VADOSE ZONE RESISTIVITY VARIATIONS ACROSS A MICHIGAN ECOTONE

Rates of groundwater recharge, evapotranspiration, and plant growth are all strongly influenced by near surface soil moisture variability. In light of future changes in land use and climate, it is of great importance to understand the nature of this variability across a range of scales. Most tools for monitoring the critical near-surface zone are limited by scaling issues (well measurements, soil moisture sensors), repeatability (ground-penetrating radar) or resolution (satellite and airborne remote sensing). Electrical resistivity tomography (ERT) can overcome some of these issues by measuring the distribution of subsurface resistivity at a wide range of spatial and temporal scales. We present results of a novel application of ERT to monitor and quantify the effects of seasonal climate variability on vadose-zone soil moisture beneath different vegetation types for a site in Mid-Michigan. The field site has been equipped with a permanent array of 84 surface electrodes, 4 sets of 14 borehole electrodes, and multiple soil moisture and temperature sensors along a transect between a deciduous forest and grassland. We will present our analysis of a series of weekly to bi-weekly wenner and dipole-dipole datasets collected over a twelve month period. Data were interpreted using 2D difference inversions and 1D and 2D inversions with a-priori geological information, aided by laboratory measurements to obtain Archie's parameters for the different soil textures. Our results highlight the pronounced effects of seasonal vegetation changes, climatic conditions, and recharge events. The presented approach should be applicable for imaging spatial and temporal soil moisture variability for a range of settings.