Abstract: In this paper, the energy balance closure of eddy covariance measurements and the effects of land surface conditions on it were analyzed in a winter wheat/summer maize double cropping system in the North China Plain from November 2003 to October 2006. The results showed that energy balance ratios (EBR) varied diurnally and the fluctuation was great in the turn of day and night. EBR in the afternoon was slightly higher than that in the morning.The energy balance closure was obviously better in the daytime than that in the nighttime.The energy balance closure was improved by shifting the phase of the turbulent fluxes (LE+H) half〖KG-*2〗-〖KG-*3〗hourly ahead. Daytime EBR varied seasonally.It was higher in autumn and winter (0.98 and 0.94,respectively) but lower in the spring and summer (0.85 and 0.70,respectively).The seasonal variability of daytime EBR was in the order of summer>autumn>winter>spring.Land surface status had significant impacts on EBR.EBR was 1.11, 0.94 and 0.74 for the periods of bare soil,winter wheat and summer maize respectively.Energy balance closure was better in the winter wheat field than that in summer maize field.The variability of EBR was in the order of bare soil>maize>wheat.Annual mean daytime EBR varied from 0.82 to 0.97 with an average of 0.89,indicating a satisfied data quality in the eddy covariance measurements in the farmland.

Key words: Eddy covariance, Energy balance ratio, Winter wheat period, Summer maize period