Survey of Transverse Range Fire Scars in 10 Years of UAVSAR Polarimetry

Because cross‐polarized radar returns are highly associated with volume scatter, radar polarimetry returns tend to show strong evidence of wildfire scars and recovery in forest and chaparral. We focus on the polarimetry images from UAVSAR (PolSAR) line SanAnd_08525, which covers a roughly 20 km wide swath over the Transverse Range including parts of the Santa Monica, San Gabriel and San Bernardino Mountains. We select images from four acquisition dates from October 2009 to September 2020, very roughly 4 years apart. These are compared to fire perimeters from the national Geospatial Multi‐Agency Coordination and NIFC databases for years 2003–2020, which shows the areas affected by the major fires (west to east) Springs2013, Woolsey2018, Topanga2005, LaTuna2017, Station2009, BlueCut2016, Pilot2016, Slide2007, Butler2007, and many smaller fires. PolSAR images are shown to be helpful in identifying types and boundaries of fire, 50‐meter scale details of vegetation loss, and variability of vegetation recovery in post‐fire years. Plain Language Summary The wildfires in the Southern California mountains that frequently threaten nearby cities and recreational resources are effectively mapped by an imaging radar on a NASA aircraft (Uninhabited Aerial Vehicle Synthetic Aperture Radar [UAVSAR]). Polarimetry creates maps of changes to the forests and chapparal from the fires. From 2009 to 2020, the urban‐facing mountains near Los Angeles have been mapped 27 times, including over 20 substantial wildfires. Comparing these maps with traditional fire maps demonstrates that fire boundaries may often be accurately determined by the airborne radar, which may be the best observing tool when other views are blocked by clouds, smoke, or darkness. Some areas suffer more severe burns than others and recover over time varies within and between fire zones. Regular UAVSAR flights record these variations and may help forecast future wildfire locations and extents. Key Points The urban‐facing part of the Transverse Range is imaged by 27 Uninhabited Aerial Vehicle Synthetic Aperture Radar visits from 2009 to 2020, including locations of over 20 wildfires PolSAR images identify types and boundaries of fire, 50 m scale details of vegetation loss, and variability of post‐fire recovery Observed signs of biomass recovery in gap areas between recent fires provide clues to the locations and extents of near‐future fires