华北平原典型冬小麦农田生态系统能量平衡与闭合研究

准确量化分析地气之间的物质和能量交换对于水资源管理和农业可持续发展是十分重要的。能量平衡闭合是评估观测数据准确性和分析地表能量平衡的一个重要的评价指数。本研究利用开路涡度相关系统和全要素自动气象站对华北平原典型冬小麦农田生态系统2013—2014年度的能量通量及常规气象要素进行了连续观测,分析了冬小麦农田各能量通量的日变化和年变化特征,计算冬小麦在4个生育时期(出苗期、越冬期、拔节期和灌浆期)的能量闭合和波文比。结果表明:在日尺度上,选取的4个生育时期净辐射和各能量分量的日变化趋势均为单峰二次曲线,净辐射、显热通量和潜热通量的峰值出现在12:00—13:00,土壤热通量的峰值出现在14:00—15:00。在年尺度上,净辐射和潜热通量的变化趋势较为一致,均在越冬期达到最低值114.51 W·m~(-2)和13.47 W·m~(-2),而在灌浆期达到最大值327.02 W·m~(-2)和116.56 W·m~(-2)。选取的4个生育时期的代表性观测日期能量闭合良好,能量闭合率分别为0.49、0.77、0.81和0.76。4个生育时期内波文比值日变化趋势均呈倒"U"型,出苗期波文比在14:00达到最大值2.12;越冬期、拔节期和灌浆期在10:00左右达到最大值,分别为1.48、0.31和0.58。本文的定量化结果可为华北平原农田生态系统水热通量等研究提供依据。

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.