Publications

Brede, Benjamin; Suomalainen, Juha; Bartholomeus, Harm; Herold, Martin (2015). Influence of solar zenith angle on the enhanced vegetation index of a Guyanese rainforest. REMOTE SENSING LETTERS, 6(12), 972-981.

Abstract
In this study, the effect of solar zenith angle (theta(s)) on enhanced vegetation index ( EVI) of a Guyanese tropical rainforest was studied. For this sub-crown resolution, hyperspectral data have been collected with an unmanned aerial vehicle ( UAV) at five different solar zenith angles in a 1-day period. The hyperspectral data were used to simulate Moderate Resolution Imaging Spectroradiometer ( MODIS) spectral bands and generate EVI. The linear trend of EVI with solar zenith angle at nadir viewing conditions was found to be 0.00285 1/degrees ( p < 0: 001). The direction of this trend was in agreement with earlier studies, but with a differing magnitude. Analysis of EVI images with sub-crown resolution pointed to strong influence of canopy shadows on EVI, which is supported by other studies. Additionally, the EVI-solar zenith angle trend was investigated in the semiempirical RossThick- LiSparse- Reciprocal ( RTLSR) model implemented in the MODIS MCD43 product suite. A database of model parameters has been created and the EVI-solar zenith angle trend was modelled with each set of parameters. The linear approximated trend was found to be -0.00219 1/degrees on average, only slightly weaker compared to the trend derived from the UAV. Further analysis of the relationship between the single RTLSR model parameters and the EVI- solar zenith angle trend showed that the RTLSR produces the trend for the right reason, namely canopy shadowing expressed by the near-infrared geometric kernel. In total, this study delivers further evidence that EVI is dependent on solar zenith angle and this effect is mediated through EVI's sensitivity to within-canopy shadows.

DOI:
10.1080/2150704X.2015.1089362

ISSN:
2150-704X