Repeated electromagnetic induction (EMI) surveys have merits of revealing temporal changes in heterogeneous soilscapes such as subsurface hydrologie dynamics. We conducted eight repeated EMI surveys from 1997 to 2009 over a 19.5-ha agricultural field that revealed soil and water patterns. The first two surveys were done in 1997 and 2006 and compared different EMI meters (EM38, EM31, and Dualem-2), dipole orientations, and geometries. Another six surveys were conducted in different seasons in 2008 to 2009 using the EM38 operated in vertical dipole orientation. Soil apparent electrical conductivity (ECa) collected during the wetter periods (>10-mm antecedent precipitation during the previous 7 d) showed greater spatial variability (greater sills and shorter spatial correlation lengths), indicating the influence of soil water distribution on soil ECa. During a relatively short time period, most soil properties controlling ECa (e.g., texture, organic matter, and depth to bedrock) remain unchanged. Thus, repeated EMI surveys can capture the dynamics of soil moisture change and related subsurface flow paths in the landscape. Significantly (p< 0.05) higher ECa was detected in areas close to simulated subsurface flow paths, especially during the wetter periods; however, such flow paths could not be pinpointed directly from the ECa maps because of the limitation in ECa spatial resolution. In wetter areas, significant correlation between the relative difference in ECa and that in measured soil moisture was observed (r2 = 0.59-0.77), but not in drier areas. During drier periods or at drier locations, the influences of soil moisture and flow path on ECa were masked by terrain and other soil properties. Thus, the optimal use of EMI for detecting subsurface hydrologic dynamics would be during wet periods or in wet areas across the landscape in this study.
All Science Journal Classification (ASJC) codes
- Soil Science