A Single-Beam Infrared Hygrometer for Evaporation Measurement

Abstract

Two instruments are described which serve as humidity sensors in conjunction with existing eddy correlation techniques. The first instrument is an infrared absorption device, with a 40 cm path length, operating in a water vapor vibrational band at 6.3 μm. The second instrument is a development of this, operating at 2.7 μm with a 20 cm path length. Both devices have been successfully field-tested in a latent heat flux format, using a propeller anemometer as a vertical velocity sensor. Satisfactory energy balances at the surface have been obtained. In the case of the 2.7 μm instrument, a specific advantage is its lack of sensitivity to ambient humidity levels, while both instruments are insensitive to slow variations of optical and electronic performance.