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NOAA-CREST GPR Study

NOAA-CREST GPR Study. Eric Harmsen, Associate Researcher Agricultural and Biosystems Engineering. Objective. Support Dr. Parsiani with soil moisture measurements using a variety of methods. Provide “true” values of soil moisture content for comparison with GPR

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NOAA-CREST GPR Study

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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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