Geoscience Reference
In-Depth Information
improve estimation of areal precipitation. What is
encouraging about the technique is that nearly all
the world is covered by satellite imagery so that it
can be used in sparsely gauged areas. The new
generation of satellite platforms being launched
in the early twenty-first century will have multiple
sensors on them so it is feasible that they will be
measuring visible, infrared and microwave wave-
bands simultaneously. This will improve the
accuracy considerably but it must be borne in mind
that it is an indirect measure of precipitation and
will still require calibration to a rain gauge set.
Precipitation also has a direct influence on water
quality through scavenging of airborne pollutants
which are then dissolved by the rain. The complex
nature of a forest topography means that trees act as
recipient surfaces for airborne pollutants. As rain
falls onto the tree, salts that have formed on leaves
and branches may be dissolved by the water, making
the stemflow and throughfall pollutant-rich. This
has been observed in field studies, particularly near
the edge of tree stands (Neal
et al
., 1991).
The best known example of pollutant scavenging
is
acid rain
. This is where precipitation in areas
polluted by industrial smokestacks dissolves gases
and absorbs particles that lower the acidity of the
rain. Naturally rain is slightly acidic with pH
between 5 and 6, due to the dissolving of carbon
dioxide to form a weak carbonic acid. The burning
of fossil fuels adds nitrogen oxides and sulphur
oxides to the atmosphere, both of which are easily
dissolved to form weak nitric and sulphuric acids.
The burning of coal is particularly bad through the
amount of sulphur dioxide produced, but any
combustion will produce nitrogen oxides by the
combination of nitrogen and oxygen (both already
in the atmosphere) at high temperatures. In areas
of the Eastern United States and Scandinavia rain-
fall has been recorded with pH values as low as 3
(similar to vinegar). In some situations this makes
very little difference to overall water quality as the
soil may have enough acid-buffering capability to
absorb the acid rain. This is particularly true for
limestone areas where the soil is naturally alkaline.
However many soils do not have this buffering
capacity due to their underlying geology (e.g.
granite areas in the north-east of North America).
In this situation the streams become acidic and this
has an extremely detrimental effect on the aquatic
fauna. The major reason for the impact on fish life
is the dissolved aluminium that the acidic water
carries; this interferes with the operation of gills and
the fish effectively drown.
It is worth noting that the dissolving of nitrous
oxide can have a positive benefit to plant life
through the addition to the soils of nitrate which
promotes plant growth (see Chapter 7). To give
PRECIPITATION IN THE CONTEXT
OF WATER QUANTITY AND
QUALITY
Precipitation, as the principal input into a catch-
ment water balance, has a major part to play in
water quantity and quality. By and large it is the
spatial and temporal distribution of precipitation
that drives the spatial and temporal distribution
of available water. Rainfall intensity frequently
controls the amount of runoff during a storm event
(see Chapter 5) and the distribution of rain through
the year controls the need for irrigation in an
agricultural sytem.
The exception to this is in large river basins where
the immediate rainfall distribution may have little
bearing on the water flowing down the adjacent
river. A good example is the Colorado River which
flows through areas of extremely low rainfall in
Utah and Arizona. The lack of rainfall in these areas
has little bearing on the quantity of water in the
Colorado River, it is governed by the precipitation
(both rain and snow) falling in the Rocky Mountains
well to the north-east.
The influence of precipitation on water quantity
directly affects water quality through dilution.
Where water quantity is high there is more water
available to dilute any contaminants entering a river
or groundwater system. It does not follow that high
water quantity equates with high water quality but
it has the potential to do so.
