Towards pairing plot and field scale measurements in managed ecosystems: Using eddy covariance to cross-validate CO2 fluxes modeled from manual chamber campaigns

The modeled seasonal course of daily NEE agreed well with the eddy covariance measurements for all five setups (R2 from 0.77 to 0.92) but with periods of systematic offsets in the range of ±5 g C m−2 day−1. Though the pattern of the offsets was different, all setups had comparable root mean square errors around 1.5 g C m−2 day-1 despite having opposite limitations. Cross-validation by simulating campaigns with artificial gaps from the continuous eddy dataset in setup 4) and 5) resulted in bias errors of around 0.4 g C m−2 day−1. This translates to a total uncertainty on annual NEE of around ±175 g C m−2 a−1 purely from the modeling, i.e. the interpolation in-between campaigns. By leave-one-campaign-out scenarios, the sensitivity to single campaigns was examined. The mean effect on the annual total was higher for setup 4 (30 g C m−2) with the original number of campaigns than for setup 5 (9 g C m−2) with four times more campaigns. Furthermore, the interpolation in-between the campaigns can be improved by deriving vegetation proxies from the continuous eddy covariance measurements, such as an effective green area index (GAI) presented herein.