An Improved Evaporimeter

Abstract

Brigogs and Shantz ('i6) have shown that evaporation from a free water surface most closely approximates the transpiration curve and hence this method of measurement is most satisfactory for ecological studies. Evaporation pans such as have been used present certain inherent difficulties. Among these are the lack of accuracy, tinder field conditions, in measuring the amount of water lost; the fact that air currents and hence evaporation are markedly modified as the water level decreases; and the impossibility of accurately correcting for rainfall if the water level is maintained near the top of the pan. The instrument herein described, which has been perfected and experimentally tested at the Alpine Laboratory, has obviated these difficulties without sacrificing durability or portability. In the text figure, A is a side elevation and B a top elevation showing the relative positions of the component parts. The instrument consists essentially of an evaporation pan (9) with an adjustable rain overflow (8) and a sealed reservoir (3) with an air intake (2) and a water connection (5) leading through a petcock (6) into the evaporation pan. A constant water level is maintained in the evaporation pan, and any increase due to rainfall promptly runs off. Two types of construction are possible. The instrument may be made from galvanized iron, 20 gauge, except for the base which is i6 gauge, or brass tubing of the proper sizes may be obtained from a plumbing supply house. The latter is more economical and satisfactory because of the exactness of dimensions, the greatly lowered labor cost, and increased durability. The water reservoir (3) has inside dimensions of two and one-half inches by twenty four inches. Circular pieces of sheet brass soldered flush with the ends of the tube close the top and bottom. A hole is cut in the top to receive a threaded piece of filling tube from an atutomiobile radiator. This should be of large diameter to permit easy measurement and filling, and as short as will allow the cap to be tightened. The fitting shotild have the male threads on the tube and the female in the cap to permit the insertion of a gasket to make an air-tight seal. The air intake (2) is one-eighth inch in diameter. It is curved at the top to prevent the entrance of dirt and water, and is soldered to the side of the reservoir to hold it firmly in position. It enters the reservoir near the bottom, and is bent in such a way that the lower end rests on the bottom. This end should be cut obliquely to permit the ready breaking of bubbles with minimum resistance, that is, the top side 288