植被二向反射特征对叶绿素荧光遥感反演的影响与校正模型

Solar-induced chlorophyll fluorescence (SIF) has become a powerful tool for the monitoring of photosynthesis by remote sensing. Accurate observation and retrieval of the SIF signal is the basis for the application of SIF. However, there are still lots of uncertainties in the algorithms for SIF retrieval to date. This project will focus on the effect of the canopy bi-directional reflectance on the retrieval of SIF, which is still an unsolved scientific issue. In this project, we will: analyze the interaction between the BRDF effect and the SIF emission at the oxygen absorption band; explain the mechanism of the artificial in-filling effect at the oxygen band by the BRDF effect; analyze the relationship between the in-filling effect and the reflectance of direct/diffuse radiation, the diffuse-to-global radiation ratio, and canopy structure parameters; optimize the methods for the estimation of the difference between the reflectance of direct and diffuse radiation under natural conditions; and hence, establish the correction model for the BRDF effect on the SIF retrieval. The outcomes of the project will be effective on improving the SIF retrieval accuracy at the oxygen absorption band, and will be able to provide more accurate and reliable data source for the application of SIF.

日光诱导叶绿素荧光是植被光合作用遥感监测的有力工具，荧光的精确观测与反演是其应用研究的基础保障，但目前的荧光遥感反演算法还存在诸多不确定性。项目将聚焦植被二向反射特征对叶绿素荧光遥感反演的影响这一有待解决的科学问题，研究植被BRDF效应与叶绿素荧光在氧气吸收波段的耦合机制，阐明植被BRDF效应对氧气吸收波段虚假“填充”效应的产生机理，研究植被直射光/漫射光反射率差异、漫射光比例、冠层结构等参数对氧气吸收波段虚假“填充”效应的影响，基于半经验BRDF模型分析优化自然条件下植被直射光/漫射光反射率差异的估算方法，并在此基础上构建植被BRDF效应对荧光反演影响的校正模型。研究成果将可有效改进基于氧气吸收波段的荧光反演精度，为荧光应用研究提供更为准确可靠的数据源。

日光诱导叶绿素荧光是植被光合作用遥感监测的有力工具，荧光的精确观测与反演是其应用研究的基础保障，但目前的荧光遥感反演算法还存在诸多不确定性。本项目针对植被二向反射特征导致的氧气吸收波段虚假“填充”效应、冠层荧光方向性发射特征等对荧光反演带来的不确定性问题，开展了以下三个方面的研究工作：1）开展了多角度冠层光谱模拟实验和长时序塔基叶绿素荧光光谱观测实验，并开展了塔基观测大气校正与荧光反演研究，为研究冠层内部荧光辐射传输过程、植被二向反射特征对荧光遥感反演的影响等科学问题积累了基础数据集；2）开展了植被二向反射特征与叶绿素荧光在氧气吸收波段的耦合机制研究，分析了氧气吸收波段虚假“填充”效应的影响因素及其测量与估算方法，构建了考虑植被二向反射特征影响的叶绿素荧光遥感反演校正模型，改进了光照条件变化情况下氧气吸收波段的叶绿素荧光反演精度；3）开展了基于光谱不变理论的叶绿素荧光冠层辐射传输机理研究，构建了冠层到叶片/光系统尺度的荧光尺度转换模型，校正了冠层内部吸收与散射作用对荧光信号的影响，将冠层尺度观测的方向性荧光信号转换为叶片内部发射的总的荧光信号，有效减少了荧光遥感研究面临的不确定性。上述研究成果为提高叶绿素荧光遥感反演精度、阐明荧光与光合作用的定量关系奠定了基础。