土壤背景对冠层NDVI的影响分析

归一化差值植被指数NDVI是植被遥感中应用最为广泛的指数之一, 但它受土壤背景等因素的干扰比较强烈。结合实测的土壤数据以及公式推导、PROSAIL 模型模拟等方法分析了这种影响。首先, 假定与土壤线性混合且叶片呈水平分布的植被冠层, 根据土壤与植被分别在红光、近红外波段处的反射率值、植被覆盖度等参数, 利用公式推导了土壤背景对不同覆盖度下冠层NDVI的影响。其次, 利用PROSAIL冠层光谱模拟模型, 模拟分析了土壤背景对不同LAI下冠层NDVI的影响。分析的结果表明:LAI 越小, 土壤背景的影响越大; 暗土壤背景下的冠层NDVI值大于亮土壤背景下冠层的NDVI值; 并且,暗土壤条件下,NDVI值对土壤亮度的变化更敏感,而亮土壤下,NDVI值则对LAI或覆盖度的变化更敏感。最后利用实测的不同土壤背景下的冬小麦冠层光谱数据, 验证了公式推导和模型模拟的结果。

Analys is on the Influence of Soil Backgrounds on Canopy NDVI

Normalized Difference Vegetation Index (NDVI) is one of the earliest vegetation indices that have been widely used in vegetation remote sensing. It can respond to changes in amount of green biomass,chlorophyll content, and canopy water status etc. However, NDVI value might not reflect the truth of vegetation canopy because it is disturbed intensively by soil background and other factors. In this study,the effect of soil background on canopy NDVI was analyzed by measured soil data, theoretic derivation and PROSAIL model simulation. First, we supposed that a vegetation canopy with horizontally distributed
leaves was linearly mixed with soil background, the influences of soil backgrounds on NDVI in different vegetation coverages were analyzed according to the equation based on red reflectance, near-IR reflectance of soils and vegetations, and vegetation coverages. Second, the influence of soil background on NDVI in different LAI was simulated and analyzed by PROSAIL model. The results indicated: ①the influence of soil backgrounds was larger when LAI was small, and the canopy NDVI with a dark soil background was bigger than it with a bright soil background. ②NDVI with a dark soil background was more sensitive to the change of soil brightness, but the canopy NDVI with a bright soil background was more sensitive to the change of LAI or vegetation coverages. Finally, the results were validated using measured wheat canopy spectrum data in different soil backgrounds.