Spatial variation of net radiation and its contribution to energy balance closures in grassland ecosystems

Introduction: Low energy balance closure (EBC) at a particular eddy-covariance flux site has increased the uncertainties of carbon, water, and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks. Methods: A series of manipulative experiments were conducted in this study to explore the role of net radiation (Rn) in the EBC in relation to spatial variability of vegetation characteristics, source area, and sensor type in three sites of the Inner Mongolian grassland of northern China. Results: At all three sites, the residual fluxes of EBC peaked consistently at 110 W m. The spatial variability in net radiation was 19 W m (5% of Rn) during the day and 7 W m −2 (16%) at night, with an average of 13 W m (11%) from eight plot measurements across the three sites. Large area measurements of Rn significantly increased by 9 W m during the day and decreased by 4 W m at night in the unclipped treatments. Net radiation decreased by 25 W m (6% of Rn) at midday and 81 MJ m −2 (6%) during a growing season with heavier regular clipping than that in unclipped treatments. The Rn was lower by 11–21 W m −2 (~20–40% of Rn) measured by CNR1 than by Q7.1 at night, while there was only 6 W m (~1–2% of Rn) difference during the daytime between these two types of commonly used net radiometers. Conclusions: Overall, the inclusion of the uncertainty in available energy accounted for 65% of the ~110 W m shortfalls in the lack of closure. Clearly, the unclosed energy balance at these three grassland sites remains significant, with unexplored mechanisms for future research.