Evaluation of an enclosed portable chamber to measure crop and pasture actual evapotranspiration at small scale

Abstract

An enclosed portable chamber was constructed and calibrated to measure actual evapotranspiration (ET) from crop and pasture and then evaluated against established methods that are used to determine evapotranspiration. The chamber was equipped with variable speed electric fans to mix the air within the chamber during each ET measurement. The most appropriate fan speed was investigated. Pasture ET measured using the enclosed portable chamber compared well with predicted water loss using the water balance method for a 6-day period during winter 1997 in Armidale (NSW, Australia). Mean cumulative pasture ET for the 6-day period was 5.8 and 5.9 mm measured with the enclosed portable chamber and water balance method, respectively. Wheat crop ET measured using the enclosed portable chamber was compared with that estimated by the Bowen ratio (BR) method for a 2-day period in the early growth stages of the crop. Mean ET using the enclosed portable chamber was 2.4 mm per day compared with 2.3 mm per day using the BR method. Results from the enclosed portable chamber method showed sensitivity to the choice of fan speed. A slow fan speed that produced an air velocity of 2.7 km/h, gave the closest agreement with the Bowen ratio method (2.3 mm per day versus 2.2 mm per day) for the wheat crop. The main attractions of the enclosed portable chamber method include: (1) its suitability for ET measurement within small areas (<1 m 2), which enables ET measurement from individual plant communities within small areas; (2) its main principles of measuring the actual water flux from transpiring vegetation rather than inferring it from climatic parameters; (3) the speed by which an instantaneous ET rate is obtained (less than 1 min); (4) instantaneous ET measurement can be repeated throughout the day from the same plant communities; and (5) the portability of the enclosed portable chamber. ET measurement using the enclosed portable chamber method may be combined with the existing soil water balance models for comparing alternative crop and pasture systems in terms of their water balance.