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