Analysis of severe droughts in Taiwan using vegetation indices from geostationary satellite observations

One of the emerging concerns of global change is the increasing frequency and intensity of severe droughts. Recently, two record-breaking droughts occurred consecutively in the spring of 2021 and 2023 across Taiwan. Motivated by the need to monitor such extreme events, this study analyzes vegetation indices from Himawari-8/9 satellite observations. The key indices, including Normalized Difference Vegetation Index (NDVI), Vegetation Condition Index (VCI), and Vegetation Health Index (VHI), are computed for subregions of Taiwan to allow an accurate characterization of drought conditions across heterogeneous landscapes. Taking advantage of the high temporal resolution of the data, daily-updated time series of the indices are analyzed to form the framework for the detection of early warning signals. As a local application, the methodology is used to analyze the effect of drought on irrigated rice fields. The result reveals overall unhealthy conditions of the crops, even though the growth of the crops was temporarily sustained through the drought by irrigation.