Evaluation on Energy Balance of Farmland in Shouxian County of Anhui Province

Energy balance means the balance between the sum of latent heat, sensible heat flux observed by Eddy Covariance (EC) technique and the variance of net radiation flux, soil heat, canopy heat. According to the first law of thermodynamics and basic assumption of EC observation, the ratio of energy balance can be used as an effective evidence to evaluate data quality and system performance of an observation system theoretically. Energy balance ratio of the field has been investigated based on the data of flux observation system on near surface layer observed at the representative station of Huaihe River Basin, Shouxian National Climate Observatory from September 2007 to August 2008 in order to assess operation status of this new system. The results show that the annual average energy balance ratio is 0.89 during the period, and it is higher in the day than that in the night, higher in spring, summer, autumn than that in winter, higher over the bare surface than that over the cropland surface, while different weather conditions make little differences. The largest energy balance ratio is 0.92 over the bare surface, and smaller ratio is 0.9 over wheat field and paddy field, while it's only 0.50 over snow surface. It also shows that energy balance is influenced by the negligence of heat reserve during exuberant growing period to some extent and absorption tem, such as thaw, freeze and sublimation. During the observation period, the observed turbulent fluxes are always less than available energy, indicating that turbulent fluxes might be underestimated. The energy balance during daytime seems better than that at night because of stronger turbulence. It can be concluded that the applicability of EC method is best in flat, bare field, better in the day than that in the night, better on the underlying surface with low vegetation than that covered with snow. All in all, the phenomenon of energy imbalance is fairly common in flux observation and it often reaches 10% to 30%. The annual energy balance ratio (0.89) of Shouxian National Climate Observatory is close to the lower limit of ranges, so the EC method of flux observation over farmland ecosystem in Huaihe River Basin is reliable.