亚热带-暖温带过渡区天然栎林的能量平衡特征

利用开路涡度相关系统和常规气象观测仪器观测了我国北亚热带-暖温带气候过渡带(河南南阳)的一片锐齿栎天然林的能量通量及常规气象。以一个完整年(2016年10月—2017年9月)的观测数据为依据,定量分析了此锐齿栎林的能量通量的日变化、季节变化以及各能量分量的分配特征,并计算了能量闭合度以及波文比。结果表明:锐齿栎林观测期间一整年净辐射为2626.17 MJ/m~2,感热通量为867.1 MJ/m~2,潜热通量为1417.25 MJ/m~2,土壤热通量为-2.60 MJ/m~2,土壤为热源;各能量分量日变化基本呈单峰型曲线,季节变化特征明显。非生长季,锐齿栎林的能量主要分配给感热通量,占净辐射的54.18%;生长季,能量主要分配给潜热通量,占净辐射的67.48%。观测期间研究区年降雨量较平均值稍大(1231.8 mm),森林蒸散量为579 mm,仅为降雨量的47%。波文比受森林物候变化影响较大,在非生长季平均值约为2.1,生长季约为0.2。土壤热通量在生长季2017年4—9月份为正值,土壤表现为热汇,其余月份皆为热源。土壤热通量的变化过程主要受净辐射调控,森林物候也起了一定的作用。河南宝天曼锐齿栎森林通量观测站全年能量闭合度为67%,在国际同类观测站的范围之内(55%—99%)。不能完全闭合的原因可能与通量源区面积不匹配、计算能量平衡时忽略冠层热存储等有关。

Energy balance characteristics of a natural oak forest (Quercus aliena) at a transitional area from a subtropical to warm temperate climate, China

In the lower atmospheric planetary boundary layer, turbulent transport is an important connection for substance and energy exchange between the atmosphere and earth''s surface. Currently it is a focus of earth system science. As a part of the ChinaFlux program, the components of energy balance fluxes were measured continuously over an oak(Quercus aliena) natural forest at a transitional climatic zone in Central China since 2009. Using an open-path eddy covariance system and micro-climate instruments, the net radiation, sensitive heat flux, latent heat flux, soil heat flux, air temperature, soil temperature, and precipitation were measured in the oak forest in Henan Province from October 2016 to September 2017. The diurnal and monthly variation of energy balance, as well as the distribution pattern of each component were analyzed, and the energy closure and Bowen ratio were also calculated. As an important index to evaluate the reliability of eddy covariance measurements, energy balance analysis has been widely accepted by the FLUXNET community. Using the ordinary least squares (OLR) method, energy balance characteristics of the oak forest were systematically analyzed. The results showed that the annual net radiation of the forest ecosystem was 2626.17 MJ/m², and the latent heat flux, sensitive heat flux, and soil heat flux were 1417.25 MJ/m², 867.1 MJ/m², and -2.60 MJ/m², respectively, suggesting that soil was a heat source. The energy balance components had obvious seasonal variations, whereas diurnal variations of energy balance components showed a single peak curve. In the dormant season, the turbulent energy exchange was dominated by an upward sensible heat flux, accounting for 54.2% of the available energy; whereas in the growth season, turbulent energy exchange was dominated by an upward latent heat flux, accounting for 67% of the available energy. The annual rainfall in the study area was slightly higher than the average (1231.8 mm); compared with 579 mm of evapotranspiration at the same time period, the results showed that only 47% of precipitation was returned to the atmosphere through evapotranspiration. The Bowen ratio was more affected by the forest phenology, with an average value of about 2.1 and 0.2 in the dormant and growing season, respectively. Soil heat flux during the growth season was positive, suggesting that soil was a heat sink, whereas it was a heat source during the dormant season. The process of soil heat exchange was mainly controlled by the net radiation, whereas the phenology of the forest also plays a role. Energy balance closure is regarded as a standard, evaluating the reliability of the eddy covariance measurement at FLUXNET sites. Energy balance closure in the Baotianman station was 67% on a half hourly basis, which fell in the range of the most FLUXNET sites (55%-99%), indicating that the flux data was basically reliable. Energy imbalance may result from the complex land surface, sampling mismatch between the flux footprint and sensors measuring other components of energy balance, negligence on the canopy heat storage, etc.