Using NDVI Differences to Measure Drought in the Russian River Watershed
On April 21, 2021, California Governor Gavin Newsom declared a drought emergency for the Russian River Watershed following two years of critically dry conditions in the region. The watershed is an essential zone within the northern Sonoma and southern Mendocino counties, covering 1,485 square miles of land and serving as the primary source of drinking water for 600,000 people. As of July 16th, Lake Mendocino and Lake Sonoma—the watershed’s two reservoirs—were at 33.8% and 51.5% water supply capacity, respectively.
Unprecedented climatic conditions in the Western United States continue to ravage the region’s ecosystems and the communities that rely upon them. Drought is the underlying factor for many of the region’s problems. As the water supply continues to deplete at alarming rates, monitoring its change has never been more imperative. Planet’s remote sensing data have been helping assess vulnerable areas where the effects of climate change are most critical.
Planet surface reflectance base maps enable the generation of Normalized Difference Vegetation Index (NDVI) values over large regions and at frequent temporal intervals to visualize California’s drying landscape. NDVI is a measurement of vegetation density and indicates plant health at a given location. The index ranges from -1 to 1, where low NDVI values (0.1 or less) represent bare rock, sand, or snow, moderate values (0.2 to 0.5) as sparse vegetation, and high values (0.6 to 0.9) as dense, green vegetation.
To visualize drought in the Russian River Watershed, NDVI values were subtracted from the first two weeks of June 2020 from the same two weeks in 2021. By taking the difference between the two sets of data changes in vegetation within the watershed over a one-year period were quantified.
Four areas of interest within the map (Wallbridge Fire, Agricultural land, Undeveloped land, and Kincade fire) were studied to highlight what the NDVI index reveals about drought response in different ecoregions of Northern California. Positive signs of regrowth were recorded in these areas over the course of a year, helping identify suitable planting dates and detailed knowledge of the region’s ecosystems.
Drought is a pervasive problem in the West that will continue to impact California’s ecosystems, industries, and residents for years to come. Leveraging satellite imagery to capture, measure, and assess the extent of its reach is a powerful and necessary tool in combating this necessary consequence of the climate crisis.
Planet’s satellites allow users to photograph, process and analyze global changes at a high spatial and temporal resolution. As climate change continues to impact Earth’s environments and the communities that rely on them, better understanding these changes as they occur is imperative for making informed decisions to benefit the world.