NOAA-CREST GPR Study Eric Harmsen, Associate Researcher Agricultural and Biosystems Engineering
Dec 17, 2015
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
Provide data to support multi-sensor fusion concept
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
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)
Street
Building
Driv
eway
Drainage Ditch
Street
SandDriv
eway
To Weather Station
8 ft
8 ft
E
w
SN
Loam Clay
8 ft
8 ft
LoamLoam
Cross Section View
Plan View
Sand Clay
Cross Section View
GPR Unit
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
Longer-Term Activities
Data fusion work Statistical analysis of first year data Perform similar tests under natural
soil conditions Present findings at national
conferences
Gravimetric Analysis
Collect soil sample Obtain wet weight Dry sample in oven Obtain dry weight Calculate volumetric moisture
vMwet Mdry
Mdry
b
water
.
Mwet
Mdry
b
water
Soil Dry Weight
Soil dry weight
Soil bulk density
Density of water
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.
Time Domain Reflectometry (TDR)
Accuracy: High Works on the principle of relative
velocity and its relation to dielectric constant
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.
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
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
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)
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
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