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Advancing Soil Moisture Monitoring Techniques: A Comprehensive Study by NOAA-CREST

This study led by Eric Harmsen, Associate Researcher at NOAA-CREST, aims to enhance soil moisture measurement techniques for agricultural applications. The project's objectives include supporting Dr. Parsiani in obtaining “true” soil moisture values through various methods including gravimetric analysis, time domain reflectometry, and neutron scatter methods. The research will compare these methods, analyze data fusion, and present findings at national conferences. Key activities include soil pit construction, calibration of sensors, field testing, and chemical/physical soil analysis to assess moisture content and other soil properties.

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Advancing Soil Moisture Monitoring Techniques: A Comprehensive Study by NOAA-CREST

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  1. NOAA-CREST GPR Study Eric Harmsen, Associate Researcher Agricultural and Biosystems Engineering

  2. Objective • Support Dr. Parsiani with soil moisture measurements using a variety of methods. • Provide “true” values of soil moisture content for comparison with GPR • Provide data to support multi-sensor fusion concept

  3. Soil Moisture Sensors • Soil Moisture • Gravimetric Analysis (standard) • Time Domain Reflectometry (TDR) • Neutron Scatter Method • Capacitance Methods (2) • Electrical Resistance • Soil Tension (suction) • Tensiometers • Electrical Resistance

  4. Soil Analysis • Chemical • pH, conductivity, point of zero net charge, anion exchange capacity, cation exchange capacity, exchangeable cations, organic C, free iron-alluminium oxides • Physical • texture, conductivity, specific surface of both the soil and the clay fraction, aggregate stability, bulk density, porosity and the soil characteristic curve (suction vs. moisture content)

  5. Near-term Activities • Finalize equipment acquisition • Construct soil pits (October) • Soil physical and chemical analysis • Calibration of soil moisture sensors • Conduct field testing • Make comparison between instruments • Present results in January

  6. Longer-Term Activities • Data fusion work • Statistical analysis of first year data • Perform similar tests under natural soil conditions • Present findings at national conferences

  7. Gravimetric Analysis • Collect soil sample • Obtain wet weight • Dry sample in oven • Obtain dry weight • Calculate volumetric moisture

  8. Soil Dry Weight Soil dry weight Soil bulk density Density of water

  9. Neutron Scatter Method • Accuracy: High • Method relies on the thermalization or slowing of neutrons when they collide with hydrogen ions. • Radioactive source is a major disadvantage.

  10. Time Domain Reflectometry (TDR) • Accuracy: High • Works on the principle of relative velocity and its relation to dielectric constant

  11. Capacitance Method • Delta-T Profile Probe • Works in an acess tube • Measure soil moisture at six depths simultaneously • This method may have not work well in clay soils.

  12. Resistance Blocks • Electrical resistance block systems use small gypsum blocks and a portable resistance meter to measure soil moisture content. • This method is subject to soil salinity. • Give a reading in percent of soil saturation

  13. Tensiometers • Tensiometers measure soil water potential • Soil water potential is also called negative soil pressure, tension or suction. • Soil water potential is related to the moisture content • Limitation: only works up to a soil tension of 1 atm

  14. Soil Moisture Characteristic Curve • Curve relates soil water potential or soil tension to volumetric moisture content. • Curve may be hysteretic (i.e., for a given tension, multiple moisture contents may be measured depending on history of wetting)

  15. Watermark Soil Moisture Sensors • Watermark soil moisture sensors are an electrical resistance block with a granular composition similar to the texture of sandy soils. • Reading are in negative pressure or tension (kPa). • Data logger can be left in the field • Readings can be taken at any time interval

  16. Near-term Activities • Finalize equipment acquisition • Construct soil pits (October) • Soil physical and chemical analysis • Calibration of soil moisture sensors • Conduct field testing • Make comparison between instruments • Present results in January

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