Geoscience Reference
In-Depth Information
nearby. This is why a maritime climate has cooler
summers, but warmer winters, than a continental
climate.
The energy required to break hydrogen bonds is
also the mechanism by which large amounts of
energy are transported away from the hot equatorial
regions towards the cooler poles. As water evap-
orates the hydrogen bonds between liquid molecules
are broken. This requires a large amount of energy.
The first law of thermodynamics states that energy
cannot be destroyed, only transformed into another
form. In this case the energy absorbed by the water
particles while breaking the hydrogen bonds is
transformed into latent heat that is then released
as sensible heat as the water precipitates (i.e. returns
to a liquid form). In the meantime the water has
often moved considerable distances in weather
systems, taking the latent energy with it. It is esti-
mated that water movement accounts for 70 per
cent of lateral global energy transport through latent
heat transfer (Mauser and Schädlich, 1998).
Water acts as a climate ameliorator in one other
way: water vapour is a powerful greenhouse gas.
Radiation direct from the sun (short-wave radiation)
passes straight through the atmosphere and may
be then absorbed by the earth's surface. This energy
is normally re-radiated back from the earth's surface
in a different form (long-wave radiation). The
long-wave radiation is absorbed by the gaseous
water molecules and consequently does not escape
the atmosphere. This leads to the gradual warming
of the earth-atmosphere system as there is an
imbalance between the incoming and outgoing
radiation. It is the presence of water vapour in our
atmosphere (and other gases such as carbon dioxide
and methane) that has allowed the planet to be
warm enough to support all of the present life forms
that exist.
river to the sea. The terminology suggests that the
area is analogous to a basin where all water moves
towards a central point (i.e. the plug hole, or in this
case, the river mouth). The common denominator
of any point in a catchment is that wherever rain
falls, it will end up in the same place: where the
river meets the sea (unless lost through evaporation).
A catchment may range in size from a matter of
hectares to millions of square kilometres.
A river basin can be defined in terms of its
topography through the assumption that all water
falling on the surface flows downhill. In this way a
catchment boundary can be drawn (as in Figures 1.4
and 1.5) which defines the actual catchment area for
a river basin. The assumption that all water flows
downhill to the river is not always correct, especi-
ally where the underlying geology of a catchment
is complicated. It is possible for water to flow as
groundwater into another catchment area, creating
a problem for the definition of 'catchment area'.
These problems aside, the catchment does provide
an important spatial unit for hydrologists to consider
how water is moving about and is distributed at a
certain time.
THE HYDROLOGICAL CYCLE
As a starting point for the study of hydrology it is
useful to consider the
hydrological cycle
. This
is a conceptual model of how water moves around
between the earth and atmosphere in different states
as a gas, liquid or solid. As with any conceptual
model it contains many gross simplifications; these
are discussed in this section. There are different
scales that the hydrological cycle can be viewed at,
but it is helpful to start at the large global scale and
then move to the smaller hydrological unit of a
river basin or catchment.
The catchment or river basin
The global hydrological cycle
In studying hydrology the most common spatial
unit of consideration is the
catchment
or
river
basin
. This can be defined as the area of land from
which water flows towards a river and then in that
Table 1.2 sets out an estimate for the amount of
water held on the earth at a single time. These
figures are extremely hard to estimate accurately.
