Geoscience Reference
In-Depth Information
1999). Microwave data offer a far better method of
detecting snow cover. Passive microwaves detected
by a satellite can be interpreted to give snow cover
because any water (or snow) covering the surface
absorbs some of the microwaves emitted by the
earth surface. The greater the amount of snow
the weaker is the microwave signal received by the
satellite. Ranzi
et al
. (1999) have used AVHRR
imagery to monitor the snowpack in an area of
northern Italy and some of Switzerland and
compared this to other measurement techniques.
Unfortunately, the passive microwave satellite
sensors currently available (e.g. AVHRR) are at
an extremely coarse spatial resolution that is really
only applicable at the large catchment scale. Active
microwave sensing offers more hope but its usage
for detecting snow cover is still being developed.
STORAGE IN THE CONTEXT OF
WATER QUANTITY AND QUALITY
The storage term within the water balance equation
is among the most important for consideration of
water quantity and quality. Its influence on water
quantity has been outlined in this chapter (e.g.
snow melt contribution to flooding, groundwater
contribution to streamflow - see Chapter 5). The
importance of storage for water quality is mostly
through the addition and removal of nutrients that
occurs when water comes into contact with soil. The
soil is an extremely active biological zone, whether
through microbes, plants or animals. Water is an
important part of the nutrient cycling that occurs
within the soil zone; the presence or absence of water
in a soil limits the biological productivity of a site.
When water passes through soil it dissolves chemi-
cal salts (both natural and introduced to the system
by humans as fertilisers). Many of these salts are used
by plants and microbes as part of their respiration
processes. Those that are not used may stay in the
water as it moves into groundwater and possibly
back into river systems. This is referred to as
nutrient leaching and is a particular issue with
nitrogen fertilisers (e.g. urea, potassium nitrate,
ammonium nitrate) used to boost pasture plant and
arable crop production (see Chapter 7). The rate of
water flow through a soil (essentially the storage
term in the water balance equation) is critical in
controlling the rate of leaching and therefore has a
major influence on water quality in a receiving
stream.
Snow melt
Of critical importance to hydrology is the timing
of snow melt, as this is when the stored water is
becoming available water. There are numerous
models that have been developed to try and estimate
the amount of snow melt that will occur. Ferguson
(1999) gives a summary of recent snow melt mod-
elling work. The models can be loosely divided into
those that rely on air temperature and those that
rely on the amount of radiation at a surface. The
former frequently use a degree days approach, the
difference between mean daily temperature and a
melting threshold temperature. Although it would
seem sensible to treat zero as the melting threshold
temperature this is not always the case; snow will
melt with the air temperature less than zero because
of energy available through the soil (soil heat flux)
and solar radiation. The degree day snow melt
approach calibrates the amount of snow that might
be expected given a certain value of degree day.
Although this is useful for hydrological studies it is
often difficult to calibrate the model without
detailed snow melt data.
SUMMARY
Water held in storage is an important part of the
water balance equation. It is of particular importance
as a change in storage, whether as an absorption
term (negative) or a release (positive). As with all
the processes in the hydrological cycle, storage
is difficult to measure accurately at a useful spatial
scale. This applies whether it is water held under-
ground or as snow and ice. The release of water from
