Climate Change and the European Water Dimension - Agri ...

water and latent heat due to precipitation suppression may have the potential to
influence circulation patterns.
Evaporation and evapotranspiration
Evaporation occurs when water is converted to water vapor. Whether occurring from
a liquid surface, from soil or from plants (in the latter case it is called transpiration,
defined as soil water reaching the atmosphere through plants), driving mechanisms
for evaporation are similar, and rates are controlled by the energy available at the
evaporating surface (radiation), air temperature, humidity and wind speed. The
physics of evaporation and transpiration rely on surface exchange processes, as
driven by latent heat flux, water molecule migration between water and air, saturated
water content of air (which in turn is influenced by removal action of air currents) and
sensible heat flux. At non-limiting water availability conditions, evaporation rates are
controlled by surface radiation balance. Plant stomatal resistance also controls
transpiration. In particular, in agricultural crops or forested areas transpiration from
canopy and evaporation from underlying soil surface occur simultaneously, and there
is no easy way to calculate each contribution separately, so that the two processes
are condensed into a single one named evapotranspiration.
Evapotranspiration has always been difficult to measure, especially on an ecosystem
or watershed spatial scale. Methods have been developed to measure
evapotranspiration at the leaf level, the tree level, and the stand level. At the stand
level, instruments mounted on a tower above the canopy are routinely used to
measure humidity and wind velocities at high frequency, with water fluxes out of the
forest canopy calculated by the eddy covariance method. Several general methods
are available for measurement of evaporation/evapotranspiration (e.g. evaporation
pans, water balance at lysimeter scale, water balance at watershed or water-body
scale). For a complete review, see Shuttleworth (1993) and references therein.
On the other side, estimates of crop evapotranspiration usually follow a two-step
procedure: i) estimation of potential evapotranspiration ET0; ii) adjustment through
appropriate coefficient for specific crop, management practices and actual water
availability to obtain actual evapotranspiration ETa. Although a large number of more
or less empirical methods have been developed over the last 50 years to estimate
ET0, none of then has proved to be universally valid. Choice of one method over the
other depends on data availability and type of application. Among the most popular
methods are: Blaney-Criddle (1950); radiation - e.g. Priestley-Taylor (1972) equation,
or Turc (1967) equation; Penman-Monteith (Penman, 1948; Monteith, 1965);
Hargreaves (1974) and pan evaporation. For a comparison among the different
methods see again Shuttleworth, 1993. Crop coefficients and other correction
procedures to derive ETa from ET0 can be derived for a large number of commercial
crops from Doorenbos and Pruitt (1977). Large-scale evapotranspiration can also be
estimated with the aid of remote sensing measurements, adopting an energy balance
calculation approach (Bastiaanssen et al., 1998a; 1998b).
Evaporation and climate change
Over the entire land surface of the globe, rainfall averages around 750 mm per year,
of which some two thirds is returned to the atmosphere as evapotranspiration,
making evapotranspiration the largest single component of the terrestrial hydrological
cycle (Baumgartner and Reichel, 1975). In Figure III.5 the annual potential
evapotranspiration for Europe is presented. The actual rate of evaporation is
constrained by water availability; due to an increase in evaporative demand as a
result increased temperatures, a decrease in actual evapotranspiration might
correspond in those areas with scarce soil moisture (arid and semi-arid areas); in
other words it could happen that a reduction in summer soil water could lead to a
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