Imaging Preferential Flow Pathways In Forested Hillslopes Using Electrical Resistivity Tomography.

Tree stumps left behind after logging are a common feature of managed forests.  Subsequent forest fires often result in the complete combustion of decayed stumps and roots, leaving behind empty soil pipes.  These soil pipes potentially provide rapid subsurface lateral flow pathways for water that play a critical role in hillslope hydrology and stability.  However, little is known about the distribution and connectivity of soil pipes, which is needed for accurate predictions of watershed responses to changes in regional climate, land-use or ecological shifts.  We used electrical resistivity (ERT) as a non-invasive technique to produce a three dimensional representation of the soil subsurface.  We imaged three 6 X 6 m plots surrounding tree stumps on a 2 hectare clear cut of coniferous forest prior to and after a prescribed burn.  A combined Wenner-β/dipole-dipole electrode configuration with a 25 cm electrode spacing produced 3D images to a depth of approximately 1 m.  Pre-burn ERT images indicated a portion of an existing soil pipe.  A post burn survey of soil pipe openings over the 2 hectares documented 198 surface soil pipe openings.  Most soil pipe openings exhibited evidence of forming from, or being enhanced by the combustion of decaying tree roots.  Comparing pre and post-burn ERT images demonstrated the degree of soil pipe formation due to the prescribed burn.  The imaged areas were excavated to validate the accuracy of ERT to detected soil pipes.  A better understanding of soil pipe distribution is crucial for water management in forested ecosystems.