Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest
Vegetation indices are widely used to derive land surface phenology (LSP). However, due to inconsistent illumination geometries, reflectance varies with solar zenith angles (SZA), which in turn affects the vegetation indices, and thus the derived LSP. To examine the SZA effect on LSP, the MODIS bidi...
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| Format: | Article |
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MDPI AG
2021-10-01
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| Series: | Remote Sensing |
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| Online Access: | https://www.mdpi.com/2072-4292/13/20/4126 |
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| author | Yang Li Ziti Jiao Kaiguang Zhao Yadong Dong Yuyu Zhou Yelu Zeng Haiqing Xu Xiaoning Zhang Tongxi Hu Lei Cui |
| author_facet | Yang Li Ziti Jiao Kaiguang Zhao Yadong Dong Yuyu Zhou Yelu Zeng Haiqing Xu Xiaoning Zhang Tongxi Hu Lei Cui |
| author_sort | Yang Li |
| collection | DOAJ |
| description | Vegetation indices are widely used to derive land surface phenology (LSP). However, due to inconsistent illumination geometries, reflectance varies with solar zenith angles (SZA), which in turn affects the vegetation indices, and thus the derived LSP. To examine the SZA effect on LSP, the MODIS bidirectional reflectance distribution function (BRDF) product and a BRDF model were employed to derive LSPs under several constant SZAs (i.e., 0°, 15°, 30°, 45°, and 60°) in the Harvard Forest, Massachusetts, USA. The LSPs derived under varying SZAs from the MODIS nadir BRDF-adjusted reflectance (NBAR) and MODIS vegetation index products were used as baselines. The results show that with increasing SZA, NDVI increases but EVI decreases. The magnitude of SZA-induced NDVI/EVI changes suggests that EVI is more sensitive to varying SZAs than NDVI. NDVI and EVI are comparable in deriving the start of season (SOS), but EVI is more accurate when deriving the end of season (EOS). Specifically, NDVI/EVI-derived SOSs are relatively close to those derived from ground measurements, with an absolute mean difference of 8.01 days for NDVI-derived SOSs and 9.07 days for EVI-derived SOSs over ten years. However, a considerable lag exists for EOSs derived from vegetation indices, especially from the NDVI time series, with an absolute mean difference of 14.67 days relative to that derived from ground measurements. The SOSs derived from NDVI time series are generally earlier, while those from EVI time series are delayed. In contrast, the EOSs derived from NDVI time series are delayed; those derived from the simulated EVI time series under a fixed illumination geometry are also delayed, but those derived from the products with varying illumination geometries (i.e., MODIS NBAR product and MODIS vegetation index product) are advanced. LSPs derived from varying illumination geometries could lead to a difference spanning from a few days to a month in this case study, which highlights the importance of normalizing the illumination geometry when deriving LSP from NDVI/EVI time series. |
| first_indexed | 2024-03-10T06:13:07Z |
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| id | doaj.art-3fc23742065c4e949391b72da97b700b |
| institution | Directory Open Access Journal |
| issn | 2072-4292 |
| language | English |
| last_indexed | 2024-03-10T06:13:07Z |
| publishDate | 2021-10-01 |
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| spelling | doaj.art-3fc23742065c4e949391b72da97b700b2023-11-22T19:54:35ZengMDPI AGRemote Sensing2072-42922021-10-011320412610.3390/rs13204126Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard ForestYang Li0Ziti Jiao1Kaiguang Zhao2Yadong Dong3Yuyu Zhou4Yelu Zeng5Haiqing Xu6Xiaoning Zhang7Tongxi Hu8Lei Cui9The State Key Laboratory of Remote Sensing Science, Beijing Normal University, Beijing 100875, ChinaThe State Key Laboratory of Remote Sensing Science, Beijing Normal University, Beijing 100875, ChinaEnvironmental Science Graduate Program, The Ohio State University, Columbus, OH 43210, USAThe State Key Laboratory of Remote Sensing Science, Beijing Normal University, Beijing 100875, ChinaDepartment of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011, USADepartment of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305, USASchool of Environment and Natural Resources, The Ohio State University, Columbus, OH 43210, USAThe State Key Laboratory of Remote Sensing Science, Beijing Normal University, Beijing 100875, ChinaEnvironmental Science Graduate Program, The Ohio State University, Columbus, OH 43210, USACollege of Urban and Environmental Sciences, Tianjin Normal University, Tianjin 300387, ChinaVegetation indices are widely used to derive land surface phenology (LSP). However, due to inconsistent illumination geometries, reflectance varies with solar zenith angles (SZA), which in turn affects the vegetation indices, and thus the derived LSP. To examine the SZA effect on LSP, the MODIS bidirectional reflectance distribution function (BRDF) product and a BRDF model were employed to derive LSPs under several constant SZAs (i.e., 0°, 15°, 30°, 45°, and 60°) in the Harvard Forest, Massachusetts, USA. The LSPs derived under varying SZAs from the MODIS nadir BRDF-adjusted reflectance (NBAR) and MODIS vegetation index products were used as baselines. The results show that with increasing SZA, NDVI increases but EVI decreases. The magnitude of SZA-induced NDVI/EVI changes suggests that EVI is more sensitive to varying SZAs than NDVI. NDVI and EVI are comparable in deriving the start of season (SOS), but EVI is more accurate when deriving the end of season (EOS). Specifically, NDVI/EVI-derived SOSs are relatively close to those derived from ground measurements, with an absolute mean difference of 8.01 days for NDVI-derived SOSs and 9.07 days for EVI-derived SOSs over ten years. However, a considerable lag exists for EOSs derived from vegetation indices, especially from the NDVI time series, with an absolute mean difference of 14.67 days relative to that derived from ground measurements. The SOSs derived from NDVI time series are generally earlier, while those from EVI time series are delayed. In contrast, the EOSs derived from NDVI time series are delayed; those derived from the simulated EVI time series under a fixed illumination geometry are also delayed, but those derived from the products with varying illumination geometries (i.e., MODIS NBAR product and MODIS vegetation index product) are advanced. LSPs derived from varying illumination geometries could lead to a difference spanning from a few days to a month in this case study, which highlights the importance of normalizing the illumination geometry when deriving LSP from NDVI/EVI time series.https://www.mdpi.com/2072-4292/13/20/4126land surface phenologysolar zenith angleNDVIEVIBRDF |
| spellingShingle | Yang Li Ziti Jiao Kaiguang Zhao Yadong Dong Yuyu Zhou Yelu Zeng Haiqing Xu Xiaoning Zhang Tongxi Hu Lei Cui Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest Remote Sensing land surface phenology solar zenith angle NDVI EVI BRDF |
| title | Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest |
| title_full | Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest |
| title_fullStr | Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest |
| title_full_unstemmed | Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest |
| title_short | Influence of Varying Solar Zenith Angles on Land Surface Phenology Derived from Vegetation Indices: A Case Study in the Harvard Forest |
| title_sort | influence of varying solar zenith angles on land surface phenology derived from vegetation indices a case study in the harvard forest |
| topic | land surface phenology solar zenith angle NDVI EVI BRDF |
| url | https://www.mdpi.com/2072-4292/13/20/4126 |
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