Soil Water Content Sensor Calibration Michael Forster ICT

Soil Water Content & Sensor Calibration Michael Forster ICT International Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 1

Soil Water Content • Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 2

Relatively Easy to Measure • Two common classes of field techniques: – Neutron Moisture Measurement – Dielectric Measurement • All field techniques are indirect: – NOTHING measures water in the field directly Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 3

Neutron Measurement Method – Neutron probe measures hydrogen atoms – Highly accurate, highly reliable, best method – Measure larges volume of soil – Time consuming, expensive, paper work, point measurement Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 4

Dielectric Measurement Method – Dielectric permittivity (ε) = the ability to hold charge Pulse EM Voltage Storage Time to Hold Charge Output Dielectric Permittivity Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 5

Dielectric Measurement Method – Dielectric permittivity (ε) = the ability to hold charge ε dry soil = ~4 ε saturated soil = ~40 to 50 Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 6

IMPORTANT! – We are measuring the ability of a substance to hold charge, the dielectric permittivity – We are NOT directly measuring water content – We infer water content from dielectric permittivity Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 7

Dielectric Sensors – Time Domain Reflectrometery (TDR) – Standing Wave Technique (MP 406) – Capacitance (Frequency Domain) Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 8

Common Capacitance Sensors Inserted into PVC Access Tube Push & Read Sentek Triscan Sentek Diviner Aquaterr Direct Soil Contact Decagon Devices 10 HS Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 9

Sensor Outputs – MOST COMMON OUTPUT: % VWC: percent volumetric water content – dielectric permittivity – Raw output; for example millivolt (m. V), raw counts etc Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 10

Where Does %VWC Come From? • Going from dielectric to volumetric water content Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 11

Where Does %VWC Come From? Raw output m. V Output 0 262. 6 395. 8 514. 7 583. 4 647. 9 709. 4 820. 6 870. 8 894. 8 904. 6 Brown Loam (%) 0 4. 12 7. 3 12. 31 16. 01 20. 82 24. 86 30. 91 41. 68 44. 34 50. 25 Lab based measurement Fancy Equation % VWC = ao + a 1χ + a 2χ2 + a 3χ3 = -0. 05334 + 1. 19650χ – 2. 5356χ2 + 1. 9961χ3 Where χ = MP 406 output in volts Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 12

Importance of Calibration • Relying on factory calibration conversions relies on uniformity across: Dielectric Permittivity Sensor Uniformity Factory Soil Volumetric Water Content Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 13

Importance of Calibration • Relying on factory calibration conversions relies on uniformity across: Sensor Dielectric Permittivity Uniformity ERRORS! Factory Soil Volumetric Water Content ERRORS! Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 14

Errors in Dielectric Permittivity – Bulk density (how much water is “pact in”) – Dielectric permittivity of water changes with temperature: εfree water = 80 at 20°C – High clay content affects bound water – Salinity Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 15

Errors in Sensor Uniformity NB: Check the manufacturer’s stated accuracy on spec sheet Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 16

Errors in Factory Calibration – Assuming calibration was done correctly – Calibration only done on a “generic” soil type – BULK DENSITY CHANGES! – Ask manufacturer about calibration Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 17

Take Home Message • For accurate, precise, repeatable, soil volumetric water content measurements, you must CALIBRATE your sensors. Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 18

Soil Water Potential • Soil water content sensors can measure soil water potential. Much better than gypsum blocks! Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 19

ICT International Pty Ltd Solutions for soil, plant and environmental research INTERNATIONAL www. ictinternational. com sales@ictinternational. com. au Phone: 61 2 6770 Fax: 61 2 6772 7616 PO Box 503, Armidale, NSW, Australia, 2350 Solutions for soil, plant & environmental monitoring 24/09/2020 www. ictinternational. com 20