Investigating the Effects of Drought on the Vegetation Activity Using Remote Sensing Data: A Case Study at Chichi-jima, Ogasawara (Bonin) Islands

The effects of drought on vegetation activity at Chichi-jima, Ogasawara(Bonin)Islands from 2015 to 2018 were investigated based on remote sensing data. Chichi-jima suffered from severe periods of drought in 2016-2017 and 2018-2019. The Normalized Difference Vegetation Index(NDVI)was calculated using data from Band 4(Red)and Band 5(Near Infrared)of the Landsat-8 Operational Land Imager. Even at Chichi-jima, which is located in the subtropics, the NDVI exhibited seasonal variations. The range of the NDVI(maximum minus minimum) is approximately 0.11 when averaged at the watersheds of reservoirs supplying drinking water. During drought periods in September 2016 and 2018, the NDVI values decreased compared to those of September in the other years, which were less affected by drought. The decrease in the NDVI is approximately 0.04, which corresponds to approximately one-third of the seasonal variations in the NDVI. Although the absolute value of this decrease is subtle, it is relatively large considering seasonal variations. From a difference map of the NDVI between September 2016 (during the drought)and September 2015(before the drought), it is observed that the decrease in the NDVI occurred across all of Chichi-jima. When comparing the vegetation map of Chichi-jima, the decrease in the NDVI did not occur in a specific species or region. This implies that the effects of drought appeared in the vegetation of Chichi-jima as a whole.