Sonoma County Wildfire Risk Index

Wildfire Risk Index (WRI) is a model that predicts relative wildfire risk. Higher index values represent a higher relative risk of wildfire. For the Wildfire Risk Analysis, the county's landmass was divided into 100-acre hexagons. Conditions will vary significantly across the area of each polygon—it is entirely possible that areas of relatively low risk could exist within a polygon whose overall risk is high. For parcel level analysis, "ground truthing" to verify data and conditions will be necessary. The value of the WRI is to identify overall trends, which then can be used to suggest the need for and nature of measures that can be taken to reduce risk. The WRI should be viewed as a high-level analysis, and is not appropriate for parcel level detail.

This story map helps explain how different factors combine to create the index and how the risk of wildfire changes across Sonoma County.

To map the Wildfire Risk Index and its component parts, first we broke the county into hexagons of 100 acres. Information that affects a hexagon's hazard or risk to wildfire is characterized for each hexagon. Each hexagon is not an island. Though the hexagons have arbitrary lines, fire does not distinguish between one hexagon or another. Therefore, to get a better understanding of how wildfire risks affect each other within communities, we have also averaged the data within those hexagons within one mile.

Many different factors contribute to wildfires. The Risk Index inputs were curated from best available data sources for Sonoma County.

The Wildfire Hazard Index (WHI) (as opposed to this risk index) took into consideration predicted flame length, transmission line location, suppression difficulty, and fire weather potential. In 2020, as part of Sonoma County's Multi-jurisdictional Hazard Mitigation planning, a Wildfire Hazard Index was developed to quantify the relative wildfire hazard within Sonoma County.

A fuller discussion on the inputs and how it was developed can be  found here .

Developed by Pyrologix, the ember load index is based on surface and canopy fuel characteristics, climate, and topography and incorporates downwind ember travel. The index also incorporates burn probability.

The model can only estimate embers created by trees and brush - wildland fuels. To date, a good model of embers produced by burning structures is not available. In urban areas, the embers produced by burning buildings will have the potential to influence fire spread, yet this value is not represented in this model.

The Ember Load Index can help identify priority areas where hardening buildings may be needed to resist ignition, yet recent fire behavior indicates that it is important for *all* Sonoma County residents to undertake structure hardening, regardless of the ember load risk. More information regarding the Ember Load Index can be  found here .

The structure density is a count of all structures found within each 100-acre hexagon. The counts were then classed into five quantiles and assigned a number from 1 through 5. Hexagons with no structures were assigned 0 (zero).

These values are based on road density, number of roads into and out of a community, and speed limits. The road network rank was developed by Kevin Lacefield of Sonoma County's ISD for a preliminary  evacuation analysis  (completed in 2019). New evacuation analyses are under development for the County. As they are developed, new data may be integrated into the CWPP Risk Index. Note that the road network rank does not impact an individual property’s or project’s compliance with Sonoma County Fire Safe Standards (Chapter 13, Section V) or Board of Forestry Fire Safe Regulations, because the road network rank provides a high-level analysis of the aforementioned data inputs whereas the local standards and state regulations govern site-specific perimeters and access.

The maps below show the inputs to the Sonoma County Wildfire Risk Index. Scroll through the 9 maps, use the + and - icons to zoom in and out. Legends and class names are explained in each maps left-hand panel.

Relative wildfire risk was calculated by simply adding up all nine inputs. Each was classed from 1 to 5 for a total range of 1 to 35. No hexagons got a 1 ranking (lowest is 5) and none got a 45 ranking (highest is 32).

The map below shows the classified relative wildfire risk index. Click the legend icon on the lower left to see the color scheme - pale yellow is very low relative risk, yellow is low relative risk, light orange is moderate relative risk, orange is high relative risk, and red is extreme relative risk.

This layer can be used to help prioritize projects within Sonoma County as part of a ranking tool soon to be develop for this project. More information to come soon.

The above Sonoma County Wildfire Risk Index was developed in cooperation with input from Sonoma County staff (Caerleon Safford and Bradley Dunn), Fire Safe Sonoma (Roberta MacIntyre), the Sonoma County CWPP Steering Committee, and the project team ( Tukman Geospatial  and  Digital Mapping Solutions ).

Important note: This is an index of wildfire risk that has not yet been formally reviewed or adopted by the resource agencies, and is subject to change prior to codification.