Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs?

article

Pereira, F.L.
Gash, J.H.C.
David, J.S.
Valente, F.

Pereira, F.L.; Gash, J.H.C.; David, J.S.; Valente, F., 2009. Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs? Agricultural and Forest Meteorology, 149(3-4): 667-679. DOI: 10.1016/j.agrformet.2008.10.013

10.1016/j.agrformet.2008.10.013

Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs?

Evaporation
Rainfall interception
Isolated trees
Quercus ilex
Wet bulb temperature
Surface temperature

This study was supported by research project SAPIENS/FCT NO. POCTI/AGG/ 39220/2001 funded by the Fundação para a Ciência e Tecnologia. Funding was also provided by the Conselho de Reitores das Universidades Portuguesas and the British Council under the Treaty of Windsor Programme 2007/08 – U27.

2022-02-01T11:17:41Z
2022-02-01T11:17:41Z
2009

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.
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