces are not well known.
It would seem that about 1/5 of the rain, which
falls on land surfaces, may be expected to
appear in the lakes and rivers. About 90 % of
the rainfall in the Mississippi comes from
maritime air. It appears that very little of this
rain can represent water which has evaporated
from the surface of the basin and has then been
re-precipitated therein. It further appears that
90 % of the rain falling within the basin is of
external origin, and 86–88 % is directly
derived from the ocean.
7.1.4 The Water Balance of Lakes
and Variations in Lake Level
The water balance of a lake is expressed by an
equation indicating that the rate of change of
volume of a lake is equal to the rate of infl ow
from all sources, less the rate of water loss.
The sources of income are:
(a) Precipitation falling on the lake surface
(b) Water in surface infl uents
(c) Goundwater seepage through the fl oor of the
lake
(d) Groundwater entering by discrete springs
It is probable that, in lakes, nearly all the water
enters in one of the above ways. It had been
reported that c 76 % of the water entering Lake
Victoria is from precipitation on the lake surface.
In the case of the Dead Sea, the proportion would
be practically zero, and, in most of the large lakes
of central Europe, only a few per cent.
Birge and Juday ( 1934 ) emphasised the distinction between drainage lakes (with an outlet) and seepage lakes (into which, groundwater
enters and from which, water leaves by seeping
through the wall of the lake basin). Further,
many lakes in semi-arid regions lie in basins
without any kind of effl uent (outlet), loosing
water only by evaporation. Such lakes may be
called ‘ closed ’, in contrast to ‘ open lakes ’ having an effl uent. All seepage lakes are almost
certainly open in this sense.
Many lakes in karstic landscapes fi ll and
empty mainly by sub-lacustrine channels. They
receive nearly all their contents from sublacustrine springs at the time of high water. Many
other lakes are spring-fed. Further, a few crenogenic meromictic lakes show adequate chemical
evidence of sub-lacustrine springs of suffi cient
magnitude to be potential water sources. Forel
( 1898 ) had opined that delayed freezing in some
of the Swiss lakes could be due to disturbance by
the waterfowls. Further, it is argued that some
kind of correlation might exist between the
ground water and the lake water. It is, probably,
determined by the irregular distribution of calcareous materials.
Concomitant to above, the modes of loss of
water from the lakes are mainly discharge at the
effl uent and evaporation. Discharge generally
occurs from a single effl uent. However, in a few
very young lakes in the Canadian Arctic and
Labrador, discharge occurs by two (Cabot 1946 )
or even by fi ve channels (Watson 1897 ).
7.1 The Hydrological Cycle and the Water Balance of Lakes