High-Resolution Monitoring of Soil Water Dynamics …Optical fibre distributed temperature sensors...
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Water and thermal conditions in the shallow vadose zone can be very complex and dynamic across a range of spatiotemporal scales. Optical fibre distributed temperature sensors (DTS) allows for precise soil temperature measurements at high spatio- temporal resolution, over kilometres of optical fibre cable buried in the soil. In addition to passive soil temperature monitoring, hybrid optical cables with embedded metal conductors can be electrically heated and allow for distributed heat pulses. Such technique is known as Active-DTS. Analysis of temperatures during the cable heating and/or cooling can provide high resolution soil moisture estimates. In summer 2015, three loops of a 500m hybrid-optical cable have been deployed at 10cm, 25cm and 40cm depths along a hillslope with juvenile forest. Active-DTS surveys have been conducted with the aim to: (i) monitor the post-installation soil settling around the cable; (ii) analyse different heating strategies (intensity, duration) of the cable; (iii) establish a method for inferring soil moisture from Active-DTS results and validate with independent soil moisture readings from point probes; (iv) monitor the soil moisture response to short forcing events such as storms and artificial irrigation. DTS readings from different models and acquisition time are interpolated using spline to analyse consistent datasets ∆T calculated in time as temperature increases compared to 3 minutes average of pre-heating temperatures for each cable loop Applied integral approach by Sayde et al., 2010,2014 : T cum is linearly scaled using the ratio between applied power rate and the power of one reference test. In this case the power conversion factor is calculated versus 4.8 W/m (June the 9 th , 2016) Performed a scatter plot of power scaled T cum in proximity of the independent soil moisture probes = =0 =900 ∆() ∙ • A-DTS provides high space-time density of temperature and soil-moisture-dependent data • Heat pulses at different powers supplied in similar soil conditions seem to be linearly scalable • Scatter plot of independent soil moisture data and T cum reveals a clear trend that could be used as site-specific calibration curve T cum (VWC). More A-DTS measurements are necessary to populate the extreme of the scale (wilting point, saturation) • Independent measurements of soil thermal conductivity and soil moisture with drying method will be performed to generate a more physically sound relation between T cum data and soil moisture. Such measurements will allow the application of the asymptotic analysis by Ciocca et al., 2012, to both the heating and the cooling phase • Connection to a fix electrical power source with higher power availability and remote control of the heat pulses via Ethernet will be provided soon to increase the number of measurements and capture both intense, rapid events and longer trends References: - Ciocca et al., Heated Optical Fiber for Distributed Soil Moisture Measurements: A Lysimeter Experiment, Vadose Zone Journal, 2012. - Sayde et al., Feasibility of soil moisture monitoring with heated fiber optics, Water Resources Research, 2014. - Sayde et al., Mapping variability of soil water content and flux across 1–1000 m scales using the Actively Heated FO method, WRR, 2014. Acknowledgements: the authors are grateful to the British National Research Council for the Environment (NERC) for the financial support to the Distributed Heat Pulse System (DiHPS) project, to Prof. Rob McKenzie and the BIFoR team for the on site support, and to Silixa engineers. Outlook • Scatter points are the average of 2 meters samples in proximity of the soil moisture point probes along the slopes • Error bars indicate the standard deviation • Soil conditions during the Active-DTS surveys performed presented limited moisture variability • Excluded from the fit the points from the left slope at 0.25m. Systematic dry conditions reported during all the surveys • High R 2 obtained fitting a third order polynomial curve. Curve fairly flat at intermediate soil moisture regimes Measured Optical fibres length [m] 4500m Electrical Parameters Voltage [V] (AC rectified) 95-118 Amperage [Amps] 17.3-21.5 Power Range [W] 1650 - 2500 Cable Electrical Resistance [Ω/km] 11 (wired in parallel) High-Resolution Monitoring of Soil Water Dynamics in a Vegetated Hillslope by Active Distributed Temperature Sensing Francesco Ciocca 1, 2 , Stefan Krause 2 , Corinna Abesser 3 , Philip Blaen 2 , David Hannah 2 , Athena Chalari 1 , and Michael Mondanos 2 Contact: [email protected] 1 Silixa Ltd., London, United Kingdom 2 School of Geography, Earth and Environmental Sciences, University of Birmingham, United Kingdom British Geological Survey, United Kingdom Date Heating Duration (per depth) Power Rate DTS configuration DTS sampling DTS time interval DTS model Power Conversion factor [dd/mm/yy] [s] [W/m] [m] [s] 23/10/15 900 3.3 Double-Ended 0.25 20 XT-DTS 1.45 08/06/16 900 3.7 Double-Ended 0.25 10 Ultima-M 1.30 09/06/16 900 4.8 Double-Ended 0.25 10 Ultima-M 1.00 25/10/16 900 5.0 Double-Ended 0.25 10 Ultima-S 0.96 Site Description Relation T cum (VWC) Electrical/Optical Parameters and Measurements DTS Response: October 2016 Data Analysis • Sequential Heating of Bottom – Middle – Top cable loop. Data shown anticlockwise in length from the DTS (i.e. right slope first) • Heating is applied only to the cable buried in the soil to save power and reduce hazards. Only small amounts of cable coming out of the soil to enter the switch box are heated more • Visible temperature increase with spatial features arising both between the different slopes and at very localized spots along the slopes Mill Haft woodland in Staffordshire, UK. This site hosts one of the two currently operating worldwide Free-Air Carbon dioxide Enrichment (FACE) experiment, under the direction of the Birmingham Institute for Forest Research (BIFoR). Fibre optics cable and soil moisture probes (5TM Decagon Devices, US) positioning. Roughly 500m of soil between rows of growing trees trenched to a depth of 0.40m. Three lines of cables laid in the trench at 0.40m, 0.25m, 0.10m during manual backfilling. 3x5TM probes inserted in the soil at the same depths in 5 different locations along the trench July 2015 June 2016 A. A. Heat Pulse Control Unit: programmable system able to take alternate input current and convoy it to an optical fibre cable rectified and at a desired power B. Splice box: to protect the connections between the optical fibres and the connectors to the DTS (not shown) C. Switch box: to convoy the electrical current to a specific cable or multiple ones B. C. Active-DTS Apparatus
Transcript of High-Resolution Monitoring of Soil Water Dynamics …Optical fibre distributed temperature sensors...
Water and thermal conditions in the shallow vadose zone can be very complex and dynamic across a range of spatiotemporal scales. Optical fibre distributed temperature sensors (DTS) allows for precise soil temperature measurements at high spatio-temporal resolution, over kilometres of optical fibre cable buried in the soil. In addition to passive soil temperature monitoring, hybrid optical cables with embedded metal conductors can be electrically heated and allow for distributed heat pulses. Suchtechnique is known as Active-DTS. Analysis of temperatures during the cable heating and/or cooling can provide high resolution soil moisture estimates. In summer 2015, three loops of a 500m hybrid-optical cable have been deployed at 10cm, 25cm and 40cmdepths along a hillslope with juvenile forest. Active-DTS surveys have been conducted with the aim to: (i) monitor the post-installation soil settling around the cable; (ii) analyse different heating strategies (intensity, duration) of the cable; (iii) establish a methodfor inferring soil moisture from Active-DTS results and validate with independent soil moisture readings from point probes; (iv) monitor the soil moisture response to short forcing events such as storms and artificial irrigation.
DTS readings from different models and acquisition time are interpolated using spline to analyse consistent datasets
∆T calculated in time as temperature increases compared to 3 minutes average of pre-heating temperatures for each cableloop
Applied integral approach by Sayde et al., 2010,2014 :
Tcum is linearly scaled using the ratio between applied power rate and the power of one reference test. In this case thepower conversion factor is calculated versus 4.8 W/m (June the 9th, 2016)
Performed a scatter plot of power scaled Tcum in proximity of the independent soil moisture probes
𝑇𝑐𝑢𝑚 𝑥 = 𝑡=0 𝑠𝑡𝐻=900 𝑠∆𝑇(𝑥) ∙ 𝑑𝑡
• A-DTS provides high space-time density of temperature and soil-moisture-dependentdata
• Heat pulses at different powers supplied in similar soil conditions seem to be linearlyscalable
• Scatter plot of independent soil moisture data and Tcum reveals a clear trend that could beused as site-specific calibration curve Tcum (VWC). More A-DTS measurements arenecessary to populate the extreme of the scale (wilting point, saturation)
• Independent measurements of soil thermal conductivity and soil moisture with dryingmethod will be performed to generate a more physically sound relation between Tcum
data and soil moisture. Such measurements will allow the application of the asymptoticanalysis by Ciocca et al., 2012, to both the heating and the cooling phase
• Connection to a fix electrical power source with higher power availability and remotecontrol of the heat pulses via Ethernet will be provided soon to increase the number ofmeasurements and capture both intense, rapid events and longer trends
References: - Ciocca et al., Heated Optical Fiber for Distributed Soil Moisture Measurements: A Lysimeter
Experiment, Vadose Zone Journal, 2012.- Sayde et al., Feasibility of soil moisture monitoring with heated fiber optics, Water Resources
Research, 2014.- Sayde et al., Mapping variability of soil water content and flux across 1–1000 m scales using the
Actively Heated FO method, WRR, 2014.
Acknowledgements: the authors are grateful to the British National Research Council for theEnvironment (NERC) for the financial support to the Distributed Heat Pulse System (DiHPS) project, toProf. Rob McKenzie and the BIFoR team for the on site support, and to Silixa engineers.
Outlook
• Scatter points are the average of 2 meters samples in proximity of the soil moisture point probes along the slopes
• Error bars indicate the standard deviation
• Soil conditions during the Active-DTS surveys performed presented limited moisture variability
• Excluded from the fit the points from the left slope at 0.25m. Systematic dry conditions reported during all the surveys
• High R2 obtained fitting a third order polynomial curve. Curve fairly flat at intermediate soil moisture regimes
Measured Optical fibres length [m] 4500m
Electrical Parameters
Voltage [V] (AC rectified) 95-118
Amperage [Amps] 17.3-21.5
Power Range [W] 1650 - 2500
Cable Electrical Resistance [Ω/km] 11 (wired in parallel)
High-Resolution Monitoring of Soil Water Dynamics in a Vegetated Hillslope by Active Distributed Temperature Sensing
Francesco Ciocca 1, 2, Stefan Krause 2, Corinna Abesser 3, Philip Blaen 2, David Hannah 2, Athena Chalari 1, and Michael Mondanos2
Contact: [email protected]
1Silixa Ltd., London, United Kingdom 2School of Geography, Earth and Environmental Sciences, University of Birmingham, United Kingdom British Geological Survey, United Kingdom
Date
Heating
Duration (per
depth)
Power
RateDTS configuration
DTS
sampling
DTS time
intervalDTS model
Power
Conversion
factor
[dd/mm/yy] [s] [W/m] [m] [s]
23/10/15 900 3.3 Double-Ended 0.25 20 XT-DTS 1.45
08/06/16 900 3.7 Double-Ended 0.25 10 Ultima-M 1.30
09/06/16 900 4.8 Double-Ended 0.25 10 Ultima-M 1.00
25/10/16 900 5.0 Double-Ended 0.25 10 Ultima-S 0.96
Site Description Relation Tcum (VWC)Electrical/Optical Parameters and Measurements
DTS Response: October 2016
Data Analysis
• Sequential Heating of Bottom –Middle – Top cable loop. Data shown anticlockwise in length from the DTS (i.e. right slope first)
• Heating is applied only to the cable buried in the soil to save power and reduce hazards. Only small amounts of cable coming out of the soil to enter the switch box are heated more
• Visible temperature increase with spatial features arising both between the different slopes and at very localized spots along the slopes
Mill Haft woodland in Staffordshire, UK. This site hosts one ofthe two currently operating worldwide Free-Air Carbondioxide Enrichment (FACE) experiment, under the direction ofthe Birmingham Institute for Forest Research (BIFoR).
Fibre optics cable and soil moisture probes (5TM DecagonDevices, US) positioning. Roughly 500m of soil between rowsof growing trees trenched to a depth of 0.40m. Three lines ofcables laid in the trench at 0.40m, 0.25m, 0.10m duringmanual backfilling. 3x5TM probes inserted in the soil at thesame depths in 5 different locations along the trench
July 2015 June 2016
A.
A. Heat Pulse Control Unit: programmable system able totake alternate input current and convoy it to an opticalfibre cable rectified and at a desired power
B. Splice box: to protect the connections between the opticalfibres and the connectors to the DTS (not shown)
C. Switch box: to convoy the electrical current to a specificcable or multiple ones
B. C.
Active-DTS Apparatus
