Comparison of Heating Strategies on Soil Water Measurement Using Actively Heated Fiber Optics on Contrasting Textured...

The actively heated fiber optics (AHFO) technique has the potential to measure soil water at high spatial and temporal resolutions, and thus it can bridge the measurement gap from point to large scales. However, the availability of power might restrict its use, since high power is required to heat long fiber optic cables under field conditions; this can be a challenge for long-term soil water monitoring under field conditions. This study investigated the performance of different heating strategies (power intensity and heating duration) on soil water measurement by the AHFO technique on three different textured soils. Different heating strategies: high power-short pulses (20 Wm^-1-3 min), low power-short pulses (10 Wm^-1-3 min, 5 Wm^-1-3 min, 2.5 Wm^-1-3 min) and low power-long pulses (10 Wm^-1-5 min, 5 Wm^-1-10 min, 2.5 Wm^-1-15 min) were tested using laboratory soil columns. The study compared the sensitivity of the thermal response, NT_cum to volumetric water content (VWC) and the predictive error of different heating strategies and soils. Results of this study showed that the sensitivity of NT_cum increased and the predictive error decreased with increasing power intensity, irrespective of the soil type. Low power-short heat pulses such as 5 Wm^-1-3 min and 2.5 Wm^-1-3 min produced high predictive errors, RMSE of 5-6% and 6-7%, respectively. However, extending the heating duration was effective in reducing the error for both 10 and 5 Wm^-1 power intensities, but not for the 2.5 Wm^-1. The improvement was particularly noticeable in 5 Wm^-1 -10 min; it reduced the RMSE by 1.5% (sand and clay loam) and 2.73% (sandy loam). Overall, the results of this study suggested that extending the heating duration of 10 and 5 Wm^-1 power intensities can improve the sensitivity of the thermal response and predictive accuracy of the estimated soil water content (SWC). The results are particularly important for field applications of the AHFO technique, which can be limited by the availability of high power, which restricts the use of 20 Wm^-1. For example, 5 Wm^-1-10 min improved the predictive accuracy to 3-4%, which has the potential to be used for validating soil water estimations at satellite footprint scales. However, the effects of diurnal temperature variations should also be considered, particularly when using low power intensity such as 5 Wm^-1 in surface soils under field conditions.