Atmometer

Abstract: Two theoretical approaches to evaporation from saturated surfaces are outlined, the first being on an aerodynamic basis in which evaporation is regarded as due to turbulent transport of vapour by a process of eddy diffusion, and the second being on an energy basis in which evaporation is regarded as one of the ways of degrading incoming radiation. Neither approach is new, but a combination is suggested that eliminates the parameter measured with most difficulty—surface temperature—and provides for the first time an opportunity to make theoretical estimates of evaporation rates from standard meteorological data, estimates that can be retrospective. Experimental work to test these theories shows that the aerodynamic approach is not adequate and an empirical expression, previously obtained in America, is a better description of evaporation from open water. The energy balance is found to be quite successful. Evaporation rates from wet bare soil and from turf with an adequate supply of water are obtained as fractions of that from open water, the fraction for turf showing a seasonal change attributed to the annual cycle of length of daylight. Finally, the experimental results are applied to data published elsewhere and it is shown that a satisfactory account can be given of open water evaporation at four widely spaced sites in America and Europe, the results for bare soil receive a reasonable check in India, and application of the results for turf shows good agreement with estimates of evaporation from catchment areas in the British Isles.

Abstract: As was first pointed out in Publication 50 of the Carnegie Institution (i906), it is only by correcting the variations in the transpiration rate to uniform conditions of evaporation (that is, to a uniform evaporating power of the air), that anything approaching quantitative information concerning the seemingly almost autonomous changes in the rate of water loss from plants may be had. To accomplish this correction it is only necessary to consider the march of the ratio of the rate of transpiration to that of evaporation, the latter determined by means of some form of atmometer. This ratio has been termed relative transpiration; it denotes simply the number of atmometers of the form used that would be necessary to evaporate the same amount of water as is lost by the transpiring plant in the same time and at the same place. In other terms, relative transpiration is a measure of the equivalent or effective evaporating surface of the plant as this varies from time to time, the unit of evaporating surface being unit area of free water surface under properly defined conditions, or any other evaporating surface which may be adequately defined. Reference to the nine graphs of relative transpiration presented in the publication just mentioned, and to the accompanying discussions, brings out the fact that the maximum of the evaporating power of the air (the evaporation rate from the porous cup atmometer in this instance) always occurred, in the cases cited, somewhat later in the day than did the maximum of relative transpiration (the ratio of transpiration rate to that of evaporation). This was interpreted to mean that some internal change had taken place in the leaves, which had begun to retard water loss even while the evaporating power of the air had still continued to increase. Such

Abstract: Because of the large area occupied by a class A pan, alternative methods have been sought to estimate reference evapotranspiration (Eto) inside greenhouses. The objective of this work was to compare ETo estimated by different methods inside and outside a greenhouse. A class A pan (CAPi), a reduced pan (RPi) and an atmometer (Ai) were installed inside a greenhouse, and another class A pan (CAPo) was installed outside. ETo estimates, obtained by CAPi, RPi, and Ai were 56%, 69% and 63% of those estimated by CAPo, respectively. A simple linear regression showed positive coefficients R = 0.94 for the RPi and the CAPi, R = 0.91 for the Ai and the CAPi, R = 0.70 for the CAPi and the CAPo, R = 0.66 for the RPi and the CAPo, and R = 0.62 for the Ai and the CAPo. ETo needs to be estimated inside greenhouses and it is possible to use reduced pans or atmometers to estimate the ETo inside the greenhouse. Equipment replacement would increase the available space inside the greenhouse.