October 2017 wildfires and their impacts on Santa Rosa, CA

Two wildfires broke out on October 8, 2017, resulting in one of the most disastrous wildfire events in California history.

Valerie Fleckenstein

Introduction

Image of the Coffey Park neighborhood taken on October 9, 2017. Photo by Chad Surmick

On October 8th, 2017, two wildfires ignited in Northern California: The Tubbs Fire and the Nuns Fire. Winds peaking at 30 - 40 miles per hour caused the fires to quickly spread to Santa Rosa, CA (Coen et al, 2018).

Fountaingrove neighborhood on October 14, 2017 (source: Flickr)

At the time, the Tubbs Fire was the most destructive fire in California history (Watkins et al, 2017). The two fires burned about 90,000 acres and over 6,000 buildings, about 2,500 of which were homes in Santa Rosa (KCRA, 2017). This project looks at the impacts of these fires on vegetation in Santa Rosa.

Study Area

Santa Rosa is located in Sonoma County, about 50 miles north of San Francisco.

The city sits in a valley surrounded by rolling hills. The climate is Mediterranean with warm, dry summers and wet, cool winters.

Image source: visitsantarosa.com/faq

Santa Rosa is in a wildland-urban interface, where developed land meets the natural environment. These areas are often at a high risk for wildfires (Cortenbach et al, 2019).

This map shows the different land cover categories according to the USGS National Land Cover Database. Pink represents developed areas, green represents forests, and beige represents shrubs. As we can see, the urban areas are in close proximity to natural environments.

Fire Perimeters

The Tubbs and Nuns fires burned portions of North and Southeast Santa Rosa, respectively. The fire perimeters can be seen in red in the image to the right.

Perimeter data retrieved from: Sonoma County Vegetation Mapping & LiDAR Program

Methods

How was vegetation in Santa Rosa impacted by the Tubbs and Nuns Fires?

To answer this question, I compared Landsat images of Santa Rosa before and after the fires. I performed supervised classifications on a Landsat 5 image from October of 2002 and on a Landsat 8 image from December 2017 to see how land cover changed due to the fires. I also conducted an NDVI analysis and NBR analysis on Landsat 8 images from September 2017 and December 2017.

NDVI stands for Normalized Difference Vegetation Index. NDVI is a way to measure healthy and unhealthy vegetation (GISGeography, 2021). Calculating the NDVI difference between two images helps to identify how vegetation health changed over time (Al-doski et al, 2013).

NBR stands for Normalized Burn Ratio. NBR is used to identify burned areas. Analyzing the difference in NBR between two images helps to identify burn severity (Wasser & Cattau, 2020).

Results

Supervised Classifications

Supervised classifications (bottom) and the corresponding Landsat images (top).

First, I performed supervised classifications on two Landsat images. The map on the left is a Landsat 5 image taken on October 2, 2002. This image was retrieved from the USGS EarthExplorer page. The map on the right is a Landsat 8 image taken on December 14, 2017. This image was also retrieved from the USGS EarthExplorer page. In both supervised classifications, dark green represents forest, light green represents grassland/other crops, gray represents urban areas, beige represents agriculture, and yellow represents bare ground. As shown in the classifications, there was an increase in bare ground coverage from 2002 to 2017. In the 2017 classification, there are large areas of bare ground (yellow) in the upper middle of the image and the lower right image of the image. These areas correspond with the areas of Santa Rosa that were burned by the Tubbs and Nuns fires.

Burn Scar Analysis

Below is a Landsat 8 image of Santa Rosa taken on October 11, 2017, three days after the fires started. This image was retrieved from the USGS EarthExplorer page. To enhance the fire areas, I created a custom band combination. The combination I used is a 752 combination. This combination was able to penetrate the smoke and highlight the burned areas, as well as the fires that were currently burning.

NBR Analysis

For the NBR analysis, I used a Landsat 8 image taken on September 25, 2017 (left) and a Landsat 8 image taken on December 14, 2017 (right). Both of these images were retrieved from the USGS EarthExplorer page. I conducted an NBR analysis on each image and then found the difference NBR between the two images

Difference NBR image

The above difference NBR image highlights the main areas of Santa Rosa that were burned in the fires. Red represents areas with high burn severity. The areas in yellow around the red areas were also land that was burned, but the severity was medium to low. The areas in green were not touched by the fires.

NDVI Analysis

I also performed an NDVI analysis to asses vegetation health before and after the fires. I use the same Landsat 8 images I used for the NBR analysis.

This is the NDVI analysis for the Landsat 8 image taken in September 2017, before the fires. Green represents areas of healthy vegetation, while red represents areas of unhealthy vegetation.

This is the NDVI analysis for the Landsat 8 image taken in December 2017, two months after the fires. As with the previous image, red represents areas of unhealthy vegetation and green represents areas of healthy vegetation.

Here is the difference between the two NDVI images. Change is shown in red and it is overlaid on the Landsat 8 image taken in December. We can see here the areas with a change in vegetation health (red) correspond to where the fires burned. In this case, the change was an increase in unhealthy vegetation.

Discussion

The results of this project show that the Tubbs Fire and Nuns Fire negatively impacted vegetation in Santa Rosa. There was a clear increase in the amount of unhealthy vegetation in the area. This was due to the severity of the burns from the fires.

Another impact from these fires was a decrease in water quality. There was a water advisory issued for the Fountaingrove neighborhood on October 10, 2017. Contaminants from burned buildings and vegetation entered the water distribution network. The water advisory was finally lifted on October 11, 2018 (Proctor et al, 2020)

Conclusion

Sugarloaf Ridge on October 19, 2017

Both fires caused significant damage to vegetation and developed areas in Santa Rosa. As previously mentioned, at the time the Tubbs Fire was the most destructive wildfire in California history. However, it was surpassed the following year by the Camp Fire (Izagguire, 2021). California continues to have record-setting high-severity fires. Climate change and drought conditions will only increase the chances of these fires in the coming years (Harris & Taylor, 2015).

Future research to determine what areas are most susceptible to wildfires can help to minimize the damage done by these fires. We also need to determine what steps we can take to prevent future wildfire events like the Tubbs and Nuns Fires from happening.

References

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Difference NBR image

Sugarloaf Ridge on October 19, 2017

Image of the Coffey Park neighborhood taken on October 9, 2017. Photo by Chad Surmick

Fountaingrove neighborhood on October 14, 2017 (source: Flickr)