A FIELD-BASED APPLICATION OF HIGH FREQUENCY GROUND PENETRATING RADAR (HFGPR) TO EVALUATE THE DEPTH, STRENGTH, SPATIAL VARIABILITY, AND DETERIORATION OVER TIME OF HYDROPHOBIC SOIL LAYERS IN BURNT WATERSHEDS IN SOUTHERN CALIFORNIA

It is widely accepted that the presence of fire related hydrophobic (water repellent) soil layers in a wide range of environmental settings can lead to significantly increased rates of storm runoff and erosion. It is also generally understood that the hydrologic and geomorphic impact of water repellent soil layers may persist for three to five years or longer and in many situations can contribute to the generation of and influence the size of debris and/or hyper-concentrated flows. As a result, the ability to evaluate the condition and persistence of hydrophobic layers both during the period immediately after a fire and also during the years following a fire, is central to the ability to determine post-fire watershed recovery rates and to monitor ongoing risks associated with fire impacted watersheds. To date, the most widely accepted method of assessing the presence, strength, extent, and persistence of hydrophobic soil layers is the in-situ water drop penetration test (WDPT). While effective, this approach is highly localized to a specific sample point on a slope, is labor and time intensive, and is a destructive test. Because of this, accurate evaluation of the changes over time in the layer at a given location or over a wider area is not possible with the WDPT. We have developed the HFGPR method to overcome some of these constraints. Specifically, the use of the non-invasive HFGPR method allows for a repeatable assessment of the hydrophobic layer over time and images a wider area of the soil subsurface while determining the depth, strength, persistence and spatial variability. After several years of development, the approach has recently been applied to several burnt watersheds following relatively recent fires in Southern California. Shortly after the fires occurred, a number of long-term monitoring sites were established where we began mapping alteration of the hydrophobic layers over time. The results of these efforts are presented. For example, the impact of variables such as root growth and tunneling by wildlife can be clearly seen in the HFGPR radargrams. Going forward the information this method provides should significantly improve the ability to evaluate the recovery and remaining risk in fire affected watersheds.