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has been discarded (it is of the order of 10-25 W m
-2
at midday, depending on inso-
lation and the type of vegetation; see Meyers and Hollinger,
2004
). Another term in
the energy balance, seldom taken into account, is the sensible heat related to the input
of precipitation into the control volume. If, for instance, during daytime conditions
the temperature of the rain is lower than that of the air that it replaces in the control
volume, a net exchange of energy out of the volume occurs. Further, if the cold rain-
water percolates into the soil, a signiicant redistribution of energy within the control
volume can occur (see Kollet et al.,
2009
).
1.2.3 The Link: Evapotranspiration
From Eqs. (
1.2
) and (
1.3
) the link between the water balance and energy balance
becomes clear: the evaporation appears as a transport of mass in the water balance
and as a transport of energy in the energy balance. The total water vapour lux that
leaves the system is made up of a number of luxes
within
the system: soil
evapora-
tion
(
E
soil
),
transpiration
by the plants (
T
), and
evaporation
of intercepted water (
E
int
).
Both transpiration and soil evaporation extract water from the soil subsystem and
release it in the air subsystem, whereas in the case of interception the soil is bypassed.
The sum of these three terms is called
evapotranspiration
and denoted by
E
(in the
hydrological literature a commonly used symbol is
ET
).
Evaporation and transpiration occur simultaneously and there is no easy way of
distinguishing between the two processes. Apart from water availability in the top
soil, the evaporation from a cropped (or more general: vegetated) soil is determined
mainly by the fraction of solar radiation reaching the soil surface. This fraction
decreases over the growing period as the crop develops and the crop canopy shades
more and more of the ground area. When the crop is small, water is lost predomi-
nantly by soil evaporation, but once the crop is well developed and completely cov-
ers the soil, transpiration becomes the main process. In
Figure 1.6
the partitioning
of evapotranspiration into soil evaporation and transpiration is plotted in correspon-
dence to leaf area per unit soil surface below it (LAI). At sowing, nearly 100% of
E
comes from soil evaporation, whereas at full crop cover more than 90% of
E
comes
from transpiration (Allen et al.,
1998
).
Apart from the direct link between water balance and energy balance through the
occurrence of evapotranspiration in both balances, there is also a more indirect link.
The two balance equations exactly stand for the two requirements needed for evapo-
transpiration: water should be available to be evaporated, and energy (through radia-
tion) should be available to actually let the evaporation happen (see also
Chapter 7
).
The availability of water in the soil in turn will largely be determined by the amount
of rainfall. These two limiting factors - radiation and rainfall - translate into different
behaviour of evapotranspiration in different regions of the world: in regions of abun-
dant rainfall (relative to the evaporative loss) evapotranspiration correlates highly
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