162
7.1.2 Atmospheric Precipitation
and Its Distribution
Supersaturation with water vapour in the air is
required for the production of rain and snow as
they originate from water vapour. In general,
supersaturation may occur whenever a mass of
air containing water vapour is uplifted.
Uplift and cooling may occur in three main ways:
(a) If solar radiation heats the ground below
calm air, convection will be started as the air
in contact with the hot ground becomes
unstable. Many of the warm steppe regions
of the world receive rainfall in this way. Such
a rain is often called convective rain .
(b) When a high mountain stands in the path of a
prevalent moist wind, the upturning of the
wind by the mountains produces supersaturation. This results in orographic rain .
(c) The formation of cyclonic eddies in the
large circulation systems and the production of discontinuities, where major elements of the systems meet, may give rise to
complex patterns of uplift. Frontal cyclonic
rain of this type is of much importance in
some regions.
It could, thus, be said that the distribution of
rainfall is determined primarily by the circulation
pattern of the atmosphere.
Concomitant to above, the most constant elements of the hydrological cycle are the trade
wind systems. These are the air currents moving
towards the equatorial belt of high temperature.
They also move towards the west because the
earth rotates. There is an equatorial region of
calm between the trade winds. This is known to
the mariners as doldrums .
Further, the regions of the westerlies lie to the
north and south of the trade winds. Here, the pattern of circulation is the reverse of that in the trade
wind belts, i.e. the SW winds blow in the northern
hemisphere and NW winds blow in the southern
hemisphere. A third system of easterlies develops
in the polar regions.
In addition to the above, there are a number
of features which are superimposed on the simple planetary system due to distribution of land
and water as it happens to be developed at the
surface of the earth. The most striking of these
secondary patterns are the monsoon systems .
These are due to the development of high pressures in winter and low pressures in summer in
the centres of continents, where the cooling
and heating of the air masses are very great.
The formation of a wet monsoon blowing
across the Indian Ocean during summer is of
great signifi cance in producing summer rains,
mainly in the tropical belt, which would otherwise remain arid.
Martonne and Aufrere ( 1928 ) had recognised
three types of hydrological regions based on land
masses of the earth, namely,
(a) Exorheic regions , from which rivers reach
the sea.
(b) Endorheic regions , within which rivers arise
but from which these rivers never reach the
sea. They lose themselves in dry water
courses or enter closed lake basins. The distribution of the endorheic regions, as transition zones between exorheic and arheic
regions, emphasises their arheic nature. A
slight change in the climate towards a more
arid condition will cause the lakes of the
endorheic regions to dry completely as the
landscape becomes arheic. A slight increase
in precipitation will cause the same lakes to
rise and, perhaps, to overfl ow, and the earlier
endorheic region may become exorheic.
Events of this kind might have been relatively frequent throughout the Pleistocene.
These, further, might have had a considerable infl uence on the chemistry of most of
the closed lakes. The lakes of endorheic
regions are astatic , as contrasted with the
more eustatic lakes (Gagl 1924 ; Hutchinson
2004 ) of exorheic regions.
(c) Arheic regions , within which no rivers arise.
The lower part of the Nile provides a good
example of this.
The distribution of these kinds of areas shows
a characteristic pattern depending on the distribution of rainfall. Two large desert zones tend to
develop in the latitudes of the trade winds. These
constitute the main arheic regions of the world.
Between them lie the zone of equatorial rains
(north and south) and the zones of temperate humid
7 Lake Hydrology
7.1.2 Atmospheric Precipitation
and Its Distribution
Supersaturation with water vapour in the air is
required for the production of rain and snow as
they originate from water vapour. In general,
supersaturation may occur whenever a mass of
air containing water vapour is uplifted.
Uplift and cooling may occur in three main ways:
(a) If solar radiation heats the ground below
calm air, convection will be started as the air
in contact with the hot ground becomes
unstable. Many of the warm steppe regions
of the world receive rainfall in this way. Such
a rain is often called convective rain .
(b) When a high mountain stands in the path of a
prevalent moist wind, the upturning of the
wind by the mountains produces supersaturation. This results in orographic rain .
(c) The formation of cyclonic eddies in the
large circulation systems and the production of discontinuities, where major elements of the systems meet, may give rise to
complex patterns of uplift. Frontal cyclonic
rain of this type is of much importance in
some regions.
It could, thus, be said that the distribution of
rainfall is determined primarily by the circulation
pattern of the atmosphere.
Concomitant to above, the most constant elements of the hydrological cycle are the trade
wind systems. These are the air currents moving
towards the equatorial belt of high temperature.
They also move towards the west because the
earth rotates. There is an equatorial region of
calm between the trade winds. This is known to
the mariners as doldrums .
Further, the regions of the westerlies lie to the
north and south of the trade winds. Here, the pattern of circulation is the reverse of that in the trade
wind belts, i.e. the SW winds blow in the northern
hemisphere and NW winds blow in the southern
hemisphere. A third system of easterlies develops
in the polar regions.
In addition to the above, there are a number
of features which are superimposed on the simple planetary system due to distribution of land
and water as it happens to be developed at the
surface of the earth. The most striking of these
secondary patterns are the monsoon systems .
These are due to the development of high pressures in winter and low pressures in summer in
the centres of continents, where the cooling
and heating of the air masses are very great.
The formation of a wet monsoon blowing
across the Indian Ocean during summer is of
great signifi cance in producing summer rains,
mainly in the tropical belt, which would otherwise remain arid.
Martonne and Aufrere ( 1928 ) had recognised
three types of hydrological regions based on land
masses of the earth, namely,
(a) Exorheic regions , from which rivers reach
the sea.
(b) Endorheic regions , within which rivers arise
but from which these rivers never reach the
sea. They lose themselves in dry water
courses or enter closed lake basins. The distribution of the endorheic regions, as transition zones between exorheic and arheic
regions, emphasises their arheic nature. A
slight change in the climate towards a more
arid condition will cause the lakes of the
endorheic regions to dry completely as the
landscape becomes arheic. A slight increase
in precipitation will cause the same lakes to
rise and, perhaps, to overfl ow, and the earlier
endorheic region may become exorheic.
Events of this kind might have been relatively frequent throughout the Pleistocene.
These, further, might have had a considerable infl uence on the chemistry of most of
the closed lakes. The lakes of endorheic
regions are astatic , as contrasted with the
more eustatic lakes (Gagl 1924 ; Hutchinson
2004 ) of exorheic regions.
(c) Arheic regions , within which no rivers arise.
The lower part of the Nile provides a good
example of this.
The distribution of these kinds of areas shows
a characteristic pattern depending on the distribution of rainfall. Two large desert zones tend to
develop in the latitudes of the trade winds. These
constitute the main arheic regions of the world.
Between them lie the zone of equatorial rains
(north and south) and the zones of temperate humid
7 Lake Hydrology