Geoscience Reference
In-Depth Information
upon the water films between the adjacent soil particles.
As these soil water films shrink, however, the tension
within them increases. If the tension of the soil films
exceeds the osmotic root tension, the continuity of the
plant's water uptake is broken and wilting occurs.
Transpiration is controlled by the atmospheric factors
that determine evaporation as well as by plant factors
such as the stage of plant growth, leaf area and leaf
temperature, and also by the amount of soil moisture
(see Chapter 12C). It occurs mainly during the day,
when the stomata (small pores in the leaves), through
which transpiration takes place, are open. This opening
is determined primarily by light intensity. Transpiration
naturally varies greatly with season, and during the
winter months in mid-latitudes conifers lose only 10 to
18 per cent of their total annual transpiration losses and
deciduous trees less than 4 per cent.
In practice, it is difficult to separate water evaporated
from the soil, intercepted moisture remaining on vege-
tation surfaces after precipitation and subsequently
evaporated, and transpiration. For this reason, evapo-
ration, or the compound term evapotranspiration , may
be used to refer to the total loss. Over land, annual
evaporation is 52 per cent due to transpiration, 28 per
cent soil evaporation and 20 per cent interception.
Evapotranspiration losses from natural surfaces can-
not be measured directly. There are, however, various
indirect methods of assessment, as well as theoretical
formulae. One method of estimation is based on the
moisture balance equation at the surface:
basis for the climate classification developed by C. W.
Thornthwaite (see Appendix 1).
In regions where snow cover is long-lasting,
evaporation/sublimation from the snowpack can be
estimated by lysimeters sunk into the snow that are
weighed regularly.
A meteorological solution to the calculation of
evaporation uses sensitive instruments to measure the
net effect of eddies of air transporting moisture upward
and downward near the surface. In this 'eddy correla-
tion' technique, the vertical component of wind and the
atmospheric moisture content are measured simul-
taneously at the same level (say, 1.5 m) every few
seconds. The product of each pair of measurements is
then averaged over some time interval to determine the
evaporation (or condensation). This method requires
delicate rapid-response instruments, so it cannot be used
in very windy conditions.
Theoretical methods for determining evaporation
rates have followed two lines of approach. The first
relates average monthly evaporation ( E ) from large
water bodies to the mean wind speed ( u ) and the mean
vapour pressure difference between the water surface
and the air ( e w - e d ) in the form:
E = K u ( e w - e d )
where K is an empirical constant. This is termed the
aerodynamic approach because it takes account of the
factors responsible for removing vapour from the water
surface. The second method is based on the energy
budget. The net balance of solar and terrestrial radiation
at the surface ( R n ) is used for evaporation ( E ) and
the transfer of heat to the atmosphere ( H ). A small
proportion also heats the soil by day, but since nearly all
of this is lost at night it can be disregarded. Thus:
P - E = r
S
This can be applied to a gauged river catchment, where
precipitation and runoff are measured, or to a block of
soil. In the latter case we measure the percolation
through an enclosed block of soil with a vegetation
cover (usually grass) and record the rainfall upon it.
The block, termed a lysimeter , is weighed regularly
so that weight changes unaccounted for by rainfall or
runoff can be ascribed to evapotranspiration losses,
provided the grass is kept short! The technique allows
the determination of daily evapotranspiration amounts.
If the soil block is regularly 'irrigated' so that the vege-
tation cover is always yielding the maximum possible
evapotranspiration, the water loss is called the potential
evapotranspiration (or PE). More generally, PE may
be defined as the water loss corresponding to the avail-
able energy. Potential evapotranspiration forms the
R n = LE
H
where L is the latent heat of evaporation (2.5
10 6 J
kg -1 ). R n can be measured with a net radiometer and
the ratio H/LE = ß, referred to as Bowen's ratio , can
be estimated from measurements of temperature and
vapour content at two levels near the surface. ß ranges
from <0.1 for water to ≥10 for a desert surface. The
use of this ratio assumes that the vertical transfers of
heat and water vapour by turbulence take place with
equal efficiency. Evaporation is then determined from
an expression of the form:
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