Precipitation and evapotranspiration - Fundamentals of the Physical Environment - page 103

to measurement have been developed, including direct
measurement (e.g. with evaporation pans, lysimeters and
eddy correlation systems), meteorological formulas and
moisture budget methods.

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 AE 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

Direct measurement
Possibly the most widely used method of direct measure-
ment is the evaporation pan . This consists of a shallow pan
filled with water. The rate at which the water is lost
through evaporation is measured with a gauge ( Plate 5.4 ).
This procedure measures only potential evaporation,
as it does not allow for limitation of moisture supply,
nor does it directly determine transpiration losses. In
addition, the results vary according to the size, 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

Plate 5.4 A well run climate station at Campinas, Brazil, with evaporation tank on the lower left. There are several rain gauges
and a thermometer screen. The area of bare soil is to note the state of the ground.
Photo: Peter Smithson