Using NDVI Differences to Measure Drought in the Russian River Watershed

The visualizations in this post were created by Planet’s Robert Simmon and Leanne Abraham. 

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.

Water is an especially vital resource in California where a complex network of reservoirs and irrigation systems help manage and distribute the state’s water supply to nearly 8 million acres of agricultural land and 39.5 million people. 

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 water supply continues to deplete at alarming rates, monitoring its change has never been more imperative. Planet’s remote sensing data can help identify vulnerable areas where the effects of climate change are most critical.

True-color comparison of the Russian River Watershed during the same two week period in 2020 and 2021. © 2021, Planet Labs Inc. All Rights Reserved.

Drought’s cascading consequences can be measured at every point of impact, from falling reservoirs to decreased agricultural yields. Some, like changes in vegetation, are strikingly apparent from space. Planet surface reflectance basemaps enable us to generate 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.

This map compares normalized difference vegetation index (NDVI) — a measure of plant health — in California’s Russian River Watershed from the first two weeks in June 2021 and 2020. Brown areas show decreased vegetation, and green areas show more vigorous vegetation. In general, drought has decreased NDVI in the watershed, but a few areas — in particular forests recovering from the 2019 Kincade Fire — show an increase. © 2021, Planet Labs Inc. All Rights Reserved.

To visualize drought in the Russian River Watershed, we subtracted the NDVI values from the first two weeks of June in 2020 from the same two weeks in 2021. By taking the difference between the two sets of data we were able to quantify changes in vegetation within the watershed over a one year period.

Northern California is home to a complex mixture of distinct ecosystems with varying levels of human involvement. Ecoregions respond to drought and climate-stress in different ways, resulting in some anomalies from year to year. But the overall picture tells a story about the alarming drying of the Russian River Watershed. 

We’ve selected four areas of interest within the map to highlight what the NDVI index reveals about drought response in different ecoregions.

Wallbridge Fire

Forests and woodlands burned by the Wallbridge fire showed a large decrease in greenness between 2021 and 2020. © 2021, Planet Labs Inc. All Rights Reserved.

The Wallbridge Fire was part of the LNU Lightning Complex that burned over 350,000 acres in Northern California between mid-August and October 2020. The areas with the greatest NDVI decrease are found in the burn scars from the 55,000 acres destroyed in the Wallbridge Fire, which are seen as brown in the true color imagery. Almost a year later the region shows few signs of regrowth, with much of the area, including parts of the region that were not burned, having decreased by an NDVI value of 0.1 to 0.3 or more. 

Agriculture

Agricultural land shows a a complex response to the drought. Dryland crops primarily show a decrease in greenness, while some irrigated fields were greener in 2021 than they were in 2020. © 2021, Planet Labs Inc. All Rights Reserved.

Drought has been particularly devastating for California’s agriculture industry, which heavily relies on extensive irrigation. Diminishing water supply and tighter regulations on its use, however, are transforming the industry, causing some farmers to consider alternative commodities. The selection in Potter Valley shows how human influence complicates NDVI readings. Many fields have a lower NDVI reading in 2021 than 2020, but some have positive signs of regrowth, possibly due to earlier harvests or later planting dates.

Undeveloped Land

Most undeveloped land in the Russian River Watershed exhibited a decrease in greenness between early June 2020 and 2021. In this area just north of Lake Sonoma grass- and rangeland appears the most severely affected, while forests and woodland are more resilient. © 2021, Planet Labs Inc. All Rights Reserved.

The largely undeveloped forests, woodlands, and grasslands just north of Lake Sonoma have all suffered from the stress of drought. Landscapes like these have largely shown decreasing NDVI values over the past year across the Russian River Watershed. A closer look, however, reveals that many forested areas show smaller decreases in NDVI than nearby grasslands. The reasons for this are complicated, and unravelling them requires comprehensive knowledge of the region’s ecosystems and the subtleties involved in measuring vegetation from space.

Kincade Fire

Oblique view of the Kincade Fire taken by a Planet SkySat on October 27, 2019. © 2019, Planet Labs Inc. All Rights Reserved.

Burn scars from the Kincade Fire that burned over 75,000 acres in October of 2019 are still visible almost 2 years later, but the NDVI also shows some regrowth. The varying NDVI values result from woodland areas growing between 2020 and 2021 (positive NDVI) and grassland areas growing quickly after the fire in 2020 but suffering in the following year due to drought. Although the region is still very dry, the detail here partly reveals how forest ecosystems recover over time after a wildfire.

The impact of 2019’s Kincade Fire varied according to land cover and the severity of the burn, and the 2021 drought has complicated the area’s recovery. Heavily-burned forest showed significant regrowth, while grassland was less vigorous in 2021 than 2020. Vineyards, untouched by the fire, displayed a large drop in greenness. © 2021, Planet Labs Inc. All Rights Reserved.

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 are occurring is imperative for making informed decisions to benefit the world. 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 crucial consequence of the climate crisis.

This is the first in a planned series of posts visualizing the drought currently impacting the Western United States. Continue to check back for more content sharing Planet’s unique perspective on the critically dry conditions in the region. To learn more about our products, visit our site.