Article

Some considerations on the behaviour of the Piche evaporimeter

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Abstract

L'influence de la radiation, de la conduction de la chaleur, du vent et de l'humidité de l'air sur l'évaporation d'eau par un évaporomètre de Piche, est décrite quantitativement. Une théorie sur l'évaporation d'une surface mouillée soumise à la radiation, élaborée par Penman, a servi de base pour ces considérations. On a obtenu expérimentalement pour les rondelles depaier vert, employées avec le Piche, un coefficient de réflection de 0.2 pour le rayonnement à courte longueur d'ondes. Cette valeur représente une moyenne satisfaisante du coefficient de réflexion d'un tapis végétal vert, telle une prairie par exemple. L'influence de la conduction de la chaleur dans l'instrument de Piche, est faible. L'influence du vent est exprimée dans la description théorique par le coefficient de transmission de la chaleur de la surface évaporante. La relation entre ces deux grandeurs est déduite des données expérimentales sur la base de la théorie. Le résultat est représenté à la figure 2. En s'appuyant sur quelques exemples, les auteurs ont montré que la théorie permet d'interpréter des phénomènes observés quantitativement (comparez les tableaux 1 et 2). En comparant les données de l'évaporomètre de Piche avec l'évaporation d'un tapis végétal, et pour autant que cette évaporation est influencée par les conditions atmosphériques (le contrôle de l'évaporation par des causes physiologiques ou autres est écarté), il résulte de la théorie, que le Piche n'est pas assez sensible à l'action de la radiation, mais par contre réagit trop fortement sous l'influence du vent.

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