真实性检验是评价遥感反演产品质量和验证遥感应用产品是否准确、真实地反映实际情况的重要途径.叶面积指数(LAI)是表征陆地植被结构和长势的关键参数,全面准确评价和验证LAI产品是产品用于陆面过程模型的前提.本文以MODIS LAI与GLASS LAI产品为研究对象,在尺度效应和尺度转换的基础上,建立了针对非均匀像元的低分辨率LAI产品真实性检验方法.在考虑空间异质性和植被长势差异的情况下,借助中分辨率的遥感影像,分别利用1 km像元平均叶面积指数和反演表观叶面积指数实现了对LAI算法和产品的真实性检验.为了比较作物长势差异和地表非均匀度对产品的影响,本文选择有代表性的河南鹤壁和甘肃张掖两个地区进行两种LAI产品真实性检验研究.研究结果表明,GLASS LAI和MODIS LAI产品均存在明显的低估现象.这并不是产品算法的问题,而是由于地表异质性和非均匀度的影响.在异质性更显著的张掖盈科灌区,低估现象更明显.GLASS LAI产品是多种LAI产品的融合,它的平均LAI比MODIS更接近真实情况,但是LAI的动态范围比MODIS窄.

Validation is one of the most important processes used to evaluate whether remotely sensed products can accurately reflect land surface configuration. Leaf Area Index (LAI) is a key parameter that represents vegetation canopy structures and growth conditions. Accurate evaluation of LAI products is the basis for applying them to land surface models. In this study, validation methods of coarse resolution MODIS and GLASS LAI products for heterogeneous pixels are established on the basis of the scaling effect and the scaling transformation. Considering spatial heterogeneity and growth difference, we transformed LAI from field measurements into a 1 km resolution scale with the aid of middle resolution images. We used average LAI and apparent LAI separately to validate the algorithms and products of MODIS and GLASS LAI. Two study areas, Hebi City and the Yingke Oasis, were selected for validation. Both MODIS and GLASS LAI products underestimate the true LAI in crop area. However, this result cannot be completely attributed to their algorithms. Instead, the primary reason is the heterogeneity and nonuniformity of the coarse pixels. Underestimation is evident in the Yingke Oasis, where heterogeneity is significant. Given that GLASS LAI product is the fusion of multiple LAI products, the mean value of this product is closer to the real situation, but the dynamic range is narrower than that of MODIS LAI product.