Appendix Figure 3. (a) Three forward simulation scenarios: I) an empty macropore filled with air, indicating the initial
soil condition before any infiltration; II) a macropore filled with water, indicating that water is passing through soil
macropore; and III) an empty macropore surrounded by saturated soil, representing the soil condition after water bypassing
though the macropore. The diameter and depth of the simulated macropores are kept the same as 5 mm and 30 cm,
respectively. (b) Simulated GPR reflection differences (after background removal and Kirchhoff migration) between the
simulated scenarios: I) simulated radar signal of an empty macropore; II-I) the radar reflection variations between
simulations II and I ; and III-I) the radar reflection variations between simulations III and I. Simulations were computed
using a common type of GPR simulation software, GprMax V2.0 [Giannopoulos, 2005]. Based on the Finite Difference
Time Domain (FDTD) theory, GprMax provides a numerical solution for Maxwell’s equations in the time domain and
simulates GPR wave propagation and signal properties [Wilson et al., 2009]. GprMax has been validated as a feasible GPR
response simulator in several studies [e.g., Galagedara et al., 2005, Brandt et al., 2009, Wilson et al., 2009]. The input
parameters of GprMax include those associated with the GPR system settings, the geologic geometries of simulation
scenarios, and the EM properties of the involved media. In our simulation, the simulated antenna center frequency was
determined at 800 MHz to accord the field experiment. The input dielectric constants of air and water were 1 and 81,
respectively [al Hagrey, 2007]. The dielectric constants of unwetted soil background (with soil volumetric water content
at 22.5% m3/m3) and saturated soil (with soil volumetric water content at 44% m3/m3) were calculated using the Topp
equation [Topp et al., 1980] based on the field soil water content measurements (Figure 2). Given that the true antenna
central frequency in soil matrix was likely different than 800 MHz, above simulation was also computed at 400 and 1200
MHz, respectively. Similar to (b), successive polarity changes were also observed for these simulations (data not shown),
indicating that the second criterion of identifying lateral preferential flow signature was robust to a range of antenna
frequencies.
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