ELECTRICAL RESISTIVITY TOMOGRAPHY TO STUDY THE EFFECT OF SLOPE ASPECT ON RECHARGE AND SOIL MOISTURE VARIABILITY

Variation in sun exposure with slope orientation has significant effects on vegetation characteristics and hydrological processes. In order to understand the varying rates of evapotranspiration and groundwater recharge for slopes of different aspect it is important to accurately characterize the variability in soil moisture. Previous studies of joined north- and south-facing slopes have examined the differences in vegetation, soil development and spectral characteristics. The objective of our study was to use electrical resistivity methods to directly monitor water infiltration and soil moisture variability in such environments. We studied an east-west trending valley bordering Lake Michigan's eastern shoreline near Ludington, Michigan. The V-shaped valley, which is about 300m long, 80m wide and 20m deep, formed in the 1940s during a washout in proglacial sandy and gravelly sediment. The north-facing slope is thickly forested with deciduous trees and abundant undergrowth, while the south-facing slope is an even mix of bare soil and small patches of grass. Mean annual rainfall for the site is 742mm and the average temperature is 8.8°C. We permanently equipped three sites (one on each slope and one in the valley bottom) with 14 surface and 14 borehole electrodes (separation 0.2 and 0.1m, respectively) as well as soil moisture probes at 0.15 and 0.3m depth. During summer precipitation events, pole-dipole electrical resistivity data sets were collected twice hourly. Difference inversions were performed to estimate changes in electrical resistivity during the rainfall events. The results were compared with data from the soil moisture probes and a nearby weather station to obtain a view of the spatial and temporal variability in soil moisture within and between the three sites.