Non-Invasive Detection of Soil Moisture in Wet Vertisols Using Electrical Resistivity Tomography

The unique characteristics of Vertisols complicate soil moisture measurements and often obscure traditional markers of wetland soils, which may result in failure to identify jurisdictional wetlands. Electrical Resistivity Tomography (ERT) has been proposed for non-invasive measurement and temporal monitoring of soil moisture in Vertisols. Resistivity measurements were conducted on two Texas Vertisols: a Typic Hapludert and a Chromic Haplustert. Water and clay content were determined from soil cores taken from corresponding ERT transects. Multiple linear regression models predicting volumetric water content from log resistivity and clay content indicate that resistivity is, at best, a weak predictor of water content in high-clay soils (p-value ≥ 0.03), while clay content is consistently highly significant (p-value < 0.001). Further, regression models were site-specific. Regression of changes in log resistivity and water content over time provides weak evidence that ERT may be used to monitor temporal trends. Resistivity was a significant predictor of changes in water content (p-value < 0.001) in a limited area at one site where clay content was well-known and changes in both resistivity and water content were large compared to other areas in the same site. Changes in soil moisture cannot be accurately quantified from changes in resistivity in wet Vertisols, though wetting and drying trends may be discernible. ERT data in wet Vertisols are prone to noise that obscures the signal from soil moisture. Accordingly, ERT is generally unfit for monitoring temporal changes in soil moisture in wet Vertisols