Heated optical fiber for distributed soil-moisture measurements: A lysimeter experiment

An Actively Heated Fiber Optics (AHFO) method to estimate soil moisture is tested and the analysis technique improved on. The measurements were performed in a lysimeter uniformly packed with loam soil with variable water content profiles. In the first meter of the soil pro- file, 30 m of fiber optic cable were installed in a 12 loops coil. The metal sheath armoring the fiber cable was used as an electrical resistance heater to generate a heat pulse, and the soil response was monitored with a Distributed Temperature Sensing (DTS) system. We study the cooling following three continuous heat pulses of 120 s at 36 W m^-1 by means of long- time approximation of radial heat conduction. The soil volumetric water contents were then inferred from the estimated thermal conductivities through a specifically calibrated model relating thermal conductivity and volumetric water content. To use the pre-asymptotic data we employed a time correction that allowed the volumetric water content to be esti- mated with a precision of 0.01-0.035 (m³ m^-3). A comparison of the AHFO measurements with soil-moisture measurements obtained with calibrated capacitance-based probes gave good agreement for wetier soils [discrepancy between the two methods was less than 0.04 (m³ m^-3)]. In the shallow drier soils, the AHFO method underestimated the volumetric water content due to the longer time required for the temperature increment to become asymp- totic in less thermally conductive media [discrepancy between the two methods was larger than 0.1 (m³ m^-3)]. The present work suggests that future applications of the AHFO method should include longer heat pulses, that longer heating and cooling events are analyzed, and, temperature increments ideally be measured with higher frequency.