涡度相关观测的能量闭合状况及其对农田蒸散测定的影响

Energy balance closure and its effects on evapotranspiration measurements with the eddy covariance technique in a cropland

涡度相关法被认为是测定农田蒸散量的标准方法。然而,能量不闭合现象在涡度相关测量中普遍存在。分析能量不闭合对涡度相关观测的影响,对于提高涡度相关观测精度具有重要意义。以蒸渗仪法为参照,探讨涡度相关观测的能量闭合状况对农田蒸散测定的影响,在导致涡度相关观测能量不闭合的诸多因素中,寻找对蒸散测定有影响的因素。结果表明:涡度相关观测的白天能量平衡比率(EBR)呈秋冬高、春夏低的变化特征,麦季日均EBR范围在0.26—2.84之间,平均1.15;玉米季日均EBR范围在0.19—2.59之间,平均0.78。无论麦季或玉米季,涡度相关法测定的平均蒸散量(ETec)均明显低于蒸渗仪法观测值(ETL),但两者显著相关(P<0.01),并有相似的季节变化。平均蒸散比(ETec/ETL)麦季约为0.61,玉米季约为0.50。在冬小麦田和夏玉米田,ETec/ETL均与EBR显著相关(P<0.01)。麦田种植密度大,下垫面较均匀,蒸散比与EBR成正比(P<0.01),且不受叶面积指数(LAI)大小影响;反之,玉米田种植密度小,只有当LAI>1,下垫面变得较均匀后,蒸散比与EBR的关系才变得显著(P<0.01)。风速小时ETec/ETL与EBR显著相关,风速增加时二者相关性减弱。尤其在玉米田,当摩擦风速(u*)大于0.3 m/s时,ETec/ETL与EBR的相关性不再显著。风速小时,大气湍流微弱,湍流的涡旋较大。在有限的观测时段(0.5h)内,涡度相关仪的传感器难以捕捉足够的湍涡能量,所测湍流能量偏低,导致能量不闭合。以上结果为应用能量平衡比率校正农田蒸散提供了可能途径。

The eddy covariance （EC） technique is generally regarded as a standard method for crop evapotranspiration measurements. However, the imbalance of energy closure prevails in the EC observations. Evaluating the effect of energy balance closure on EC measurement is critical for improving the accuracy of this method. In this paper, a weighting method （the Lysimeter method ） was used as reference to evaluate the effect of energy balance ratio （EBR） on EC evapotranspiration. The results revealed that daytime EBR varied seasonally in the field, where wheat and maize were rotated in the winter and summer, respectively. The EBR was higher in autumn and winter but lower in the spring and summer. For the wheat field, mean daytime EBR varied from 0.26 to 2.84 with an average of 1.15. As for the maize field, EBR varied from 0.19 to 2.59 with a mean value of 0.78. The evapotranspiration （ET） using EC （ETec） was clearly lower than derived by Lysimeter （ETL）. The mean ratio of EToo to ETL（ ETec/ETL） was 0.61 and 0.50 during the wheat growing season and the maize season, respectively. The ET observed with these two approaches significantly correlated with each other （P 〈 0. 01 ）, with their characteristics of seasonal variation performing in a similar manner. The ETec/ETL was found to be proportional to EBR （P 〈 0.01 ） in both the winter wheat field and the summer maize field. Furthermore, the effect of the leaf area index （LAI） on the relationship between ET ratios and EBR was significant in the even crop field during the entire growing season of winter wheat and the maize growing stage when the LAI was higher than 1. However, the effect was insignificant in the uneven maize field when the LAI was less than 1. On the other hand, friction velocity （ u ＊ ） exerted a strong impact on EBR and its relationship with ET ratios. EBR was observed to be proportional to u ＊ in both the winter wheat field and the summer maize field. ET ratios were proportional to EBR when u＊ was small. Nonetheless, the correlation between these two parameters showed recession while u＊ increasing, especially in summer maize field, ETec/ETL was not significantly correlated with EBR when u＊ was bigger than 0.3 m/s. Due to the weak turbulent intensity and the large eddy under the condition of low friction velocity, the EC sensor could not catch enough turbulent energy in a limited observation period （30 minutes）, which caused lower measured turbulent fluxes and imbalanced energy closure. The results offer a possible way to correct the underestimated ETo using EBR.