MONITORING SEASONAL CHANGES IN SOIL MOISTURE ACROSS A FOREST-GRASSLAND BOUNDARY USING ELECTRICAL RESISTIVITY SOUNDINGS

In recent years electrical resistivity has been increasingly used to study spatial and temporal changes in vadose zone soil moisture. Many of these studies, however, involved controlled injection or infiltration experiments. In this study we analyze a series of electrical resistivity soundings across a deciduous forest-grassland boundary to examine the influence of climate and vegetation characteristics on seasonal variations in vadose zone soil moisture. Data have been collected bi-weekly over a 6 month period from late fall through early spring. The field site located in glacial till near East Lansing, Michigan, has been permanently instrumented with an 84-electrode resistivity array (a-spacing 1.5m) and multiple sensors for monitoring soil moisture and temperature. Observed apparent and inverted resistivity values are well correlated with precipitation events and seasonal changes in vegetation (leaf cover). Using laboratory measurements of field samples to establish parameters in Archie's equation, we show that seasonal soil moisture patterns differ significantly under the two vegetation types, which we attribute to varying root zone depth and evapotranspiration rates.