Heated fibre optics to monitor soil moisture under successive saturation-drying cycles: An experimental approach

Author(s)

Bertotto, Luis Eduardo ; Reis, Alan ; Cobalchini, Erick Rubens Oliveira ; Schwamback, Dimaghi ; Uchoa, Jose Gescilam Sousa Mota ; Wendland, Edson Cezar

• Document type: Journal article
• Source: European Journal of Soil Science; v. 75, n. 4, p. 17-pg., 2024-07-01.

Abstract

In recent decades, distributed temperature sensing (DTS) has emerged as a robust technology for environmental applications, enabling high-resolution temperature measurements along fibre optic cables (FOCs). The actively heated fibre optic (AHFO) method is employed to monitor soil moisture (theta, m(3) m(-3)), wherein the soil temperature subsequent to the application of a heat pulse is measured by a DTS (AHFO-DTS approach). Despite significant improvements in the application of AHFO-DTS under controlled and natural conditions, the thermal behaviour of soil during multiple saturation-natural drying cycles has been insufficiently evaluated. This study aimed to address this gap by constructing an experimental horizontal soil profile in the laboratory for the application of the AHFO-DTS method during two successive saturation-drainage-evaporation (SDE) cycles. Three heating strategies were applied to a metallic alloy in contact with a FOC, and calibration models were used to correlate theta with the thermal conductivity (lambda ), cumulative temperature increase (T-cum), and maximum temperature increase (T-max). The results indicated that during the second SDE cycle, the highest errors in theta estimates were observed with the low power-short heat pulse, whereas the application of the low power-long duration and high power-short duration pulses improved the accuracy of calculations. Additionally, errors in theta estimates escalated under wetter conditions, attributed to a shift in soil heat transfer capacity from the first to the second SDE cycle for theta > 0.10 m(3) m(-3). This behaviour was ascribed to thermal hysteresis, arising from the contact resistance of the FOC and the alloy with the surrounding soil. Furthermore, the T-max method exhibited the least sensitivity to this effect and yielded reliable theta estimates, thus its adoption is recommended. Moreover, the use of the low power-long duration heating strategy is suggested as it promotes a trade-off between energy saving and accurate estimates. We concluded that assessing soil thermal response under multiple SDE cycles enhances the comprehension of the AHFO-DTS method. Overall, our findings provide insights into enhancing the applicability of this approach under field conditions, particularly following irrigation schedules and natural rainfall events. (AU)

• FAPESP's process: 15/03806-1 - Water availability and quality threats in a Guarani Aquifer System outcrop zone
• Grantee: Edson Cezar Wendland
• Support Opportunities: Program for Research on Bioenergy (BIOEN) - Thematic Grants
• FAPESP's process: 19/24292-7 - Infiltration and runoff dynamics under different land uses and climate change in the Brazilian Cerrado area
• Grantee: Dimaghi Schwamback
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 23/08756-9 - Spatio-temporal characterization of water dynamics in a vertical profile of unsaturated soil, utilizing the temperature measurement in heated fiber optics
• Grantee: Luis Eduardo Bertotto
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 21/14016-2 - Hydrus model for the theoretical representation of vertical flow under different land uses commonly found in the Brazilian Cerrado biome
• Grantee: Dimaghi Schwamback
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate