THE USE OF HIGH FREQUENCY GROUND PENETRATING RADAR (HFGPR) TO EVALUATE THE RATE OF RECOVERY OF FIRE-IMPACTED WATERSHEDS IN SOUTHERN CALIFORNIA

Wildfires remove vegetation and alter habitats, change the soil surface and subsurface conditions, and can also generate hydrophobic (water repellent) soil layers. When combined with significant post-fire precipitation events the result can be catastrophic floods and debris flows. With the increased frequency and scale of wildfires in ecosystems ranging from the Mediterranean to the Arctic, the need to assess both the immediate post-fire biologic, hydrologic and geomorphic impacts of wildfires and long term post-fire recovery rates has taken on added significance. In terms of assessing immediate wildfire impact on vegetation and subsequent recovery rates, a combination of remote sensing methods such as Normalized Differential Vegetative Index (NDVI), hyperspectral techniques, and field-based calibration, have proven effective. Surveys of recovery rates in animal populations have also been useful. However, the assessment of post-fire soil surface and subsurface processes and the formation and persistence of hydrophobic soil layers, remains a challenge. To date, the most widely accepted methods of assessing soil surface and subsurface conditions and recovery rates are the field based: water drop penetration test (WDPT); the molarity of an ethanol drop (MED); and the mini-disc infiltrometer (MDI). While effective, these approaches are all highly localized to a specific sample point on a slope, are labor and time intensive, and are destructive tests. As a result, accurate and representative evaluations of immediate and long term recovery rates are difficult. High frequency ground penetrating radar (HFGPR) is non-invasive and enables repeatable assessment of the subsurface and hydrophobic layers over time. Moreover, by imaging a wider area of the soil subsurface it can better document changes in the depth, strength, persistence and spatial variability of a much larger and more representative area of subsurface. We present here the results of the repeated use of this technique, both immediately post-fire and for a period up to 10 years following wildfires, at selected sites in Southern California. The radargrams presented indicate both the rate of deterioration of the hydrophobic soil layers over time and the impact of vegetation, animal activity and precipitation on post-fire recovery rates.