Geoscience Reference
In-Depth Information
Estimating water beneath the surface
In the previous section it was stated that several of
the methods listed were indirect measurement
methods or estimation techniques. They certainly
do not measure soil moisture content directly, but
they have a good degree of accuracy and are good
measures of soil moisture, albeit in a surrogate form.
Estimating the amount of water beneath the surface
can also be carried out using either numerical mod-
elling or remote sensing techniques. The main
groundwater modelling techniques focus on the
movement of water in the subsurface zone, using
different forms of Darcy's law. There are also models
of soil water balance that rely on calculating inflows
(infiltration from rainfall) and outflows (seepage
and evaporation) to derive a soil water storage value
for a given time and space.
A less reliable technique for the estimation of
soil moisture is through remote sensing. There are
three satellite remote sensing techniques of rele-
vance to soil moisture assessment: thermal imagery,
passive microwave and active microwave. All of
these techniques sense the soil moisture content
at the very near surface (i.e. within the top 5 cm),
which is a major restriction on their application.
However, this is an important area in the generation
of runoff (see Chapter 5) and is still worthy of
measurement.
Figure 4.12
A single ring infiltrometer. The ring has
been placed on the ground and a pond of water is
maintained in the ring by the reservoir above. A bubble
of air is moving up the reservoir as the water level in
the pond has dropped below the bottom of the
reservoir. A reading of water volume in the reservoir is
taken and the time recorded.
Thermal imagery
The high heat capacity of water means that it has
considerable effect on the emission of thermal infra-
red signals from the earth. These can be detected
by satellites and an inference made about how wet
the soil is. This is especially so if two images of the
same scene can be compared in order to derive a
relative wetness. Satellite platforms like LANDSAT
and SPOT are able to use this technique at spatial
resolutions of around 10-30 m. The major difficulty
with this technique is that it relies on a lack of cloud
cover over the site of investigation, something that
is not easy to guarantee, especially in hydrologically
active (i.e. wet) areas.
surface. To overcome this a double ring infiltrometer
is sometimes used. With this, a second wider ring
is placed around the first and filled with water so
that the area surrounding the measured ring is
continually wet. The second problem is that ponded
infiltration is a relatively rare event across a
catchment. It is more common for rainfall to
infiltrate directly without causing a pond to form
on the surface. To overcome this a rainfall simulator
may be used to provide the infiltrating water.
