Optimization Setting Of Eddy Covariance Technique And Its Application In Water Use Efficiency

The eddy covariance method is an important technique for investigating energy and substance exchange between the atmosphere and an ecosystem.However,an inappropriate average period causes inaccurate fluxes and a low energy balance ratio(EBR).(1)Based on flux data over the entire growth stage of maize measured by an eddy covariance system in northern China in 2014,the effects of various average periods on flux and EBR were analyzed.The results indicate:The relative error of the flux between an average period of 10-60 min and the commonly used average period(30 min)was within 3%.When the average period was between 10 and 60 min,the fluxes increased with an increasing average period at various growth stages.When the average period exceeded 60 min,the fluxes fluctuated greatly,particularly at 120 min.The EBR tended to increase within periods of 10-60 min,but it decreased rapidly at various growth stages when the average period was longer than 120 min.As calculated using the Ogive function,the optimal averaging period of seedling-shooting stage and shooting-heading stage is approximately 10-30 min and that of the heading-filling and filling-maturity stages is 30-60 min.(2)Based on the flux data over the entire growth stage of winter wheat measured by an eddy covariance system in Beijing,applying the analytical FSAM and Lagrangian KL models,the flux footprints were analyzed in different atmospheric conditions,different growth stages and different wind directions.Results indicate that:The prevailing wind direction was 270°?360°,and the wind frequency of stable atmospheric condition and unstable atmospheric condition were 42%and 37%respectively in this direction.The flux footprint in prevailing wind direction had differences at different growth stages and different atmospheric conditions.When the atmosphere condition was stable,the upwind range of flux footprint was 16.07-167.05 m.When the atmosphere condition was unstable,the upwind range of flux footprint was 13.73-153.66 m.And the flux footprint in stable atmosphere condition was bigger than that in unstable atmosphere condition.The change of upwind footprint was first increasing then decreasing with the change of growth stages.From comparative analysis of the FSAM and KL model,the largest distances away from the observation point estimate by FSAM model and KL model were 215.8 m and 291.8 m respectively.There were different estimation results between the two kinds of model at different growth stages.(3)Based on the summer corn flux data in the north China plain in 2014,2015 and 2016,the variations of water use efficiency were analyzed at different time scales.The results indicate the following:At daily scales,the flux curves were unimodal,their maximum appeared at around 13:00.Flux variations at different stages were heading-filling stage>shooting-heading stage>filling-maturity stage>seedling-shooting stage;Water use efficiencies were "inverted U" type,and were shooting-heading>heading-filling stage>filling-maturity stage>seedling-shooting stage at different stages;Water use efficiency showed "inverted U" type changed with days,the WUE increased after seedling gradually,reached to the maximum in the end of shooting-heading stage,smoothly changed at heading-filling stage,and decreased in the end of filling-maturity stage;At year scales,the average evaporation ET and dry matter accumulation were:heading-filling stage>shooting-heading stage>filling-maturity stage>seedling-shooting stage,water use efficiencys were:shooting-heading stage>heading-filling stage>filling-maturity stage>seedling-shooting stage at different stages.