小麦LAI-2000观测值对辐亮度变化的响应

利用LAI-2000植物冠层分析仪和ASD光谱仪,通过固定点拔节期冬小麦叶面积指数(LAI)观测实验和同步光谱辐亮度实验,测量了晴天条件下,固定点冬小麦从中午至傍晚24个不同时刻的LAI值及对应的辐亮度。继之分析了此段时间内测得的定点冬小麦LAI值分别与对应时刻可见光和近红外谱段的天空光辐亮度、总辐亮度和太阳直射辐亮度值之间的相关性。结果表明,无论波长小于490nm的谱段,还是波长大于490 nm的谱段,LAI与天空光辐亮度、总辐亮度和太阳直射辐亮度都呈负相关,相关系数(R2)高达0.8左右;尤其LAI与天空光辐亮度的负相关性最高,这种相关性随着波长的增大而减小。LAI与各谱段天空光辐亮度的相关性特征可为LAI-2000晴天观测的LAI值归一化修正处理提供一种新思路和技术途径,以消除太阳直射光的影响,从而解决LAI-2000只能在晴天观测的局限性,拓展LAI-2000在晴天观测条件下的适用性。

Response of change of wheat LAI measured with LAI-2000 to the radiance

The radiance from the sun and the sky is varying with solar altitude angle, and the solar altitude angle is varying with time. That is to say the radiance is varying with time. The LAI-2000 measures the canopy light interception at five view angles using a "fisheye" optical sensor. From all the above, it can be inferred that the LAI is related to the radiance. And then, a fixed plot observation experiment of winter wheat was conducted from 12:15 to 17:40 on sunny with LAI-2000 plant canopy analyzer and ASD spectrometer to explain that inference. In this experiment, the wheat LAI and the radiance were measured continuously at 24 different moments which were corresponding to 24 different solar altitude angles. And then the wheat LAI measured at different moments was computed with FV-2200. The total radiance and sky radiance was got by ASD spectrometer. After that, the direct solar radiance was got by the total radiance subtracting the sky radiance. Next, the radiance in 380-430 nm spectrum region, 430-470 nm spectrum region, 470-490 nm spectrum region, 500-560 nm spectrum region, 620-760 nm spectrum region and 760-1000 nm spectrum region were extracted. At last, the correlation between wheat LAI and the sky radiance, the total radiance and the direct solar radiance was separately analyzed at different spectrum regions. The results of the study showed that either in spectrum regions below 490 nm or above 490 nm, the R² between LAI and the sky radiance, the total radiance and the direct solar radiance were all high which were up to 0.8. In addition, the R² between LAI and the sky radiance was highest among each spectrum regions, while the R² between LAI and the direct solar was lowest. Moreover, the R² between LAI and the sky radiance decreased with the increasing of wavelength, while the R² between LAI and the total radiance and the direct solar radiance increased with the increasing of wavelength. And the results of the study also showed that LAI-2000 would be affected greatly by the direct sunlight and caused large error on sunny condition. Therefore, it is necessary that the LAI measured on sunny condition should be corrected. This research provided a thought and a technological approach for the normalization and correction of the LAI measured with LAI-2000 on sunny condition. And then the influence of the direct sunlight could be eliminated, which was benefit of getting over the limits of weather condition for LAI-2000's use, and the suitability of using LAI-2000 under sunny condition could also be expanded.