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Foi realizado um estudo com o objectivo de quantificar os consumos de água por transpirarão em dois povoamentos adultos de eucalipto (Eucalyptus globulus Labbill) e de pinheiro bravo (Pinus pinaster Aiton), respectivamente. A estimativa destes consumos foi feita recorrendo a dois métodos distintos: o balanço de água no solo e o modelo de Penman-Monteith (aplicado numa versão unilaminar). Os resultados obtidos indicam que, em qualquer dos povoamentos, a transpiração está sobretudo dependente da disponibilidade de água no solo, tendo-se verificado que a transpiração acumulada ao longo dos anos de 1992 e 93 é muito próxima do valor da precipitação efectiva no mesmo período. A realização de medições de condutância estomática revelou que ambas as espécies apresentam padrões de variação horária e sazonal, que traduzem a capacidade destas espécies em controlarem eficazmente as perdas de água por transpiração, quer quando aumenta a secura do ar, quer quando se reduz a humidade no solo. A comparação directa das estimativas da transpiração fornecidas pelos dois métodos utilizados revelou algumas diferenças que, no entanto, parecem aceitáveis tendo em conta que estes métodos são concebidos para aplicações em escalas temporais distintas.

In closed canopy forests the energy absorbed by the trees can be adequately estimated solely from the vertical radiation fluxes. However, in isolated or widely spaced trees this approach is no longer valid and radiation fluxes in all directions must be accounted for. An adequate estimate of the tree available energy is critical to model and calculate both interception losses and transpiration. Within a study where interception loss in a sparse evergreen oak woodland (montado) of Southern Portugal is evaluated and mod¬elled, the net amount of radiant energy absorbed by an isolated holm oak tree (Q) was measured under different radiation conditions. The measuring and calculating proce¬dure was based on the integration of the flux density of net radiation (Rn) at different points of a cylindrical surface (S) enclosing the tree crown. A set of 4 net radiome¬ters were used: one at a fixed position, on the top of the crown, and the remaining 3 mounted on a standing structure that could be moved around the tree to measure Rn fluxes through the inferior and lateral sides. Measurements of Q were made for 8 dif¬ferent days, during the first 3 months of 2006. Night time measurements of Rn were also done, but with the net radiometers at fixed positions around the tree. The meteoro¬logical conditions during the measurements included clear sky and cloudy days, some of which with light rain. Net radiation at the top of the crown accounted for about 72 % of the total energy absorbed by the tree, and this is reflected by the good linear fit between Q and Rn above the crown. Meteorological conditions seem to have some influence on this relationship, as suggested by the differences on the adjusted linear models when total, clear sky, cloudy or rainy data sets were used. The occurrence of rain tends to cause a slight increase in Q in comparison to dry conditions, for identical levels of Rn. Q also shows a strong linear response to solar radiation (Rs), given the dependence of net radiation upon short wave radiation. The same happens with the component of Q received by the top crown surface. However, energy absorbed lat¬erally is much less dependent on Rs, and the inferior component of Q is completely independent of solar radiation. Under conditions when rainfall interception is most likely to occur, i.e. cloudy/rainy days, the daily time-course of Q follows closely those of Rs and Rn, with a maximum of only 75 W m-2 (expressed per unit of leaf area). Similar maximum daily values were observed in other studies with different species but under similar weather conditions. During the night, net radiation should not have a significant spatial variability and Rn around the canopy should be relatively homo¬geneous. Accordingly, night time estimates of Q were obtained from measurements of Rn at fixed positions, which were considered representative of the Rn fluxes around the tree.

The study of heat and mass exchange between the vegetation and its local environment plays a central role in the analysis of plant-atmosphere interactions. These studies can be undertaken at different scales, ranging from individual leaves to isolated trees or even the canopy scale. In each of these cases, heat and mass fluxes depend on the use of adequate values of transfer conductances. Within a broader study on interception loss from a sparse cork and holm oak woodland (montado) of Southern Portugal, aerodynamic conductances were determined for the boundary layers of both leaves (LBL) and the entire canopy.

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.

Evaporation of rainfall intercepted by tree canopies is usually an important part of the overall water balance of forested catchments and there have been many studies dedicated to measuring and modelling rainfall interception loss. These studies have mainly been conducted in dense forests; there have been few studies on the very sparse forests which are common in dry and semi-arid areas. Water resources are scarce in these areas making sparse forests particularly important. Methods for modelling interception loss are thus required to support sustainable water management in those areas. In very sparse forests, trees occur as widely spaced individuals rather than as a continuous forest canopy. We therefore suggest that interception loss for this vegetation type can be more adequately modelled if the overall forest evaporation is derived by scaling up the evaporation from individual trees. The evaporation rate for a single tree can be estimated using a simple Dalton-type diffusion equation for water vapour as long as its surface temperature is known. From theory, this temperature is shown to be dependent upon the available energy and windspeed. However, the surface temperature of a fully saturated tree crown, under rainy conditions, should approach the wet bulb temperature as the radiative energy input to the tree reduces to zero. This was experimentally confirmed from measurements of the radiation balance and surface temperature of an isolated tree crown. Thus, evaporation of intercepted rainfall can be estimated using an equation which only requires knowledge of the air dry and wet bulb temperatures and of the bulk tree-crown aerodynamic conductance. This was taken as the basis of a new approach for modelling interception loss from savanna-type woodland, i.e. by combining the Dalton-type equation with the Gash’s analytical model to estimate interception loss from isolated trees. This modelling approach was tested using data from two Mediterranean savanna-type oak woodlands in southern Portugal. For both sites, simulated interception loss agreed well with the observations indicating the adequacy of this new methodology for modelling interception loss by isolated trees in savanna-type ecosystems. Furthermore, the proposed approach is physically based and requires only a limited amount of data. Interception loss for the entire forest can be estimated by scaling up the evaporation from individual trees accounting for the number of trees per unit area.

The Penman–Monteith equation has been widely used to estimate the maximum evaporation rate (E) from wet/saturated forest canopies, regardless of canopy cover fraction. Forests are then represented as a big leaf and interception loss considered essentially as a one-dimensional process. With increasing forest sparseness the assumptions behind this big leaf approach become questionable. In sparse forests it might be better to model E and interception loss at the tree level assuming that the individual tree crowns behave as wet bulbs (“wet bulb approach”). In this study, and for five different forest types and climate conditions, interception loss measurements were compared to modelled values (Gash’s interception model) based on estimates of E by the Penman–Monteith and the wet bulb approaches. Results show that the wet bulb approach is a good, and less data demanding, alternative to estimate E when the forest canopy is fully ventilated (very sparse forests with a narrow canopy depth). When the canopy is not fully ventilated, the wet bulb approach requires a reduction of leaf area index to the upper, more ventilated parts of the canopy, needing data on the vertical leaf area distribution, which is seldom-available. In such cases, the Penman–Monteith approach seems preferable. Our data also show that canopy cover does not per se allow us to identify if a forest canopy is fully ventilated or not. New methodologies of sensitivity analyses applied to Gash’s model showed that a correct estimate of E is critical for the proper modelling of interception loss.

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.