Abstract-This study represents the first attempt to investigate the C;PR direct ground wave sampling depth by comparing GPR estimated soil moisture contents with data from horizontally and vertically installed TDR probes at different depths. The GPR direct ground wave method (200 MHz centre frequency) was used to estimate the temporal soil moisture dependence during uniform irrigation and drainage. Uniform irrigation and drainage experiments were conducted in an experimental pit (2.5 x 1.01 x 0.8 m) filled with repacked sandy loam soil. The GPR moisture contents measurements were more consistent with the moisture contents from vertically installed TDR than horizontally installed TDR. An analytical solution for onedimensional drainage of water was used to estimate the change in GPR ground wave sampling depth during drainage. The analytical solution was first fit to vertical TDR data to obtain an estimate of the soil hydraulic parameters and the GPR sampling depth was then estimated by fitting the drainage solution to the measured GPR data. The GPR direct ground wave sampling depth using the analytical solution during drainage varied from -20 cin at high moisture content to -50 cm at the lowest moisture conten
