Characterizing Soil Pipe Networks On 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 tomography (ERT) as a non-invasive technique to produce a three dimensional representation of the soil subsurface.  Prior forward and inverse modeling suggested that soil pipes are detectable as spatially-correlated high electrical-resistivity anomalies.  We imaged three 6 X 6 m areas surrounding tree stumps on a clear cut portion of coniferous forest prior to a prescribed burn.  A combined Wenner-β/dipole-dipole electrode configuration at 25 cm electrode spacing produced a 3D image approximately 1 m deep.  Pre-burn ERT images indicated the presence of existing soil pipes and decaying tree roots.  A comparison of pre-burn and post-burn ERT images of the sub-plots will demonstrate the degree of soil pipe formation and distribution due to the prescribed burn.  The imaged areas will be 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 and will contribute to watershed scale measurement tools for the estimation of physical characteristics driving the transient hydrologic response of soil pipes.