Geoscience Reference
In-Depth Information
SUMMARY
ESSAY QUESTIONS
The evaporation process involves the transfer of
water from a liquid state into a gaseous form in
the atmosphere. For this to happen requires an
available energy source, a water supply and the
ability of the atmosphere to receive it. Evaporation
is difficult to measure directly and there are
various estimation techniques. These range from
water budget techniques, such as evaporation pans
and lysimeters, to modelling techniques, such as
the Penman-Monteith equation. As a process,
evaporation suffers from the same problems with
measurement and estimation as does precipitation
(i.e. extreme variability in space and time). This
variability leads to difficulties in moving from point
measurements to areal estimates such as are required
for a catchment study. These can be overcome by
using spatial averaging techniques or using evap-
oration estimations that assume a large base area
(e.g. Priestly-Taylor). Forests have an important
role to play in evaporation, particularly through
interception loss. In general, more water is lost from
a forested catchment than a non-forested catchment.
This is through evaporation off wet leaves, but this
is not always the case - there are cases where a tree
canopy leads to more water in the catchment. The
importance of canopy interception in a catchment
water balance is dependent on the size and extent of
vegetation cover found within a watershed.
1
Give a detailed account of the factors
influencing evaporation rate above a
forest canopy.
2
Compare and contrast the use of
evaporation pans and lysimeters for
measuring evaporation.
3
Outline the major evaporation
estimation techniques and compare
their effectiveness for your local
environment.
4
Describe the factors that restrict actual
evaporation (evapotranspiration) from
equalling potential evaporation in a
humid-temperate climate.
FURTHER READING
Allen, R.G., Pereira L.S., Raes, D. and Smith, D.
(1998) Crop evapotranspiration - Guidelines for
computing crop water requirements.
FAO Irrigation
and drainage paper 56
(available at www.fao.org/
documents).
Brutsaert, W. (1982)
Evaporation into the atmosphere:
theory, history, and applications
. Kluwer, Dordrecht.
A detailed overview of the evaporation process.
Calder, I.R. (1990)
Evaporation in the uplands
.
J. Wiley & Sons, Chichester.
Although concerned primarily with upland
evaporation it covers the issues of estimation well.
Cheng, M. (2003) Forest hydrology: an introduction
to water and forests. CRC Press, Boca Raton,
Florida.
An overview of forest hydrological processes,
including evaporation and interception loss.
