| In the context of September 2006-August 2010 eddy covariance data of the Semi-Arid Climate Change and Environment Observatory, Lanzhou University (SACOL), an intensive study is performed to analyze the exchange characteristics of energy and matter in air-land interaction over the native grassland on Semi-Arid Loess Plateau of Northwest China. Specially emphasizing on the issue of energy balance unclosure in observation experiment of land surface, not only the condition of energy balance unclosure and all the factors affecting it have been analyzed in detail by using the eddy covariance data, but also the model of atmospheric motion in the ideal and the actual status and the transport of matter and energy in surface layer is summarized theoretically. The following are the main conclusions:(1) The data obtained by SACOL site can well represent the climate characteristic of this region both at time and space by analyzing the different meteorological elements and footprint; further more, by evaluating and controlling the quality of eddy covariance data, the valid data are 77.5%,75.4%,68.3% and 51.9% in spring, summer, autumn and winter, respectively.(2) By using the EdiRe software to deal with the data, we found that the reasonable average interval of the eddy covariance data at SACOL site is 30min.(3) The turbulence intensity over the native grassland underlying surface on Semi-Arid Loess Plateau satisfied the general law Iu＞Iv＞Iw；the dimensionless standard deviation of w and u components in all seasons are closed to the coefficients of homogeneous surface, while the v component is relatively larger than homogeneous surface.(4) The energy balance closure (EBC) is closely related with the flux contribution from the target area, large-scale eddy, calculation of surface soil heat flux, atmospheric stability and friction velocity, in different seasons these factors have different effect on EBC, but more is the uniform. With the flux contribution from the target area increasing, the energy balance closure was enhanced, and this factor was associated with the underlying surface heterogeneity but independent on season. On the SACOL sites, the sensible heat flux of average time,30min, in spring, summer, autumn and winter included all the data of turbulence spectrum (case 1) by and large, and the radios were 77.7%,75.4%,69.4% and 60.3%, respectively, the radios for latent heat flux were 56.0%,54.5%,56.7% and 42.0%. The low frequency part had relatively little effect in spring and summer but large in autumn and winter, and the latent heat flux underestimated more than sensible heat flux; Furthermore, increasing the average time interval can improve the EBC; the calculation of soil surface heat flux have significant effect to EBC, but also independent on season. The atmospheric stability is of great importance to EBC, on the whole, the EBC gradually decreased with the stability change from unstable by neutral to stable in each season. EBC is well in all seasons and stable in daytime and not change with friction velocity, but increased with friction velocity in nighttime.(5) This paper theoretically summarized the characteristics of atmospheric motion and the transport of energy and matter in surface layer, as well as the ideal and the actual model. Based on it and deduced that if the atmosphere motion is more close to the turbulence, then the closure of energy in the surface layer calculated and observed by present theory is well. On the view of the atmosphere motion itself, defined the relative vertical turbulence intensity RIw to characterize the strength of atmospheric turbulence, so as to study the impacts of atmospheric motion states on energy balance closure. By using the SACOL sites data, it proved that the actual observed degree of energy balance closure on surface layer was determined by the state of atmospheric motion. Overall, EBC was increased with the increasing degree of turbulent mixing. After the data classifying of quality control, it showed that although the available data and calculation methods of surface soil heat flux can affect the energy balance closure, it cannot change the essence that the state of atmospheric motion determined the EBC degree on surface layer. In addition, the energy balance closure changing with the strength of atmospheric turbulence had little effect by the season. When the relative vertical turbulence intensity was around 0.16 RIw, the EBC reached best and closed to stability.(6) All the energy balance components on Semi-arid Loess Plateau have obvious characteristics of diurnal and seasonal variation, and they have reached the maximum at noon in different seasons. Bowen ratio also has obvious diurnal variation which shows as "n" reaching the highest value at noon, and when day and night is alternate, it will be relatively small. Sensible heat flux in the dry seasons was dominant in energy balance, the average Bowen radio was 5.0 in winter and 2.0 in spring; in wet seasons, sensible heat flux and latent heat flux had a considerable effect on energy balance, and the average Bowen radio was around 1.0 in summer and autumn. In the four seasons of spring, summer, autumn and winter, the residual of the energy (the radio of the residual and available energy) during the day were 13.5%,21.6%,23.2% and 32.9%, respectively; the night were 74.2%,76.0%73.4% and 72.9%. Energy balance closure has obvious seasonal variation, and it became worse from spring to winter. All seasons'situation of energy balance closure had obvious diurnal variation which changed as the same trend. When the OLS slope value was big, the energy closure condition was good, but it was bad at night.(7) Although the energy balance unclosure is obvious, by analyzing the seasonal characteristics and the short-term transfer features of energy balance on surface layer at SACOL station, the observation at SACOL station is successful, and the observation results of turbulence on surface layer can represent the land-air exchange characteristics of underlying surface over the native grassland on Semi-Arid Loess Plateau of Northwest China.(8) When the atmosphere is near-neutral stratification, the momentum transfer coefficients CDN in spring, summer, autumn and winter were 7.71×10-3,6.63×10-3, 7.20×10-3 and 7.55×10-3, respectively; the heat transfer coefficients CHN were 3.41×10-3,3.76×10-3,3.82×10-3 and 2.60×10-3, respectively.
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