A NEW EVAPORIMETER FOR USE IN FOREST STUDIES

Abstract

SYNOPSIS This article treats of the subject of evaporation or transpiration from plants, of the factors which influence it, and of the conditions which must be met before water losses from plants can be approximately determined through instrumental evaporation. It is pointed out that the ?evaporation stress? or tendency to evaporate, is produced by a different combination of factors in each body from which evaporation may occur. Because of the fact that the vaporizing process in leaves takes place on the surface of moist cell walls which are not directly exposed to the moving air, the theory would lead us to expect that the effect of wind would be greatly minimized in evaporation from leaves, the rate of diffusion of the vapor being almost completely controlled by vapor pressures in the leaf (inter-cellular) spaces. On the other hand, the leaf is admirably adapted fro absorbing the sun rays of all wave lengths; hence evaporation from leaves will be more directly controlled by the supply of radiant energy than perhaps will that from a body which does not absorb so readily and which may obtain considerable heat from the air, especially if a strong wind brings new supplies of air rapidly to the evaporating surface. The article then describes the efforts which were made to devise an instrument having about the same relation to wind and to radiant energy as do leaves of plants in general. The idea of an ?inner cell? for the vaporizing process, rather than a freely exposed moist surface, was the basis for these efforts. The result was a very practical metallic instrument known as the ?Type 4 evaporimeter,? whose behavior and operation are fully described. The essential feature of this instrument is a moist layer of linen between two metal plates, the upper of which protects the which protects the wick from rain, is coated with lampback and transmits absorbed heat to the wick; the lower plate is thick and contains a number of small perforations simulating the stomata of leaves. Vapor formed in the moist linen escapes through these perforations. The layer of linen is above a well-insulated tank, from which it is fed by a stem wick. Distilled water is used and evaporation losses are obtained by weighing before and after exposure. It is shown that considering either a large number of daily periods having a variety of weather conditions, or shorter periods at different times of the day, the evaporation from the Type 4 evaporimeter parallels the total transpiration of 12 small coniferous trees more closely than does the evaporation from other instruments commonly used in ecological study, or the earlier types which led up to Type 4. ?The others show wider variation (from the plants) about in proportion to the degree in which they expose the evaporating surface to moving air, and fail to absorb fully the heat of sunlight.? This parallelism between plants and the new instruments comprises the sole technical argument in favour of its use, at the same time demonstrating the correctness of the theory on which the instrument was constructed. On the other hand, the new instrument has an apparently objectionable future in exposing a horizontal surface to evaporation. This, it is shown, may possibly be an advantage, if, as it appears, activity becomes less as the season advances, and the instrument at the same time exposes its absorbing surfaces less squarely to the sun's rays. In the closing paragraphs the practical features of the instruments are stressed, and precautions in its use are given.