Abstract: Based on the measurement of the latent and sensible heat flux by open-path eddy covariance(OPEC) system and meteorological observation system in Horqin grassland,a semi-arid sandy grassland in Inner Mongolia,during 1st and 26th Sept.2007,the status of energy balance closure was evaluated and the relationship between the diurnal variations of grassland latent heat flux and meteorological factors was analyzed.The results showed that the energy balance closure was 82.7%,which suggested that the latent heat flux and sensible heat flux measured by OPEC system at the grassland site were reasonable and the result was accorded to the range of literature reported energy closure measurements.The latent heat flux was linearly related to the net radiation by the 30 minutes mean value.The latent heat flux was higher in the daytime than at night and reached at the peak value at noon,while the daily peak of the latent heat flux and the net radiation appeared at same time.The peak value of the temperature and the vapor pressure deficit had a two to three hours lag behind the peak value of the latent heat flux.The relative humidity at the peak value of the latent heat flux was the lowest within a day.The average total daily latent heat flux during the growing period was 5.44MJ·m-2,which was equivalent to 2.21mm evapotranspiration.The total daily latent heat flux had an exponential relationship with the total daily net radiation.

Key words: Semi-arid sandy grassland, Semi-arid sandy grassland, Eenergy balance, Latent heat flux, Eddy covariance method