Soil Material, Temperature, and Salinity Effects on Calibration of Multisensor Capacitance Probes

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Abstract
Multisensor capacitance probes (MCAP) are an alternative to gravimetric or nuclear soil water content (θv, m3 m−3) measurements. Their θv measurements are more convenient than gravimetric, and don't carry the nuclear regulatory burdens. Previous studies noted potential salinity and temperature effects on MCAP θv determinations. Our objectives were to calibrate and verify MCAP θv measurement accuracy in two soil materials, two water salinities (1.3 and 11.3 dS m−1), and with diurnal temperature fluctuations. The surface and calcic horizons of an Olton soil (fine, mixed, superactive, thermic Aridic Paleustoll) were packed into triplicate, 0.5‐m‐tall, 100‐L columns and wetted. We compared θv determined by volumetric measurements, time domain reflectometry (TDR), and MCAPs. The TDR θv were within ±0.01 m3 m−3 of volumetric determinations for air‐dry and saturated soil. The factory supplied universal MCAP calibration provided accurate θv estimates for air dry (±0.01 m3 m−3) surface and calcic soil materials but not after wetting (≈−0.05 m3 m−3). Also, imprecise MCAP sensor positioning during water frequency parameter determination was problematic and biased initial θv measurements. After calibration against TDR, the MCAP θv varied ±0.01 m3 m−3 from measured θv for air‐dry and saturated conditions for both soil materials, which were then pooled to obtain one calibration. Column resaturation with saline water affected permittivity and elevated MCAP θv ≈0.25 m3 m−3 above the available pore space. Cyclical soil temperature fluctuations of 15°C induced similar fluctuations in indicated θv throughout the column (0.04 m3 m−3 for MCAP and 0.02 m3 m−3 for TDR), which was attributed to variations in permittivity.