Geoscience Reference
In-Depth Information
At present, remote sensing techniques are not
able to provide reasonable estimates of canopy inter-
ception. They do provide some useful information
that can be incorporated into canopy interception
models but cannot provide the detailed difference
between above- and below-canopy rainfall. In
particular, satellites can give good information on
the type of vegetation and its degree of cover.
Particular care needs to be taken over the term 'leaf
area index' when reading remote sensing literature.
Analysis of remotely sensed images can provide a
good indication of the percentage vegetation cover
for an area, but this is not necessarily the same as
leaf area index - although it is sometimes referred
to as such. Leaf area index is the surface area of leaf
cover above a defined area divided by the surface area
defined. As there are frequently layers of vegetation
above the ground, the leaf area index frequently has
a value higher than one. The percentage vegetation
cover cannot exceed one (as a unitary percentage) as
it does not consider the third dimension (height).
Mass balance estimation
In the same manner that evaporation pans and
lysimeters estimate evaporation rates, evaporation
at the large scale (catchment or lake) can be esti-
mated through the water balance equation. This is
a relatively crude method, but it can be extremely
effective over a large spatial and/or long temporal
scale. The method requires accurate measure-
ment of precipitation and runoff for a catchment or
lake. In the case of a lake, change in storage can be
estimated through lake-level recording and know-
ledge of the surface area. For a catchment it is
often reasonable to assume that change in storage is
negligible over a long time period (e.g. one year)
and therefore the evaporation is precipitation minus
runoff.
Canopy interception loss estimation
Empirical models that link rainfall to interception
loss based on regression relationships of measured
data sets have been developed for many different
types of vegetation canopy (see Zinke (1967) and
Massman (1983) for examples and reviews of these
types of model). Some of these models used log-
arithmic or exponential terms in the equations but
they all rely on having regression coefficients based
on the vegetation type and climatic regime.
A more detailed modelling approach is the Rutter
model (Rutter
et al
., 1971, 1975) which calculates
an hourly water balance within a forest stand. The
water balance is calculated, taking into account the
rate of throughfall, stemflow, interception loss
through evaporation and canopy storage. In order
to use the model a detailed knowledge of the canopy
characteristics is required. In particular the canopy
storage and drainage rates from throughfall are
required to be known; the best method for deriv-
ing these is through empirical measurement. The
Rutter model treats the canopy as a single large leaf,
although it has been adapted to provide a three-
dimensional canopy (e.g. Davie and Durocher,
1997) that can then be altered to allow for changes
and growth in the canopy.
EVAPORATION IN THE CONTEXT OF
WATER QUANTITY AND QUALITY
Evaporation, as the only loss away from the surface
in the water balance equation, plays a large part in
water quantity. The loss of water from soil through
direct evaporation and transpiration has a direct
impact on the amount of water reaching a stream
during high rainfall (see Chapter 5) and also the
amount of water able to infiltrate through into
groundwater (see Chapter 4). The impact of evap-
oration on water quantity is not as great as for
precipitation but it does have a significant part to
play in the quantity and timing of water flowing
down a river.
The influence of evaporation on water quality
is mostly through the impurities left behind after
water has evaporated. This may lead to a concen-
tration of impurities in the water remaining behind
(e.g. the Dead Sea between Israel and Jordan) or
a build up of salts in soils (salination). This is
discussed in more detail in Chapter 8.
