Sensitivity of Seven MODIS Vegetation Indices to BRDF Effects during the Amazonian Dry Season

Caio Arlanche Petri,Lênio Soares Galvão

doi:10.3390/rs11141650

Abstract

We used Moderate Resolution Imaging Spectroradiometer (MODIS) data, processed by the multi–angle implementation of atmospheric correction (MAIAC) algorithm, to investigate the sensitivity of seven vegetation indices (VIs) to bidirectional reflectance distribution function (BRDF) effects in the dry season (June–September) of the Brazilian Amazon. The analysis was first performed over three sites, located from north to south of the Amazon, and then extended into the entire region. We inspected for differences in viewing–illumination parameters and pixel quality retrievals during MODIS data acquisition over the region. By comparing and correlating corrected and non–corrected data for bidirectional effects, we evaluated monthly changes in reflectance and VIs (2000–2014). Finally, we computed the effect size of the BRDF correction using non–parametric Mann–Whitney tests and Cohen’s r metrics. The results showed that the most anisotropic VIs were the enhanced vegetation index (EVI), photochemical reflectance index (PRI), and shortwave infrared normalized difference (SWND). These VIs presented the largest relative changes and the lowest correlation coefficients, between corrected and non–corrected data, because of the large effect size of the BRDF. The least anisotropic VI was the normalized difference water index (NDWI). The anisotropy of these VIs was stronger in the northern Amazon. It increased from the beginning to the end of the dry season, following changes in the relative azimuth angle (RAA) toward the BRDF hotspot in September. The modifications in the relative proportions of backscattering observations used in composite products caused a reflectance increase in all MODIS bands at the end of the dry season, especially in the near infrared (NIR). The reflectance decreased after BRDF correction. Because of the atmospheric effects, the view zenith angle (VZA) of the pixels selected in composite products decreased toward the south of the Amazon. In the southern Amazon, the seasonal amplitude in the solar zenith angle (SZA) reached values close to 18°. For the most anisotropic index, the BRDF correction removed, on average, 30% of the EVI signal in June, and 60% of the EVI signal in September, reducing dry season variations over time. The results reinforce the need for bidirectional correction of MODIS data before the seasonal and inter–annual analyses of the most anisotropic VIs.

Highlights

Phenological studies on tropical forests of the Amazon are very important to understand the vegetation response to climatic variations over time, carbon cycling, eco–physiological and hydrologic processes, and land–atmosphere interactions [1,2,3,4]
We focused our analysis on the dry season (June to September), a more favorable period of satellite observations in the Amazon, when important changes in vegetation phenology have been reported in the literature
view zenith angle (VZA) values higher than 25◦ produce bidirectional reflectance distribution function (BRDF) effects, but this viewing parameter is generally constrained in composite products during pixel selection

Summary

Introduction

Phenological studies on tropical forests of the Amazon are very important to understand the vegetation response to climatic variations over time, carbon cycling, eco–physiological and hydrologic processes, and land–atmosphere interactions [1,2,3,4]. While large FOV instruments have the advantage of increasing the revisit time of the scene (almost daily observations), they have the disadvantage of acquiring data that are much more sensitive to bidirectional effects than the data obtained by nadir viewing sensors [6]. In Amazonian tropical forests, BRDF effects are still present even in MODIS VI composite products because of the adverse conditions of the atmosphere to allow retrievals of nadir observations [9]

Results

Discussion

Conclusion