The use of ground penetrating radar to determine the presence, extent, and spatial variability of fire related hydrophobic soils in fire impacted watersheds in southern California

Wildfires are responsible for the creation of water repellent soils (also known as hydrophobic soils) that can inhibit or completely prevent the infiltration of water into the soil profile, which increases surface runoff dramatically. Enhanced surface runoff over de-vegetated and water repellent land surfaces, especially at locations with a large amount of topographic relief, can lead to very high levels of sediment transport, and in some instances dangerous mass wasting events such as debris flows, both of which have dramatic and long lasting effects for the watersheds in which they occur. Current methods to evaluate post fire hydrophobic soils do exist, however they are limited in their evaluation capabilities and cannot spatially image the depth and variability at which a hydrophobic soil layer is created. In this study, Ground Penetrating Radar (GPR) methods have been used to successfully evaluate the presence, extent, and spatial variability of hydrophobic soils in selected Southern California Watersheds. Evaluation of both the applicable conditions as well as capability constraints associated with using high frequency GPR to evaluate the presence, extent, and spatial variability of hydrophobic soils are presented. The development of this entirely new method to identify, spatially image, and quantify the depth of hydrophobic soils should fill an acknowledged capability gap that currently exists in the study of and assessment of post wildfire hydrophobic soils. The methods outlined in this research may potentially lead to significant improvements over existing practices. The use of GPR can provide a much more accurate and comprehensive method of evaluating the nature of hydrophobic layers in such environments. As a result, the use of near surface GPR may significantly advance our capacity to assess the potential for increased erosion and the generation of hazardous debris flows in fire impacted watersheds in Southern California.