Frequency domain analysis for extending time domain reflectometry water content measurement in highly saline soils

Water content and electrical conductivity of soils are routinely determined using time-domain reflectometry (TDR) based on analysis of signal travel time and attenuation along embedded probes. In soils with appreciable salinity, time domain analysis becomes progressively inaccurate due to signal attenuation to the point of failure. Our objectives were to test whether dielectric permittivity, which is inextricable using TDR travel time analysis (TTA) in saline soils could be extracted using frequency domain techniques applied to waveforms from shorter TDR probes that reduce signal attenuation. The methodology was tested using coaxial cells and three-wire TDR probes in sand and silt loam soil under a wide range of saturated solution soil electrical conductivities. Three different interpretation techniques were compared; conventional ITA, scatter function fitting (SFF), and resonant frequency analysis (RFA). Using a range of probe lengths (2, 3, 6, 10, and 15 cm) soil bulk dielectric permittivity estimates were obtained using all three techniques in solution electrical conductivities up to 48 dS m(-1). Both SFF and RFA produced similar permittivity estimates, which generally increased with increasing solution electrical conductivity. Network analyzer permittivity measurements (0.5-1.5 GHz) were greater than estimates using TTA, which were both greater than values from SFF and RFA in a saturated silt loam soil. Although dependent upon dielectric permittivity and electrical conductivity, frequency domain analysis results indicate a 3-cm probe is optimal for maintaining a interpretable scatter function in the TDR frequency band while providing maximum extension of dielectric determination under lossy conditions in saturated soils.