The Characteristics Of Turbulence Fluxes In The Near Surface Layer Over The Wheat Field In Huaihe River Basin

The features of surface albedo, water and heat flux, CO2 flux, bulk transfer coefficients and energy balance in different underlying surface and growing period have been investigated based on the data of turbulent fluxes observed by Eddy Covariance method, radiation balance components and profile data over cropland in Huaihe River Basin from Sept.2007 to Aug.2008 in order to complete insight into the features of turbulent transfers over the cropland ecosystem. The seasonal and diurnal variation of the interaction between surface and atmosphere were also elucidated from these data. Furthermore, the sensible heat, latent heat and momentum fluxes are calculated by the eddy correlation and vertical gradient method are compared. The main results can be listed as follows:(1)It is found that diurnal and seasonal variation features of wind speed, temperature and humidity in different height is obvious, Wind velocity at daytime is larger than night which increase with elevate observation height. The air temperature decrease as elevate observation height at daytime and temperature inversion state appear at night, Humidity decrease frequently as elevate observation height, but humidity inversion state is also observed. The diurnal and seasonal variation features of soil temperature from 0cm layer to 10cm layer is obvious, however, only monthly variation regulation exist surpass that depth. The soil humidity of cropland is dependent on precipitate and underlying surface state.(2)The analysis of radiation balance components and energy flux in Huaihe River basin shows that: the range and pattern variation of radiation balance components and energy flux are influenced by the change of season, weather and underlying surface state. Annual total solar radiation amount in this area reaches around 4878.8 MJ·m-2, indicated that the Huaihe River basin has an abundance heat resource. We also found that allocation of net radiation among various energy and surface albedo were strongly influenced by underlying surface state. The heat fluxes of sensible and latent are mainly budget items of net radiation. The largest ratio of the sensible heat flux to the net radiation is about 30% to 50% on bare soil. The Latent heat flux is a prevailing item of the net radiation over the cropland, which is 50% to 80% of net radiation, whereas soil heat flux account for less than 10%. The results from the surface albedo investigation suggest that: bare field< wheat field< paddy field < snow field. The bulk transfer coefficients of momentum is larger than the bulk transfer coefficients of sensible heat at the same observation height. To some extent, CO2 flux is determined by underlying surface state, and the variation of CO2 flux with different underlying surface and crop growing stage is obvious. The net CO2 assimilation reached maximum value of 2.86 g/(m2·h) in jointing stage during the crop growing period, and study further shows that water, heat and CO2 flux vertical transportation was in favor of near neutral conditions.(3)The analysis of the surface energy balance shows that the phenomenon of energy imbalance exists over cropland. The average energy closure rate was 0.91 in bald field, whereas only 0.46 in snow field and the study shows that: bare field> wheat field> paddy field> snow field during the observed period. The average energy closure rate was 0.89 during the observed period, which 0.78 at daytime and 0.24 at night. The energy in-closure of cropland was influenced by energy imbalance at night, the ignorance of some energy item and turbulent intensity impact on turbulent transfers. During the observation period, the turbulent fluxes(LE+H)that observed by eddy covariance method always less then available energy( Rn ? G),which indicated that turbulent fluxes might be underestimated.(4)Sensible heat, latent heat, momentum flux observed by vertical gradient method was consistent with eddy covariance method. The multiple correlation coefficients between data acquired by these two methods were greater than 0.6 and reliability test indicated that there was a fine correlation between two methods.(5)Using Z-test at 0.05 level to analysis the difference of EC data before and after interpolate by MDV method, it shows that the difference of CO2 positive value region was obvious.