Geoscience Reference
In-Depth Information
5
Precipitation and
looking at these features of the hydrological cycle in turn
throughout the topic, but in this chapter we will now
concentrate on precipitation and evaporation.
HYDROLOGICAL CYCLE
As we have seen in
Chapter 4,
moisture is a vital element
of the Earth system. In reality, vast flows of moisture
move through this system and are stored in some areas,
in some cases, like ice caps, for thousands of years. This
chapter is largely concerned with only two of these flows
but, as an introduction, we need to mention their role in
the overall system, often termed the hydrological cycle.
We will refer to this again in
Chapter 14
when dealing
with flowing water in the landscape.
Figure 5.1
illustrates the main features of the hydrolog-
ical cycle. We have seen that the atmosphere contains mois-
ture, though in absolute terms the amounts are small com-
pared with the oceans. Nevertheless as this provides the
essential element for precipitation it is extremely impor-
tant. Precipitation distribution across Earth's surface is
varied, as we will find in this chapter. Without precipita-
tion, vegetation growth is impossible and human activi-
ties are minimal unless supported by water from elsewhere.
Much of the precipitation is evaporated where it falls, but
some soaks into the soil to form part of the ground water,
then making its way either to rivers to become run-off or
directly into lakes or the sea. A small amount falls as snow
which will eventually melt or it may become compressed
into ice to become part of a glacier or ice cap. All this mois-
ture is eventually evaporated back into the atmosphere as
evaporation or evapotranspiration. The wind systems
of the world can then distribute this water vapour until
it eventually condenses back into cloud droplets and
perhaps precipitation to start the cycle again. We will be
PRECIPITATION
For those who live in the humid regions of the world,
precipitation is normally so frequent that it is taken for
granted. During times of drought the importance of
precipitation and its role in feeding the hydrological
system and providing water for human use and plant life
become all too apparent. In recent years the problems
raised by drought in areas such as China, Somalia,
Australia and even parts of north-west Europe have
become publicized in the media as demand for water has
increased at a time when some areas have experienced
below average precipitation.
Precipitation represents the vital input of water to the
surface hydrological system. It is the nature of this input
- the character and distribution of precipitation - which
we will consider in the first part of this chapter.
Types of precipitation
To most people three types of precipitation come
immediately to mind: rain, snow and hail. If we looked at
these more closely, the distinction between them is not
always clear. Moreover, they are not the only forms in
which moisture is input to the surface. Dew, fog-drip and
rime all transfer water from the atmosphere to the ground
