Geoscience Reference
In-Depth Information
conclude that in general there is little, if any,
evidence that forests can increase rainfall. However
Calder (1999: 24, 26) concludes, 'Although the
effects of forests on rainfall are likely to be
relatively small, they cannot be totally dismissed
from a water resources perspective . . . Further
research is required to determine the magnitude
of the effect, particularly at the regional scale.'
wind speed and temperature profiles. These have
to be measured at extremely short timescales (e.g.
microseconds) to account for eddies in vertical wind
motion. Consequently, extremely detailed micro-
meteorological instrumentation is required with
all instruments having a rapid response time. In
recent years this has become possible with hot wire
anemometers and extremely fine thermistor heads
for thermometers. One difficulty is that you are
necessarily measuring over a very small surface area
and it may be difficult to scale up to something of
interest to catchment-scale hydrology.
The aerodynamic profile (or turbulent transfer)
method is based on a detailed knowledge of the
energy balance over a surface. The fundamental
idea is that by calculating the amount of energy
available for evaporation the actual evaporation rate
can be determined. The measurements required
are changes in temperature and humidity giving
vertical humidity gradients. To use this method it
must be assumed that the atmosphere is neutral
and stable, two conditions that are not always
applicable.
The Bowen ratio method is similar to the aero-
dynamic profile method but does not assume as
much about the atmospheric conditions. The Bowen
ratio is the ratio of sensible heat to latent heat
and requires detailed measurement of net radiation,
soil heat flux, temperature and humidity gradient
above a surface. These measurements need to be
averaged over a 30-minute period to allow the
inherent assumptions to apply.
All of these micro-meteorological approaches
to measuring evaporation use sophisticated instru-
ments that are difficult to leave in the open for
long periods of time. In addition to this they are
restricted in their spatial scope (i.e. they only
MEASUREMENT OF EVAPORATION
In the previous chapter there has been much empha-
sis on the difficulties of measuring precipitation due
to its inherent variability. All these difficulties
also apply to the measurement of evaporation, but
they pale into insignificance when you consider
that now we are dealing with measuring the rate
at which a gas (water vapour) moves away from a
surface. Concentrations of gases in the atmosphere
are difficult to measure, and certainly there is no
gauge that we can use to measure total amounts in
the same way that we can for precipitation.
In each of the process chapters in this topic there
is an attempt to distinguish between measurement
and estimation techniques. In the case of evap-
oration this distinction becomes extremely blurred.
In reality almost all the techniques used to find an
evaporation rate are estimates, but some are closer
to true measurement than others. In this section
each technique will include a sub-section on how
close to 'true measurement' it is.
Direct micro-meteorological
measurement
There are three main methods used to measure
evaporation directly: the eddy fluctuation (or corre-
lation), aerodynamic profile, and Bowen ratio
methods. These are all micro-meteorological
measurement techniques and details on them can be
found elsewhere (e.g. Oke, 1987). An important
point to remember about them all is that they are
attempting to measure how much water is being
evaporated above a surface, a very difficult task.
The eddy fluctuation method measures the water
vapour above a surface in conjunction with a vertical
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