An Inter Comparison of Fast-Response Hygrometers (Invited Presentation)

Energy balance can change following dramatic disturbance to a plant community. With a loss of canopy, regardless of a crop succumbing to drought or a forest to bark beetles, we hypothesize that ecosystems will trend toward greater energy balance closure as transpiration declines. This motivates the following initial research question: How adequate are the current fast-response hygrometer technologies for eddy covariance measurements of evapotranspiration? To investigate this, we compare seven fast-response hygrometers of both open and closed path designs that were produced during three different eras. These are the oldest technologies (> 20 years) including the closed path Resonance Lyman-alpha and the open-path Campbell Scientific KH2O, the intermediate technologies (~ 15 years) comprising the closed path LI-COR LI7000 and open path LI-COR LI7500, and recent technologies (< 10 years) containing the closed path LI-COR LI7200 and Campbell Scientific EC155 and the open path Campbell Scientific EC150. During three weeks in September-October 2015, a comparison was conducted atop (> 23 m) of the GLEES AmeriFlux scaffold in southeastern Wyoming where a slow-response chilled-mirror hygrometer was employed as an in-situ calibration standard. Preliminary results show that when calibrated against a dew point generator, the closed path analyzers were more accurate in measuring the slow-response changes in ambient humidity (average RMSE = 0.21 mmol/mol) than the open path instruments (average RMSE = 0.86 mmol/mol), with the exception of the Lyman-alpha which was more similar to the open path designs. This performance was improved by in situ calibration against the chilled-mirror hygrometer, after which all hygrometers averaged RMSE = 0.20 mmol/mol. The power spectra were all similar, except for notable high-frequency noise in the oldest hygrometers and some high-frequency dampening in the intermediate aged closed path analyzer.