Environmental Engineering Reference
In-Depth Information
depth, colour, composition and position of the pan, so it is not always easy to compare
results from different sites.
In some countries the
atmometer
is used. There are various types but essentially they
consist of a porous ceramic or paper evaporating surface and are generally coloured
either black or white. The evaporating surface is continuously supplied with water. They
are sensitive to variations in wind speed but, properly cared for and calibrated, they can
provide reasonable estimates of PE. Their main advantage is cheapness.
An alternative system of direct measurement is the
lysimeter
. Essentially it is a device
to record the changes in moisture of a column of soil. It may be employed to measure
either potential or actual evapotranspiration. To measure PE the column of soil is kept
constantly moist so that water deficiencies do not occur. To measure actual
evapotranspiration the column is allowed to respond naturally to atmospheric conditions.
Regular weighing allows the moisture content to be determined. If the amount of
precipitation is known, the moisture loss through evapotranspiration can be calculated.
Unfortunately there are many difficulties in obtaining accurate results. More recently a
neutron probe system has become available to record changes in soil moisture content or
ground water from which estimates of evapotranspiration can be made.
With improvements in instrumentation and data collection it is technically possible to
obtain values for evapotranspiration directly through the measurement of the turbulent
fluxes of water vapour, sensible heat and momentum. Sensitive and precise sensors of
temperature, vertical and horizontal wind speed and humidity are required, together with
a suitable logging system to collect the vast amounts of data generated. Such instruments
are now used for research projects (Plate 5.5)
METEOROLOGICAL FORMULAS
Because evapotranspiration is greatly dependent upon atmospheric conditions, it is
possible to derive good estimates of PE from data on meteorological conditions. A wide
range of formulas have been developed to do this, some of them so complex that it is
almost impossible to use them under normal circumstances; the necessary data are not
collected except during special programmes.
This problem is illustrated by what at first seems to be a very simple approach. As we
saw in Chapter 3, the energy budget can be expressed as follows:
where
Q
* is the net radiation,
Q
H
is the sensible heat flow,
Q
E
is the energy use through
evaporation and
Q
G
is the heat flow into the ground. If we could determine all the other
components of the equation we could find
Q
E
by difference, and that would tell us how
much evaporation was occurring. Unfortunately we rarely know the value of the other
components precisely.
