Calibration of a combined dielectric probe for soil moisture and porewater salinity measurement in organic and mineral coastal wetland soils

Accurate measurement of soil moisture (θ), bulk electrical conductivity (σb), and porewater electrical conductivity (σw) in the vadose zone is critical for a wide range of environmental monitoring applications. The use of combined dielectric probes allows for the automated collection of high-resolution, long-term data, however variation in probe response to different soil types can lead to unacceptably large measurement errors, especially in soils with high organic content such as those found in wetlands. The objectives of this study were to calibrate and field-test a combined, capacitance-based dielectric probe for three soil series encountered in the floodplain of a southeastern (USA) coastal river where watershed modifications have led to reduced freshwater flow and saltwater intrusion. To calibrate the probe, floodplain soils were categorized into three groups: a low organic content fine sand; a moderately organic, depositional fluvent soil; and a highly organic muck. PVC soil cores were packed at field bulk density, and θ and σb were measured in the lab over a range of soil moistures (pressure potentials of 0–333 cm H2O) and σw values (0.01–1.0 S/m). Soil dielectric properties measured with the probe were used to test several potential models relating real and imaginary dielectric constants to θ, σb, and σw. Soil-specific calibrations improved θ estimation over standard manufacturer calibrations, particularly for the more organic soils. Of all θ–σb–σw models tested, the empirical relationship proposed by Vogeler et al. (1996) performed the best (overall R2 = 0.97 for the three soils), though all models performed well in all soils (0.94 ≤ R2 ≤ 0.98) and each can be selected for the specific range of σb expected in the field. Calibrations were successfully tested in the field by comparing in-ground probe estimates of σw with collocated soil water samples. These calibrations add to the limited published data available for soils of high organic content and support accurate monitoring of the vadose zone in coastal wetlands to inform restoration and management of these valued ecosystems.