Energy balance and closure of typical winter wheat farmland ecosystem in the North China Plain

The accurate quantification of energy and mass exchange between terrestrial ecosystem and the atmosphere is important for water resources management and sustainable agricultural development. Energy balance closure is also a vital index for assessing the accuracy of measurements data and analyzing surface energy balance. In order to evaluate energy balance and energy closure in farmland ecosystems, the open eddy covariance system and total factor automatic weather station were used to observe continuous surface energy flux and conventional meteorological elements of typical winter wheat in farmland ecosystems in the North China Plain for the period 2013-2014. In the study, four typical growth stages (seeding, overwintering, jointing and grain-filling stages) were investigated for diurnal and annual variations in energy flux of winter wheat. Also the diurnal variations in Bowen ratio for four typical growth stages were calculated. The results showed that the trends in diurnal variations in net radiation and energy component of the four growing stages of winter wheat were unimodal in shape. The peak values of net radiation, sensible heat flux and latent heat flux were observed between 12:00 and 13:00. The maximum value of sensible heat flux was at 11:30 and the peak value of soil heat flux occurred between 14:00 and 15:00; which was about 1.00 hour later than sensible heat flux and latent heat flux. For annual variation, the trends in net radiation and latent heat flux were strongly consistent. The minimum values of net radiation and latent heat flux at overwintering stage were 114.51 W·m^-2 and 13.47 W·m^-2, respectively. However, sensible heat flux at overwintering stage was relatively higher than latent heat flux. Sensible heat flux and soil heat flux were respectively 33.61 W·m^-2 and 13.05 W·m^-2. The maximum values of net radiation and latent heat flux were observed at grain-filling stage, with respective values of 327.02 W·m^-2 and 116.56 W·m^-2. After winter wheat harvest, sensible heat flux increased rapidly, while latent heat flux rapidly decreased. Energy closures of representative observation dates selected for the four growing stages were also good. The energy closure ratios were 0.49, 0.77, 0.81 and 0.76, respectively. The energy closure ratio was high in summer and relatively low in winter. Diurnal variations in Bowen ratio during the four growing stages had an inverted U-type curve. While Bowen ratio values for daytime were relatively stable, those for nighttime were negative and relatively unstable. The Bowen ratio curves agreed well with sensible heat flux curve. Bowen ratio was positive when sensible heat flux was positive and it increased with sensible heat flux. The maximum value of Bowen ratio at seeding stage (2.12) occurred at 14:00, while the maximum values of Bowen ratio at overwintering (1.48), jointing (0.31) and grain-filling stages (0.58) all occurred at 10:00. The results of the study set the basis for research on heat and water vapor fluxes in farmland ecosystems in the North China Plain.