Each fall, Western Arctic caribou migrate across frozen Northern Alaskan rivers to reach wintering grounds. Regional climate changes have altered ice and vegetation phenology dynamics throughout caribou habitats, preventing caribou from crossing rivers and successfully calving. Caribou cannot cross partially frozen rivers and require healthy vegetation in the spring to calve. The Massachusetts NASA DEVELOP Team partnered with the National Park Service in Alaska to analyze changes in river ice formation and terrestrial vegetation. Landsat 8 and 9 Operational Land Imagery, Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), Sentinel 1-Synthetic Aperture Radar (SAR) C-band, and Sentinel 2A/B Multispectral Instrument imagery were used to assess ecological conditions relevant to caribou survival. The timing and duration of ice formation was assessed using the Normalized Difference Infrared Index (NDII), Relative Difference River Ice (RDRI), and Vertical-Vertical/Vertical-Horizontal (VV/VH) backscatter values based on the spectral and surface characteristics of rivers. Similarly, phenology cycles were quantified using the Nadir Bidirectional Reflectance Distribution Function Adjusted Reflectance (NBAR) for Normalized Difference Vegetation Index (NDVI) time-series data. The team produced annual freezing timelines, time series plots and maps, and composite imagery at various caribou locations to analyze annual changes. This research compared various optical and radar remote sensing methods in detecting high ice extent and high-quality vegetation over remote Alaska regions where in situ measurements are often infeasible. These study results can help evaluate spatiotemporal migratory shifts and contextualize recent shifting population dynamics.
