基于SAILH模型的植被冠层NDVI二向性分析

Analysis of NDVI Bidirectionality Based on SAILH Model

NDVI是植被遥感中最为常用的一种植被指数,建立NDVI与其他冠层参数模型必须考虑其方向性问题。本文基于SAILH模型讨论了连续植被冠层NDVI的二向性特征,并分析了叶面积指数、叶倾角分布、热点参数以及太阳天顶角和相对方位角对NDVI的影响。研究表明冠层NDVI在主平面观测方向存在一个明显的负热点,前向散射方向的NDVI大于后向散射方向;冠层NDVI的二向性分布随外界参数的改变而发生变化,但每个参数对NDVI二向性的影响程度不同,并且通常后向散射方向受各参数的影响要小于前向散射方向。

NDVI（Normalized Difference Vegetation Index） is an important vegetation index,extensively used to indicate vegetation status.But due to its anisotropy or bidirectionality,there are many limitations when to develop the model between NDVI and vegetation parameters and the precision of model is low.In order to develop reliable model,it is necessary to know about the bidirectionality of NDVI.SAILH model can model BRDF of vegetation canopy,so based on the BRDF of canopy the bidirectional distribution of NDVI can be calculated.The result shows that NDVI has very obvious bidirectionality or anisotropy and it can be influenced by different parameters such as LAI（Leaf Area Index）,LAD（Leaf Angle Distribution）,hot parameter,solar zenith and relative azimuth.The difference between bidirectional distribution of NDVI and BRDF is that NDVI has the smallest value in the hot spot,but canopy BRDF is contrary.In the principal plane,the value of NDVI in forward scattering direction is larger than in backward scattering direction.With the increase of LAI,the NDVI increases,but saturation occurs.When LAI is constant,other parameters can change the value and angle distribution of NDVI.Because the hot spot effect only occurs when observer is in the principal plane,bidirectionality of NDVI will weaken in other planes.The increase of solar zenith makes NDVI lower in the hot spot.The increase of average leaf inclination angle makes NDVI increase in forward scattering direction,while NDVI in backward scattering changes very little.