Department of Natural Resources Sciences, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC H9X 3V9, Canada.
Department of Soil Science, University of Peradeniya, Peradeniya 20400, Sri Lanka.
Sensors (Basel). 2021 Feb 1;21(3):962. doi: 10.3390/s21030962.
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-3 min), low power-short pulses (10 Wm-3 min, 5 Wm-3 min, 2.5 Wm-3 min) and low power-long pulses (10 Wm-5 min, 5 Wm-10 min, 2.5 Wm-15 min) were tested using laboratory soil columns. The study compared the sensitivity of the thermal response, NT 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 increased and the predictive error decreased with increasing power intensity, irrespective of the soil type. Low power-short heat pulses such as 5 Wm-3 min and 2.5 Wm-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 power intensities, but not for the 2.5 Wm. The improvement was particularly noticeable in 5 Wm -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 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. For example, 5 Wm-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 in surface soils under field conditions.
主动加热光纤（AHFO）技术具有以高时空分辨率测量土壤水分的潜力，因此可以弥合从点到大规模测量的差距。然而，由于在野外条件下需要高功率来加热长光纤电缆，因此电源的可用性可能会限制其使用；这可能是野外长期土壤水分监测的一个挑战。本研究调查了不同加热策略（功率强度和加热持续时间）对三种不同质地土壤中 AHFO 技术测量土壤水分的性能的影响。使用实验室土壤柱测试了不同的加热策略：高功率-短脉冲（20 Wm-3 min）、低功率-短脉冲（10 Wm-3 min、5 Wm-3 min、2.5 Wm-3 min）和低功率-长脉冲（10 Wm-5 min、5 Wm-10 min、2.5 Wm-15 min）。研究比较了不同加热策略和土壤的热响应灵敏度 NT 与体积含水量（VWC）的关系以及不同加热策略的预测误差。本研究结果表明，无论土壤类型如何，随着功率强度的增加，NT 的灵敏度增加，预测误差减小。低功率-短热脉冲（如 5 Wm-3 min 和 2.5 Wm-3 min）会产生较高的预测误差，分别为 5-6%和 6-7%。然而，延长加热时间对于 10 Wm 和 5 Wm 的功率强度都可以有效降低误差，但对于 2.5 Wm 则不行。在 5 Wm-10 min 中，这种改进尤为明显；它将 RMSE 降低了 1.5%（砂壤土和粉质壤土）和 2.73%（砂壤土）。总体而言，本研究结果表明，延长 10 Wm 和 5 Wm 功率强度的加热时间可以提高热响应的灵敏度和估计土壤含水量（SWC）的预测精度。这些结果对于 AHFO 技术的野外应用尤其重要，因为野外应用可能会受到高功率可用性的限制，这限制了 20 Wm 的使用。例如，5 Wm-10 min 提高了预测精度至 3-4%，这有可能用于验证卫星足迹范围内的土壤水分估计值。然而，还应考虑昼夜温度变化的影响，特别是在野外条件下，表面土壤中使用低功率强度（如 5 Wm）时。
