四种Ci/Ca模型在FVS法分离人工林生态系统蒸散发过程中的适用性评价与优化

Evaluation and Optimization of the Applicability of Four Ci/Ca Models in the FVS Method for Partitioning the Evapotranspiration of Plantation

通量方差相似法(FVS)可基于单层涡度相关法观测数据,直接拆分生态系统蒸散组分。该方法需计算关键参数Ci/Ca(胞间二氧化碳浓度与大气二氧化碳浓度之比),因此,研究构建Ci/Ca模型对FVS拆分蒸散的应用研究具有重要支撑作用。本研究以杨树人工林生态系统为研究对象,采用双层涡度相关法(CEC)计算蒸腾,评价现有Const_Ci模型、Const_k模型、Linear模型和Katul模型共4种Ci/Ca模型,并优化Ci/Ca模型中的关键参数。结果表明:基于CEC拆分的生态系统林木蒸腾和土壤蒸发日变化均表现为明显的单峰曲线,且蒸发峰值出现在蒸腾峰值之前。连续83d拆分结果表现出明显的日内变化特征,日平均TR/ET(蒸腾/蒸散)、日蒸腾和日蒸发分别为0.78、2.46和0.63mm;基于Const_Ci模型、Const_k模型、Linear模型和Katul模型的FVS方法估算的TR模拟值与双层涡度相关法实测值的偏差率分别为59%、-16%、-70%、-31%,MAE分别为0.18、0.069、0.21和0.085mm·h^(-1),RMSE分别为0.25、0.089、0.27和0.10mm·h^(-1)。参数优化后的Katul模型和VPD^(0.26)模型估算冠层蒸腾的精度较原始Katul模型有明显提高,MAE分别为0.048和0.047mm·h^(-1),RMSE分别为0.058和0.057mm·h^(-1);皮尔逊相关系数分别为0.83和0.73。说明Katul优化模型应用于FVS直接分离杨树人工林生态系统蒸散组分具有可行性。

The Flux Variance Similarity(FVS)method can directly decompose the ecosystem evapotranspiration components based on the observation data of the eddy covariance method.This method needs to calculate the key parameter Ci/Ca(ratio of intercellular CO2 concentration to atmospheric CO2 concentration),so the research and construction of the Ci/Ca model plays an important supporting role in the application research of FVS method.In this study,the poplar plantation was taken as the research object,and the transpiration was calculated by the concurrent eddy covariance(CEC)method.Four existing Ci models were inverted,evaluated and optimized:Constant Ci model,Constant Ci/Ca model,Linear model and Katul model.The results showed that the diurnal changes of ecosystem transpiration and soil evaporation based on CEC split showed obvious unimodal curves,and the peak of evaporation appeared before the peak of transpiration.The partitioning results for 83 consecutive days also showed obvious diurnal variation characteristics.TR/ET(transpiration/evaportranspiration),daily transpiration and daily evaporation are 0.78,2.46 and 0.63mm,respectively;linear regression deviation was calculated between TR measured by CEC and simulated of FVS method based on Const_Ci model,Const_k model,Linear model and Katul model.The deviation rates were 59%,−16%,−70%,and−31%,MAE were 0.18,0.069,0.21 and 0.085mm·h^(-1),and RMSE were 0.25,0.089,0.27 and 0.10mm·h^(-1),respectively.Compared with the original Katul model,the optimized Katul model and VPD^(0.26)model after parameter optimization have significantly improved accuracy in estimating canopy transpiration.MAE were 0.048 and 0.047mm·h^(-1),and RMSE were 0.058 and 0.057mm·h^(-1),and Pearson correlation coefficients were 0.83 and 0.73,respectively.It is feasible to apply the optimized Katul model for FVS to directly separate evapotranspiration components of poplar plantation ecosystem.