Using capacitance sensors tomonitor soil moisture

Interpreting the numbers

Terry PrichardTerry PrichardWater Management SpecialistWater Management Specialist

Dept. Land, Air, and Water ResourcesDept. Land, Air, and Water ResourcesUC DavisUC Davis

MEASURES OF SOIL WATER MEASURES OF SOIL WATER STATUSSTATUS

There are two ways to describe the soil There are two ways to describe the soil moisture:moisture:

Volumetric Water content (%, in/ft)Volumetric Water content (%, in/ft)QuantitativeQuantitative

Soil Water Potential. (Centibars suction)Soil Water Potential. (Centibars suction)QualitativeQualitative

Volume UnitsVolume Units

RainfallRainfall inches/depthinches/depthCrop Water UseCrop Water Use inches/depthinches/depthSoil moistureSoil moisture inches/depthinches/depth

% = in / in % = in / in % x 12 inches = % x 12 inches = inchesinches / foot soil/ foot soil% x % x rootzonerootzone depth = inches in depth = inches in rootzonerootzone

Soil Constituents by VolumeSoil Constituents by VolumeAt field capacityAt field capacity

Mineral50%

Air 25%

Water 25%

Organic matter <1%

Direct / Indirect MethodsDirect / Indirect Methods

DirectDirectSoil sampling by volumeSoil sampling by volume•• OrOr------ by weight x soil bulk densityby weight x soil bulk density

IndirectIndirectany method which relates a any method which relates a ““readingreading”” to soil to soil sampling moisture contentsampling moisture content

Indirect MethodsIndirect Methods

Soil DielectricSoil DielectricTime Domain Reflectometry (TDR)Time Domain Reflectometry (TDR)Ground Penetrating Radar (GPR)Ground Penetrating Radar (GPR)Frequency Domain Reflectometry (FDR Frequency Domain Reflectometry (FDR or capacitance)or capacitance)

Neutron ScatterNeutron Scatter

Soil DielectricSoil Dielectric

The dielectric permittivity is a measure of the The dielectric permittivity is a measure of the capacity of a noncapacity of a non--conducting material to conducting material to transmit electromagnetic waves or pulses.transmit electromagnetic waves or pulses.

Dielectric Permittivity Dielectric Permittivity Air = 1 Air = 1 soil minerals = 3 to 5soil minerals = 3 to 5

(denser soils have higher apparent (denser soils have higher apparent permittivitiespermittivities). ).

Water 81Water 81

Influencing Factors Influencing Factors

Water Content Water Content Soil Temperature Soil Temperature (small in most cases)(small in most cases)

Soil Porosity and Bulk DensitySoil Porosity and Bulk DensityMinerals (2:1 clays)Minerals (2:1 clays)Measurement FrequencyMeasurement FrequencyAir Gaps (Air Gaps (instalationinstalation–– swelling soils)swelling soils)

Frequency Domain/CapacitanceFrequency Domain/Capacitance

A couple different methods are used A couple different methods are used however, they all use:however, they all use:

Electronic circuit in which the two plates, Electronic circuit in which the two plates, rods or rings use the soil between them as rods or rings use the soil between them as dielectric of a capacitordielectric of a capacitor

The change in the circuit output is The change in the circuit output is related to the dielectric permittivityrelated to the dielectric permittivity

Capacitance (Capacitance (CC), measured in ), measured in Farads (F), is defined as:Farads (F), is defined as:

the amount of charge (the amount of charge (QQ) required to ) required to increase the voltage (increase the voltage (VV) by one volt ) by one volt between two plates separated by a known between two plates separated by a known distance containing an insulating material distance containing an insulating material

Electrode

Access Tube

Data Logger

Solar Panel

Cable

Electrode

Ground Surface

Permanent/loggingMulti depth

Single point measurePortable

Capacitance Probe

FDRFDR

Soil specific calibration curves are Soil specific calibration curves are needed for soils that are highly needed for soils that are highly conductive, have high organic content, conductive, have high organic content, or contain 2:1 clays or contain 2:1 clays

Vol

umet

ric W

ater

Con

tent

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Period (msec)

0.4

0.3

0.2

0.1

0.0

0.6

After Kim and Benson 2002

Soil Specific CalibrationSoil Specific Calibration

FDRFDR

Silt Loam

FDRFDR

C-Probe

0

10

20

30

40

50

60

70

80

90

12/9 12/14 12/19 12/24 12/29 1/3 1/8

Soil

Moi

stur

e

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Rai

nfal

l (in

)

Rainfall 36 in 24 in 48 in

C-Probe

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

70.0

75.0

6/13 6/20 6/27 7/4 7/11

Moi

stur

e

12in 24 in 36 in 48 in 60 in

Irr = 1.4 in

C-Probe

25.0

75.0

125.0

175.0

225.0

275.0

6/13 6/20 6/27 7/4 7/11

Moi

stur

e12in 24 in 36 in 48 in 60 in Sum

Irr = 1.4 in

FDR AdvantagesFDR Advantages

Relatively inexpensiveRelatively inexpensivelow frequency standard circuitrylow frequency standard circuitry

No radiation hazard / hassles No radiation hazard / hassles Fast response timeFast response timeLogging capableLogging capablePortablePortable

FDR DisadvantagesFDR Disadvantages

Small measurement volume sensitive to smallSmall measurement volume sensitive to small--scale soil variations (most in 5cm)scale soil variations (most in 5cm)

Sensitivity to installation Sensitivity to installation

Site specific calibration is necessary for Site specific calibration is necessary for accurate soil volumetric water contentaccurate soil volumetric water content

Tends to have larger sensitivity to salinity, Tends to have larger sensitivity to salinity, temperature, bulk density, clay content and air temperature, bulk density, clay content and air gaps than TDR gaps than TDR

Figure B-2. Winegrape non-irrigated in/ft by depth

0.000.200.400.600.801.001.201.401.601.80

Date

Tot

al W

ater

(in/

hr) 9"

21" 33" 45" 57" 69" 82" 94" 106"

Using Neutron Probe DataUsing Neutron Probe Data

Available Soil MoistureAvailable Soil Moisture

Field Capacity Field Capacity –– Perm wilt pointPerm wilt point

Field CapacityField CapacityUpper limit when drainage ceasesUpper limit when drainage ceases

Permanent Wilting pointPermanent Wilting pointLower limit when plants cannot extract moistureLower limit when plants cannot extract moisture

water content (m3 m-3)

Vine Irrigation StrategiesVine Irrigation Strategies

Full potential water useFull potential water useWithhold irrigation Withhold irrigation

Moderate vine water deficitModerate vine water deficitSevere vine water deficit Severe vine water deficit

Early seasonEarly seasonMid season Mid season Late seasonLate season

Precision Micro-irrigation

Sensor PlacementSensor Placement

Depends on the goal of the measurementDepends on the goal of the measurement

If measuring soil water depletion before If measuring soil water depletion before irrigationirrigation------ not too importantnot too important

If measuring after irrigationIf measuring after irrigation–– proximity to proximity to the emitter will effect the readingthe emitter will effect the reading

When To Measure Soil MoistureWhen To Measure Soil MoistureQuantitative( N Probe)Quantitative( N Probe)

Most valuable times:Most valuable times:Bud breakBud breakJust prior to 1st irrigation Just prior to 1st irrigation Dry pointDry point

Bud break Bud break –– Dry Point = Available waterDry Point = Available waterBud break Bud break –– Prior to 1Prior to 1stst irrirr = Water consumed= Water consumedPrior to 1Prior to 1stst irrirr –– Dry Point = water remainingDry Point = water remaining

Looking AheadLooking Ahead

Increased use of devices which can log Increased use of devices which can log transmit and allow automatic data transmit and allow automatic data processing. processing. ------ Dielectric methodsDielectric methods