Energy Balance Closure And Its Effects On Evapotranspiration Measurements With The Eddy Covariance Technique In A Mixed Plantation In Xiaolangdi Area

Energy balance in the terrestrial ecosystem plays an important role in regional climate and water balance. By using the open-path eddy covariance and micrometeorological observation systems, turbulent energy fluxes and available energy were measured in a mixed plantation in the Xiaolangdi area in the North China during the growing season of 2012 (May-September) and the evapotranspiration were measured during the period 2011.11-2012.10 by the two methods. The diurnal and monthly variations of energy balance terms were analyzed, energy closure and Bowen ratio were discussed,and the evapotranspiration were discussed.The results showed that:(1) The diurnal variation of energy balance terms were similar to that net radiation, and the variation order of energy balance terms was net radiation>latent heat flux>sensible heat flux>soil heat flux. The time of keeping positives for energy balance terms in a day became shorter from May to September as energy balance terms were influenced by sunshine hour. The maximal net radiation occurred in June with a value of 418.5 MJ·m-2·month-1, and the minimum in September with a value of 309.3 MJ·m-2·month-1. Latent heat flux peaked in July, with a value of 320.8 MJ·m-2·month-1. The maximal sensible heat and soil heat fluxes appeared in June, with the values of 231.4 and 12.5 MJ·m-2·month-1, respectively. During the growing season, the proportions of latent heat, sensible heat and soil heat fluxes to net radiation ranged between 0.48 and 0.62,0.15 and 0.55,0.02 and 0.05, respectively. It is indicated that energy distribution was mainly latent heat and sensible heat fluxes in the growing season of the mixed plantation, and latent heat flux was as twice as sensible heat flux. Bowen ratio (β) can be used to indicate energy exchange between the land surface and the atmosphere. The monthly variation of Bowen ratio was obvious in the growing season, and the maximum and minimum occurred in June and August, respectively. Energy balance closure is regarded as a standard evaluating the reliability of the eddy covariance measurement at FLUXNET sites.(2) Energy imbalance is about 10%～30% in the most FLUXNET sites. For the mixed plantation during the growing season, energy balance closure in the whole day was 79% on a half hourly basis, and with the highest in the daytime and lowest at nighttime. Energy imbalance was about 21% in the Xiaolangdi station in the growing season of 2012. Energy imbalance may result from the complex land surface, the sampling mismatch between the flux footprint and the sensors measuring other components of energy balance, ignoring heat storage of the canopy, the phase difference of turbulent energy. By shifting the phase of the turbulent fluxes(LE+H) half an hour ahead, the energy closure condition was improve.(3) The diurnal and seasonal variation of ETec and ETb(ETec and ETb were ET measured using EC and BREB methods, respectively) were obvious, and performed in a similar way in the period(2011.11-2012.10).ET observed with these two methods were significantly correlated with each others in daily and monthly time sscales.ET had significantly correlation with solao radiation(Rn) and air temperature (T) in both daily and monthly scales.