The vadose zone is a dynamic environment in which water is retained or transferred into the saturated zone or atmosphere. Knowledge of water content in the near-surface soil layers is important for improving our understanding of groundwater recharge, evaporation, and uptake by crops or natural vegetation (Vereecken et al., 2008). Ground-penetrating radar (GPR) measurements are capable of estimating the subsurface radar velocity, which can be converted to water content using Topp's equation. The direct ground wave is often used to provide radar velocities for the very shallow part of the vadose zone (Huisman 2003). However, this method can only be applied when the subsurface can be approximated by a homogeneous half-space (e.g., a subsurface with a uniform water content profile). Due to the large permittivity of water (ϵr = 80), the permittivity of soils can change dramatically when precipitation or irrigation is occurring at a site.