Combined High Hazard Zones (HHZ)

This layer is a combination of Tier 1 and Tier 2 High Hazard Zones. They were merged together into this dataset to simplify use. The individual Tier 1 and Tier 2 datasets can be found in the Reference Layers feature dataset. Please see the metadata for Tier 1 High Hazard Zones and Tier 2 High Hazard Zones in the reference dataset for methods of creation of each layer. See below for a brief description of each layer. Tier 1 High Hazard Zones: Tier 1 High Hazard Zones are where tree mortality directly coincides with critical infrastructure. They pose a threat to life and property. Tier 1 High Hazard Zones were last updated in 2020. CAL FIRE mapped Tier 1 High Hazard Zones by identifying assets to be protected as: communities, powerlines, roads, communication structures, and recreation facilities as well as miscellaneous other critical infrastructure. These assets to be protected were then buffered; communities were buffered by 1,000 feet and all other assets were buffered by 200 feet. Tree Mortality data was provided by the USDA Forest Service Forest Health Protection staff Aerial Detection Surveys from 2012-2015. Field review and revision was needed to refine and finalize HHZ areas. These zones were periodically updated to reflect new mortality or infrastructure assets at risk. Tier 2 High Hazard Zones: Tier 2 High Hazard Zones represent areas for forest health restoration and fire planning. Tier 2 High Hazard Zones were updated in 2025. They were composed from the combination of ranked watersheds, FHSZ, and wildfires from 2012 forward (outlined below). Please see the Reference Layers for these input layers; their metadata contain further details about the methods by which they were created. Watersheds: Areas with elevated tree mortality and high fire threat that are a hazard to public safety, community assets, and related infrastructure represent the primary focus of these zones. CAL FIRE mapped Tier 2 High Hazard Zones through a risk assessment model that scored watersheds based on prevalence of hazards and the amount of asset to be protected. This used the National Hydrography Dataset Watershed Boundaries (WBD) hydrologic unit code (HUC) 12 watersheds; these are the smallest consistently delineated watersheds in California and average around 24,000 acres in this model. The hazards utilized were: number of dead trees (from the USFS Aerial Detection Survey, 2015), acres of Tier 1 High Hazard Zones, and fire threat -- represented in the Sierra Nevada by WHRTYPE and fire return interval departure (USFS FRID, 2014) and in other areas of the state by CAL FIRE's Fire threat layer (FRAP, 2005). The assets to be protected were: amount of critical Infrastructure, represented by roads, communities, recreation facilities, communications); water related natural resources (represented statewide by acres of riparian and in the Sierra Nevada by acres of riparian and montane meadows); and amount of water infrastructure, represented by water conveyances, and water storage facilities. Each hazard and each asset was scored individually, and an overall composite score was achieved. The overall scores for hazard and assets were ranked and then also combined for an overall score. For more information on the datasets used and the scoring methodology, please contact [e-mail]. These zones may be periodically updated to reflect new mortality or infrastructure assets at risk. Wildfire Perimeters: As of 2020, Tier 2 also includes wildfire perimeters from 2012 to the present publish date. Starting with 2021, only fires over 100 acres are included. Fire Hazard Severity Zone: As of 2020, Tier 2 also includes 'High' and 'Very High' Fire Hazard Severity Zones in forested areas. --------------- Emergency Declaration: On October 30, 2015 Governor Brown issued an emergency declaration requiring public agencies to identify areas of tree mortality that hold the greatest potential to result in wildfire and/or falling trees and threaten people and property in these areas. Once identified, these areas will be prioritized for removal of dead and dying trees that present a threat to public safety. https://www.caloes.ca.gov/wp-content/uploads/Legal-Affairs/Documents/Proclamations/20151030-Tree_Mortality_State_of_Emergency.pdf