Geoscience Reference
In-Depth Information
Severn (65% forest; 35% grassland)
Wye (100% grassland)
20000
Precipitation
Forest
Grassland
Severn
10000
Evaporation
Wye
0
1975
1980
1985
Year
Figure 7.12
Results of the
Plynlimon
paired catchment study demonstrating the additional water loss as evaporation for
the partly forested catchment.
differences for different vegetation covers. Figure 7.12 shows the results of a good
example, the
Plynlimon
catchments, which demonstrate a clear difference in the
area-average evaporation loss from two adjacent catchments, one entirely grassland
and one partially forested, in the high rainfall maritime climate of Wales.
Lysimeters
Lysimeters
are sometimes regarded as providing the standard evaporation
measurement against which alternative measurement methods can be validated.
High quality lysimeter systems are expensive, however, and their use requires
great care. Accurate lysimeters have been much used in research to calibrate
some of the empirical formulae used to estimate evaporation from irrigated
crops, see Chapter 23. The method requires isolating a sample volume of soil and
vegetation, typically 0.5-2.0 m in diameter, in a container from which there is no
leakage and from which runoff is measured. The incoming water as precipitation
is measured, and the change in the water inside the container determined, often
by weighing the whole lysimeter. Lysimeters must be installed carefully to leave
the natural soil and vegetation undisturbed, and if a crop is grown in the
