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Recursos e gestão da água
Recursos e gestão da água
A new approach is suggested for estimating evaporation of intercepted rainfall from single
trees in sparse forests. It is shown that, theoretically, the surface temperature of a wet tree
crown will depend on the available energy and windspeed. But for a fully saturated canopy
under rainy conditions, surface temperature will approach the wet bulb temperature when
available energy tends to zero. This was confirmed experimentally from measurements of
the radiation balance, aerodynamic conductance for water vapour and surface temperature
on an isolated tree crown. Net radiation over a virtual cylindrical surface, enclosing the tree
crown, was monitored by a set of radiometers positioned around that surface. Aerodynamic
conductance for the tree crown was derived by scaling up measurements of leaf boundary
layer conductance using the heated leaf replica method. Thermocouples were used to
measure the average leaf surface temperature. Results showed that a fully wet single tree
crown behaves like a wet bulb, allowing evaporation of intercepted rainfall to be estimated
by a simple diffusion equation for water vapour, which is not restricted by the assumptions
of one-dimensional transfer models usually used at the stand scale. Using this approach,
mean evaporation rate from wet, saturated tree crowns was 0.27 or 0.30mm h 1, when
surface temperature was taken equal to the air wet bulb temperature or estimated accounting
for the available energy, respectively.
In a previous study, it was shown that an isolated, fully saturated tree-crown behaves like a
wet bulb, allowing evaporation of intercepted rainfall to be estimated by a simple diffusion
equation for water vapour. This observation was taken as the basis for a new approach in
modelling interception loss fromsavanna-type woodland, whereby the ecosystem evaporation
is derived by scaling up the evaporation from individual trees, rather than by considering
a homogeneous forest cover. Interception loss from isolated trees was estimated by
combining the aforementioned equation for water vapour flux with Gash’s analytical model.
A new methodology, which avoids the subjectivity inherent in the Leyton method, was used
for estimating the crown storage capacity. Modelling performance was evaluated against
data from two Mediterranean savanna-type oak woodlands (montados) in southern Portugal.
Interception loss estimates were in good agreement with observations in both sites. The
proposed modelling approach is physically based, requires only a limited amount of data
and should be suitable for the modelling of interception loss in isolated trees and savannatype
ecosystems.