Use of change-point detection for friction–velocity threshold evaluation in eddy-covariance studies

The eddy-covariance method often underestimates fluxes under stable, low-wind conditions at night when turbulence is not well developed. The most common approach to resolve the problem of nighttime flux underestimation is to identify and remove the deficit periods using friction–velocity (u*) threshold filters (u*Th). This study modifies an accepted method for u*Th evaluation by incorporating change-point-detection techniques. The original and modified methods are evaluated at 38 sites as part of the North American Carbon Program (NACP) site-level synthesis. At most sites, the modified method produced u*Th estimates that were higher and less variable than the original method. It also provided an objective method to identify sites that lacked a u*Th response. The modified u*Th estimates were robust and comparable among years. Inter-annual u*Th differences were small, so that a single u*Th value was warranted at most sites. No variation in the u*Th was observed by time of day (dusk versus mid or late night), however, a few sites showed significant u*Th variation with time of year. Among-site variation in the u*Th was strongly related to canopy height and the mean annual nighttime u*. The modified u*Th estimates excluded a high fraction of nighttime data – 61% on average. However, the negative impact of the high exclusion rate on annual net ecosystem production (NEP) was small compared to the larger impact of underestimating the u*Th. Compared to the original method, the higher u*Th estimates from the modified method caused a mean 8% reduction in annual NEP across all site-years, and a mean 7% increase in total ecosystem respiration (Re). The modified method also reduced the u*Th-related uncertainties in annual NEP and Re by more than 50%. These results support the use of u*Th filters as a pragmatic solution to a complex problem.