Abstract:

Eddy covariance (EC) technique has become a standard method to measure the carbon, water vapor and energy flux in the heterogeneous area, and the energy balance closure (EBC) is an important mean to test the reliability of EC technique. At the same time the EBC is an important content in the flux research. To evaluate the EBC, the methods of linear regression, the energy balance ratio (EBR) and the energy imbalance standard deviation were adopted in this paper. In this study site the sum of available energy and turbulent energy in a whole year were 2 798.28 and 2 332.70 MJ·m^-2, respectively. In the whole year, the EBR was 65%, and the values of each month were 52%～76%, the minimum was appeared in January, and maximum in June. Generally, the EBR in summer was better than that in winter, and in daytime, which could reach 80%, it was better than that in nighttime. For the EBR varied with the friction velocity, it concluded that there had a significant relationship between the turbulent fluctuation and EBR. The energy closure increased with friction velocity increased when the latter was less than 0.4 m·s^-1. In many case studies the ratio of EBR is always less than 1, it indicate that the turbulent energy is underestimated and the available energy is adverse, from this point, it may educe that the CO₂ flux is underestimate under the weak turbulent condition and low photosynthesis activate radiation condition. The main reasons which resulted in the imbalance of EBC were the advection ignored and the difference of footprint between turbulent energy measurement and available energy measurement.

Key words: eddy-covariance technique, energy balance closure, available energy, turbulent energy