Natural Hazards: Wildfire

How much development has occurred in California's wildfire-prone areas over the past 20 years? With over 10 million Zillow property records, Dylan Connor, Peter Kedron, Amy Frazier, & I remapped California's wildland-urban interface (WUI) from 2000–2020. We built upon moving window-based WUI mapping methods using a new data integration scheme we termed the Time Step Moving Window (TSMW) framework, which leveraged Zillow's Transaction and Assessment Data (ZTRAX) to enrich FEMA building footprints to map the WUI through time.

The time-stepped maps gave us a building-level profile of how CA's WUI changed over two decades. WUI grew from 2000–2010 (441,531 more structures, +12.6%), though growth slowed substantially in the following decade (58,725 more structures, +1.5%). Despite slower growth, California's 4 million WUI properties represent $1.34 trillion — 40% of the state's improved residential property value. These homes tend to be newer, more expensive, and single-family; over 9,000 are worth more than $5 million.

Our TSMW maps also outperformed existing approaches: they captured 94.6% of burn-exposed structures from 2010–2020, compared to 87.2% for census block-based WUI maps. In a rapidly changing, more fire-prone environment, the TSMW framework demonstrates how leveraging big data — such as property records — can provide a more nuanced, dynamic understanding of wildfire exposure.