黄河小浪底人工混交林生长季能量平衡特征

Characteristics of energy balance of a mixed plantation in the Xiaolangdi area in the growing season

利用涡度相关系统和小气候梯度观测系统,对黄河小浪底人工混交林2012年生长季(5—9月)各能量通量进行了连续观测,分析了该生态系统能量平衡各项的变化特征,讨论了能量闭合状况。结果表明:潜热通量、感热通量和土壤热通量均与净辐射有类似的日变化特征。各项的绝对值大小表现为净辐射>潜热通量>感热通量>土壤热通量。受日照时数的影响,5—9月能量平衡各项正值的日持续时间逐渐缩短。生长季,净辐射、感热通量和土壤热通量在6月份最大,最大值分别为418.5、231.4和12.5 MJ m-2month-1);潜热通量在7月份达到最大,最大值为320.8 MJ m-2month-1)。潜热通量、感热通量和土壤热通量占净辐射的比例分别在0.48—0.62、0.15—0.55、0.02—0.05之间。人工混交林生长季的能量分配主要以潜热通量和感热通量为主,且潜热通量为感热通量的2倍。波文比呈单峰曲线:6月最大,8月最小。黄河小浪底人工混交林生长季全天能量闭合度为79%。其中,白天闭合程度较高(81%),夜晚较低(41%)。本研究站点存在21%的能量不闭合。其原因可能与通量源区面积不匹配、忽略冠层热储存、湍流能的相位差等有关。

Energy balance in the terrestrial ecosystem plays an important role in regional climate and water balance. By using the open-path eddy covariance and micrometeorological observation systems, turbulent energy fluxes and available energy were measured in a mixed plantation in the Xiaolangdi area in the North China during the growing season of 2012 （May-September）. The diurnal and monthly variations of energy balance terms were analyzed, and energy closure and Bowen ratio were discussed. The results showed that the diurnal variation of energy balance terms were similar to that of net radiation, and the variation order of energy balance terms was net radiation 〉 latent heat flux 〉 sensible heat flux 〉 soil heat flux. The time of keeping positives for energy balance terms in a day became shorter from May to September as energy balance terms were influenced by sunshine hour. The maximal net radiation occurred in June with a value of 418.5 MJ m^-2 month^-1, and the minimum in September with a value of 309.3 MJ m^-2 month^-1. Latent heat flux peaked in July, with a value of 320.8 MJ m^-2 month^-1. The maximal sensible heat and soil heat fluxes appeared in June, with the values of 231.4 and 12.5 MJ m-2 month-1, respectively. During the growing season, the proportions of latent heat, sensible heat and soil heat fluxes to net radiation ranged between 0.48 and 0.62, 0.15 and 0.55, 0.02 and 0.05, respectively. It is indicated that energy distribution was mainly latent heat and sensible heat fluxes in the growing season of the mixed plantation, and latent heat flux was as twice as sensible heat flux. Bowen ratio （β） can be used to indicate energy exchange between the land surface and the atmosphere. The monthly variation of Bowen ratio was obvious in the growing season, and the maximum and minimum occurred in June and August, respectively. Energy balance closure is regarded as a standard evaluating the reliability of the eddy covariance measurement at FLUXNET sites. Energy imbalance is about 10%-30% in the most FLUXNET sites. For the mixed plantation during the growing season, energy balance closure in the whole day was 79% on a half hourly basis, and with the highest in the daytime and lowest at nighttime. Energy imbalance was about 21% in the Xiaolangdi station in the growing season of 2012. Energy imbalance may result from the complex land surface, the sampling mismatch between the flux footprint and the sensors measuring other components of energy balance, ignoring heat storage of the canopy, the phase difference of turbulent energy.