Inter-annual drought monitoring in northern mixed grasslands by a revised vegetation health index from historical Landsat imagery

Abstract

Drought is defined as a natural phenomenon with comprehensive influences on ecosystems, economies, and the livelihood and health of people resident in the affected regions. Severe drought has affected the grassland ecosystem in the Canadian prairie region with increased frequency. Thus, it is essential to monitor the drought effects. With various temporal, spectral, and spatial resolutions, remote sensing data can detect vegetation's dynamic responses to drought. The objective of this study was to monitor inter-annual drought in the northern mixed grassland, over the last thirty-six years by using remote sensing approach through a revised vegetation health index (VHIr), which combined the vegetation condition index (VCI), the temperature condition index (TCI), and the dead fuel condition index (DCI) together. Historical Landsat imagery was explored to establish the VHIr and this index was tested in the Grasslands National Parks (GNP), a typical conserved northern mixed grasslands region. The results suggested that the VHIr can distinguish dry and wet years effectively and a VHIr map was able to depict the spatial distribution pattern of the drought severity. Compared with the original vegetation health index (VHI), VHIr was more correlated with the dead-to-green biomass ratio. Meanwhile, the spatial pattern indicated that the valley grasslands had relatively low drought sensitivity; the upland grasslands experienced severe to extreme drought severity during dry years, due to a large amount of dead vegetation existed; and the drought effect on sloped grasslands was due to relatively sparse vegetation cover. In addition, VHIr was significantly correlated with greenness and wetness components derived from the tasseled cap (K-T) transformation but the correlation coefficients (r) varied between dry and wet years and among different grassland landscapes. This study expanded our understanding of the spatial and temporal distribution of drought severity in the northern mixed grasslands, which can facilitate natural resource management in the arid and semi-arid regions.