High‐Resolution Measurement of Soil Thermal Properties and Moisture Content Using a Novel Heated Fiber Optics Approach

Abstract

Hydrological parameters are scale dependent. Efficient monitoring techniques capable of measuring hydrological parameters, such as soil moisture content (θ), over a wide range of spatial scales are essential for understanding the complexity of water and energy movement across the landscape. Techniques to measure θ over spatial scales in the range from centimeters to thousands of meters, however, are sorely lacking. Recent improvements in the distributed temperature sensing (DTS) technology supported the development of novel techniques to fill that gap. However, improvements in the accuracy and applicability of DTS techniques are still needed. This study investigates the possibility of improving the accuracy of the fiber optics dual‐probe heat‐pulse (FO‐DPHP) DTS technique by using a new design to maintain the spacing between the FO‐DPHP probes and by introducing a novel data interpretation approach. The accuracy of the novel FO‐DPHP design was tested at different θ in a sand column experiment. The FO‐DPHP measurements obtained using traditional and novel data interpretation approaches were compared against independent measurements from several calibrated soil water content (EC5) sensors. Monte‐Carlo analyses were also performed to assess the impact of DTS measurement errors on the accuracy achieved using the data interpretation approaches. The novel design and data interpretation approach allowed for accurate measurements of soil thermal properties and θ without the need to perform a hard‐to‐achieve soil‐specific calibration. Measured θ had mean errors and standard deviations <0.03 and <0.01 m3 m−3, respectively, for moisture conditions ranging from dry to near saturation. The standard deviation in the measured heat capacity was <0.01 MJ m−3 K−1.

High‐Resolution Field Measurement of Soil Heat Capacity and Changes in Soil Moisture Using a Dual‐Probe Heat‐Pulse Distributed Temperature Sensing Approach

“…(2014) and Shehata et al. (2020). It consisted of a 1.2 mm stainless steel tube filled with a gel that surrounds two fiber‐optics, covered by a polyvinylidene difluoride jacket (custom made‐AFL Global).…”

“…The principle of superposition (Shehata et al., 2020) was applied to account for the limited duration of the applied heat pulse: $$$T(t)=\left\{\begin{array}{lc}{T}_{r}(t)\quad \hfill & \hfill 0< t\le {t}_{0}\hfill \\ {T}_{r}(t)-{T}_{r}\left(t-{t}_{0}\right)\hfill & \hfill t > {t}_{0}\hfill \end{array}\right.$$$ where t is the time in seconds measured from the heat‐pulse start and t 0 is the heat pulse duration ( t 0 = 120 s). The final resulting model was fitted to the sensed temperature increase during the first 300 s from the heat pulse start.…”

“…Shehata et al. (2020) proposed a new approach to maintain the spacing between the probes using a plastic structural mesh and tested different approaches to estimate C v from the observed heat‐pulse signal. The new system was tested in a sand column experiment and the system performance was compared against independent thermal property and θ sensors.…”

“…The results showed that the C v and θ measurements obtained from the optimized system deviated from the measurements of the independent point sensors by <0.2 MJ m −3 K −1 and <0.03 m 3 m −3 , respectively. Shehata et al (2020) also concluded that fitting an accurate semi-analytical model to the heat pulse was more efficient in reducing the effect of the DTS noise on the estimated thermal properties in comparison with the other simplified models that rely solely on the peak temperature variation and its time of occurrence.…”

“…Maintaining the spacing between the probes is the main limitation of applying the DPHP technique in the field over long distances (Benítez-Buelga et al, 2014;Sayde et al, 2010). Shehata et al (2020) proposed a new approach to maintain the spacing between the probes using a plastic structural mesh and tested different approaches to estimate C v from the observed heat-pulse signal. The new system was tested in a sand column experiment and the system performance was compared against independent thermal property and θ sensors.…”