Implementation of the dual-probe heat-pulse (DPHP) approach for measurement of volumetric heat capacity (C) and water content (q) with distributed temperature sensing heated iber optic (FO) systems presents an unprecedented opportunity for environmental monitoring (e.g., simultaneous measurement at thousands of points). We applied uniform heat pulses along a FO cable and monitored the thermal response at adjacent cables. We tested the DPHP method in the laboratory using multiple FO cables at a range of spacings. The amplitude and phase shift in the heat signal with distance was found to be a function of the soil volumetric heat capacity. Estimations of C at a range of moisture contents (q = 0.09-0.34 m 3 m −3 ) suggest the feasibility of measurement via responsiveness to the changes in q, although we observed error with decreasing soil water contents (up to 26% at q = 0.09 m 3 m −3 ). Optimization will require further models to account for the inite radius and thermal inluence of the FO cables. Although the results indicate that the method shows great promise, further study is needed to quantify the effects of soil type, cable spacing, and jacket conigurations on accuracy.

No abstract

The heat pulse probe method can be implemented with actively heated fiber optics (AHFO) to obtain distributed measurements of soil water content (θ) by using reported soil thermal responses measured by Distributed Temperature Sensing (DTS) and with a soil‐specific calibration relationship. However, most reported applications have been calibrated to homogeneous soils in a laboratory, while inexpensive efficient in situ calibration procedures useful in heterogeneous soils are lacking. Here we employed the Hydrus 2‐D/3‐D code to define a soil‐specific calibration curve. We define a 2‐D geometry of the fiber optic cable and the surrounding soil media, and simulate heat pulses to capture the soil thermal response at different soil water contents. The model was validated in an irrigated field using DTS data from two locations along the FO deployment in which reference moisture sensors were installed. Results indicate that θ was measured with the model‐based calibration with accuracy better than 0.022 m3 m−3.

No abstract