Gap filling strategies for defensible annual sums of net ecosystem exchange

Abstract

Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (F NEE ) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of F NEE ; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required.We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of F NEE is investigated. The difference between annual F NEE filled by MDV compared to F NEE filled by Regr. ranged from −45 to +200 g C m −2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m −2 per year.We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u * ) shifted annual F NEE on average by +77 g C m −2 per year, but in certain cases as much as +185 g C m −2 per year.Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.

Ozone concentrations, flux and potential effect on yield during wheat growth in the Northwest-Shandong Plain of China

“…Gaps in O 3 concentrations caused by instrument malfunction and power shortage were filled using two methods. If missed data spanned less than 4 sequential gaps, the linear interpolation method was used, otherwise, the mean diurnal variation method was applied (Falge et al, 2001). (Fig.…”

Monitoring the Multi-Year Carbon Balance of a Subarctic Palsa Mire with Micrometeorological Techniques

“…The CH 4 fluxes were gap filled based on the exponential soil temperature relationship presented by Jackowicz-Korczyński et al (2010) while with the CO 2 flux signal two different approaches were applied. The daytime CO 2 fluxes recorded during the growing season were gap filled by applying the light response equation (1) while the rest of the fluxes were parameterized with the night respiration equation (2) (Falge et al 2001).…”

Improved data gap-filling schemes for estimation of net ecosystem exchange in typical East-Asian croplands

“…For both sites, the smaller classes of 8, 4 and 2 K have a similar performance, indicating that it is unnecessary to bin the data into temperature classes smaller than 8 K. This range is larger than the 4 K temperature class used by Falge et al (2001) for a variety of sites including croplands and the 2 K temperature class used by Ruppert et al (2006) for a forest site. They also reported that additional time windows do not significantly improve the temperature binning method, because the existing long-time seasonal temperature response of the long-living and slow-growing coniferous forest is already covered by the time-independent allocation of the values into the temperature bins.…”

“…However, data gaps in the eddy-covariance technique are unavoidable and limit the carbon budget estimate. An average of 35% air or soil temperature, vapor pressure deficit (VPD), and soil water availability (Greco and Baldocchi, 1996;Falge et al, 2001). Amongst all of these, mean diurnal variation, look-up table, and non-linear regression are the most commonly used methods.…”