Uncertainties in Measuring Soil Moisture Content with Actively Heated Fiber-Optic Distributed Temperature Sensing

Area of Science:

• Environmental Science
• Geophysics
• Soil Science

Background:

• Actively heated fiber-optic distributed temperature sensing (aFO-DTS) is a promising technology for measuring soil moisture over large distances.
• Field-scale calibration of aFO-DTS for soil moisture is challenging due to variable soil conditions.

Purpose of the Study:

• To quantify errors in aFO-DTS soil moisture measurements using numerical modeling.
• To evaluate the impact of various error factors, including soil heterogeneity and recharge dynamics, on aFO-DTS accuracy.

Main Methods:

• A thermo-hydrogeologic, unsaturated numerical model was developed to simulate soil and fiber-optic cable interactions.
• Model simulations were compared against soil moisture values derived from the Tcum calibration method for aFO-DTS.
• Performance was evaluated across scenarios with varying recharge durations and soil properties.

Main Results:

• The numerical model demonstrated high accuracy for static hydrologic conditions but showed discrepancies with active recharge.
• aFO-DTS accuracy diminished with increased soil property variability and recharge event intensity.
• Fiber-optic cable burial depth was found to potentially impact measurement accuracy.

Conclusions:

• Numerical modeling provides valuable insights into aFO-DTS soil moisture measurement errors.
• The accuracy of aFO-DTS is sensitive to soil heterogeneity and dynamic hydrologic conditions.
• Careful selection of calibration data is crucial for reliable aFO-DTS soil moisture assessment.