Spectral vegetation index (VI) time series data from coarse resolution satellite sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), have been utilized in studying vegetation dynamics. Numerous studies have evaluated how well VI products capture variations in vegetation biophysical or physiological conditions. Equally important is to evaluate VI products over “zero vegetation” surfaces consisting of soils, litters, and/or rocks, as they define the lower bound for vegetation detection. VIs, however, vary over zero vegetation surfaces as a function of soil moisture content and surface roughness. In this study, we evaluated the behavior of VIs from Terra MODIS (T-MODIS), Aqua MODIS (A-MODIS), and Suomi-National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (S-VIIRS) at Railroad Valley Playa, Nevada for a period from April 2013 to September 2019. The playa is a dried lakebed devoid of vegetation throughout the year. Long-term in situ reflectance measurements acquired over the 1 km-by−1 km Radiometric Calibration Test Site (RadCaTS) located on the playa were obtained from the Radiometric Calibration Network (RadCalNet) portal and used as a reference. Three VIs were analyzed, including the normalized difference VI (NDVI), enhanced VI (EVI), and two-band EVI (EVI2). RadCaTS NDVI, EVI, and EVI2 of the playa surface increased and decreased occasionally for the time period examined in this study, and the satellite NDVIs, EVIs, and EVI2s had comparable temporal signatures to the RadCaTS counterparts. T-MODIS and A-MODIS NDVI and EVI2 values were comparable to the RadCaTS counterparts, whereas T-MODIS and A-MODIS EVI values were lower than the RadCaTS counterparts by ~0.006 and ~ 0.01 EVI units, respectively. All the three VIs of S-VIIRS were consistently higher than their RadCaTS counterparts by ~0.008 VI units, due to the higher near-infrared (NIR) reflectances of S-VIIRS than the RadCaTS NIR reflectance. The red and NIR, and red and blue reflectances each formed linear relationships (i.e., soil lines) for each of the three sensors. Variations in reflectance due to surface conditions and observation geometries all appeared as variations along these soil lines. The satellite red-NIR soil lines were comparable to the RadCaTS counterparts, whereas the satellite red-blue soil lines had steeper slopes than the RadCaTS counterparts due to a negative bias in the satellite blue reflectances. This translated into the T-MODIS and A-MODIS EVI behaviors different from those depicted by RadCaTS EVI, and the satellite NDVI and EVI2 behaving more comparably with the RadCaTS counterparts and across the three sensors than the satellite EVI.

来自粗分辨率卫星传感器（例如中分辨率成像光谱仪（MODIS））的光谱植被指数（VI）时间序列数据已用于研究植被动力学。许多研究评估了VI产品如何很好地捕获植被生物物理或生理条件的变化。同样重要的是评估由土壤，垫料和/或岩石组成的“零植被”表面上的VI产品，因为它们定义了植被检测的下限。但是，VI在零植被表面上随土壤水分含量和表面粗糙度的变化而变化。在这项研究中，我们评估了铁路谷Playa的Terra MODIS（T-MODIS），Aqua MODIS（A-MODIS）和Suomi-National极轨合作伙伴可见红外成像辐射计套件（S-VIIRS）的VI的行为，内华达州的时间从2013年4月到2019年9月。普拉亚海滩是干燥的湖床，全年没有植被。长期原位可从辐射校准网络（RadCalNet）门户获取位于海滩上的1 km x -1 km辐射校准测试站点（RadCaTS）上获得的反射率测量值，并将其用作参考。分析了三个VI，包括归一化差异VI（NDVI），增强型VI（EVI）和两波段EVI（EVI2）。在这项研究中，Playa表面的RadCaTS NDVI，EVI和EVI2偶尔增加和减少，并且卫星NDVI，EVI和EVI2具有与RadCaTS对应物相当的时间特征。T-MODIS和A-MODIS的NDVI和EVI2值与RadCaTS对应物相当，而T-MODIS和A-MODIS的EVI值分别比RadCaTS对应物低〜0.006和〜0.01 EVI单位。由于S-VIIRS的近红外（NIR）反射率高于RadCaTS NIR反射率，因此S-VIIRS的所有三个VI始终比其RadCaTS对应物高出〜0.008 VI单位。红色和NIR以及红色和蓝色反射率均形成线性关系（（即土壤线）的三个传感器。由于表面条件和观测几何形状而引起的反射率变化均表现为沿这些土壤线的变化。卫星红色近红外土壤线与RadCaTS对应线相当，而卫星红色蓝色土壤线的倾斜度比RadCaTS对应线陡，这是由于卫星蓝色反射率存在负偏差。这转化为与RadCaTS EVI所描绘的行为不同的T-MODIS和A-MODIS EVI行为，并且与卫星EVI相比，卫星NDVI和EVI2与RadCaTS对应物在三个传感器上的行为更为相似。