A comparison of daily representations of canopy conductance based on two conditional time- averaging methods and the dependence of daily conductance on environmental factors

Dans les modeles hydrologiques qui prennent en compte les surfaces vegetales, les estimations de la conductance du couvert pour la vapeur d'eau faites a partir des mesures de flux de seve et de la demande climatique peuvent conduire a des valeurs instantanees et des moyennes journalieres erronees, a cause d'un regime de flux hydrique non conservatif. Un calcul de la conductance base sur les moyennes journalieres de certaines variables est une methode pour resoudre ce probleme. Toutefois, la comparaison entre les deux methodes n'avait pas encore ete effectuee. Ce travail compare ces deux methodes de calcul de la conductance de couvert pour une foret de pins. L'equation de Penman-Monteith a ete simplifiee, en supposant un fort couplage entre le couvert et l'atmosphere, et utilisee ici pour calculer les valeurs instantanees de conductance de couvert. Un ecart important entre les deux methodes a ete mis en evidence, a cause des biais apparaissant en conditions de flux de seve ou de deficit de saturation de l'air faibles. Deux procedures de moyennage conditionnel ont ete developpees pour exclure ce type de donnees, et une relation etroite entre les deux methodes a ete trouvee. La conductance de couvert journaliere a ete ainsi calculee sur une saison de vegetation complete dans un peuplement de Pinus taeda âge de 15 ans. En depit de la diminution de conductance entre les episodes de pluie, conduisant a des deficits hydriques dans le sol, et de variations importantes d'indice foliaire au cours de la saison, la conductance du couvert est restee uniforme jusqu'aux basses temperatures de la fin de la saison.

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