Download - MS 03 Soil Water Content
-
7/30/2019 MS 03 Soil Water Content
1/13
ETSEA
Soil Water Content
Jorge Lampurlans ([email protected])Department of Agricultural and Forestry Engineering
ETSEA
Soil Water Content Measurement
Thermogravimetric Method Neutron ProbeTime Domain Reflectometry (TDR) Capacitance Method
Frequency Domain Reflectometry (FDR) Ground Penetrating Radar (GPR) From soil water potential
ETSEAThe Water in Soil
Soil Water Content:
Gravimetric basis: [g water/g water]
Volumetric basis: [m3 water/ m3 water]
Related by bulk density: [g soil/ m3 soil]
=
On the soil profile: W [m3 water/ m2 water]
W = i zi (zi, soil depth in mm)
3Soil Water Content Department of Agricultural and Forestry Engineering
ETSEA
4Soil Water Content Department of Agricultural and Forestry Engineering
The Water in Soil
-
7/30/2019 MS 03 Soil Water Content
2/13
ETSEA
Lysmeter
5Soil Water Content Department of Agricultural and Forestry Engineering
The Water in SoilETSEA
Lysimeter
6Soil Water Content Department of Agricultural and Forestry Engineering
The Water in Soil
ETSEA
Lysimeter
7Soil Water Content Department of Agricultural and Forestry Engineering
The Water in SoilETSEA
Soil Water Balance
Equations
Complete: ET = Wi + [P + I +Re] - [Rl + D] - Wf Simplified: ET = W i + P - Wf
I+Re RlP
W i W f
D
ET
8Soil Water Content Department of Agricultural and Forestry Engineering
The Water in Soil
ET: EvatopranpirationW: Soil Water Content (inicial, final)
P: PrecipitationI: IrrigationR: Surface Runoff (entering, leaving)
-
7/30/2019 MS 03 Soil Water Content
3/13
ETSEAThermogravimetric Method
PRINCIPLE
Soil water content definition: the water that may beevaporated from a soil by heating between 100 y 110 C(105 C) until there is no further weight loss.
9Soil Water Content Department of Agricultural and Forestry Engineering
ETSEAThermogravimetric Method MATERIAL & METHODS
Soil sampling (50 to 100 g of soil): auger (hand operated)
10Soil Water Content Department of Agricultural and Forestry Engineering
Edelman Riverside Stony soil
ETSEAThermogravimetric Method MATERIAL & METHODS
Soil sampling (50 to 100 g of soil): auger (soil column cylinder auger)
11Soil Water Content Department of Agricultural and Forestry Engineering
ETSEAThermogravimetric Method MATERIAL & METHODS
Soil sampling (50 to 100 g of soil): auger (soil column cylinder auger)
12Soil Water Content Department of Agricultural and Forestry Engineering
-
7/30/2019 MS 03 Soil Water Content
4/13
ETSEAThermogravimetricThermogravimetric MethodMethod MATERIAL & METHODS
Soil sampling (50 to 100 g of soil): auger (hydraulic drive)
hermetic containers (plastic pots)
13Soil Water Content Department of Agricultural and Forestry Engineering
ETSEAThermogravimetric Method MATERIAL & METHODS
Soil water evaporation at 105 C until constant weight(weight change in 0,5 to 4 hours < 0,1% of the initialweight): 24 to 48 hours. Forced air oven Laboratory balance
14Soil Water Content Department of Agricultural and Forestry Engineering
ETSEA
WARNINGSAt 110 C not all water removed
T > 110 C organic mater oxidation
T > 80 C gypsium hydratation water lost
CALCULATIONS = (MwetMdry)/Mdry
STONY SOILS: = (
-
7/30/2019 MS 03 Soil Water Content
5/13
ETSEANeutron Probe PRINCIPLE
H slows down fast neutrons (thermalization)
Most H in soil is in soil water
Changes in soil H Changes in water content
[Thermalized Neutrons] = f(t)
17Soil Water Content Department of Agricultural and Forestry Engineering
ETSEANeutron Probe PRINCIPLE
Calibration: = R/R s a + b
: Volumetric water contentR: Neutron count rate in soilRs: Count rate in a standard medium (Water)R/Rs: Count-rate ratioa y b: Calibration constants
18Soil Water Content Department of Agricultural and Forestry Engineering
ETSEANeutron Probe MATERIAL & METHODS
Access Tubes Aluminium, brass, stainless steel, plastic.
Transparency to neutrons (not PVC).
Mechanical strength.
Resistance to corrosion.
Stoppers to seal the top and end pieces to close the botton.
Access Tube Installation Minimize dirturbance to the soil, soil surface and vegetation to obtain
truly representative measurements of the area
Acces tube fit tightly into the soil to prevent voids and channeling ofwater dow besides it.
19Soil Water Content Department of Agricultural and Forestry Engineering
ETSEA
MATERIAL & METHODS Neutron probe
Prove (Sourve & Detector)
Prove cable (strong, waterproof,with stoppers)
Stable power supply (1-2 kV)
Counter unit (8 to 64 s)
Prove carrier (limits radiation)
20Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe
-
7/30/2019 MS 03 Soil Water Content
6/13
ETSEA
WARNINGS
Sphere of importance Decreases with soil water content
(0.15 m in wet soil0.5 in very dry soil)
Depth increments > 0.1 m
Smoot water content profile
Total profile water content: OK
Errors at the soil surface(< 0.15-0.30 m)
21Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe ETSEAWARNINGS
Random Counting Errors Increase the counting time (64 s)
Weekly standard counts (1 hour, in water)
Need of site-specific calibration Presence of strong neutron absorbers: iron, chlorine
Bulk density increases count rate specially in wet conditions
Marked textural boundaries
Stones
Access tubes
22Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe
ETSEA
FIELD CALIBRACINTake soil samples (water content determination)
during the excavation for the access tube installation. Record neutron counts at the required depths.
Determine volumetric water content of the samplesthermogravimetrically.
Repeat at different access tubes when the soil is atdifferent water contents (drying, irrigation).
Take undisturbed samples of known volume near theaccess tube at the same depths.
23Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe ETSEA FIELD CALIBRATION
24Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe
-
7/30/2019 MS 03 Soil Water Content
7/13
ETSEA
Commercial neutron probes: Troxler 3% (Troxler) (Cambell Pacific)
25Soil Water Content Department of Agricultural and Forestry Engineering
Neutron Probe ETSEAMethods based on K PRINCIPLE
Include de moist soil as part of the dielectric of a capacitor and
measure its capacitance that gives the dielectric constant (K) orrelative permittivity ():
capacitance with soil
capacitance with air
K is frequency (F) dependent: F < 30 MHz: proportion and configuration of air filled pores
F between 30-3.000 MHz K = f(water, soil, air)
(Kwater 80, Ksoil 3-5, Kair= 1) F > 3000 MHz: water dipoles do not follow the electromagnetic fields
K = =
26Soil Water Content Department of Agricultural and Forestry Engineering
ETSEA
PRINCIPLE
27Soil Water Content Department of Agricultural and Forestry Engineering
Methods based on K ETSEA METHODS
TDR (Time Domain Reflectometry): measurement of
the travel time of and electromagnetic wide range highfrequency pulse (step voltage) through the soil.
Frequency Domain (FD)fixed low frequency (around 100 MHz) Capacitance: measurement of the capacitance of the soil.
FDR (Frequency Domain Reflectometry): Measurement ofthe complex impedance.
WARNINGS Measurement of free water only
28Soil Water Content Department of Agricultural and Forestry Engineering
Methods based on K
-
7/30/2019 MS 03 Soil Water Content
8/13
ETSEATime Domain Reflectometry (TDR) PRINCIPLE
The propagation velocity of an electromagnetic wave down atransmission line in a nonmagnetic medium (soil) is related
with the dielectric constant (K) or permittivity () of themedium
A high-frequency electromagnetic pulse is fed into the soilbetween two metal rods. Part of the pulse is reflected back upthrough the soil from the bottom of the rods, and the time
interval between the incident and reflected pulses is measured.
If Kv t
K
cv =
29Soil Water Content Department of Agricultural and Forestry Engineering
t
Lv
2=
(c = 3108 m/s)
ETSEATime Domain Reflectometry (TDR) PRINCIPLE
30Soil Water Content Department of Agricultural and Forestry Engineering
2
2
=
L
tcK
2
tcLAB =
2
=
L
LK AB
K
cv =
t
Lv
2=
ETSEA
MATERIAL & METHODS Different proves (transmission lines)
31Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR) ETSEA MATERIALS & METHODS
Transmission line or wave guide Coaxial Cable 50 (no more than 2 m)
2 to 3 rigid parallel metallic rodsinserted into the soil Rod diameter: 3 - 5 mm.
Rod spacing 1.5 - 10 cm(Rod spacing / Rod diameter < 10).
Rod length: 5 - 50 cm (attenuation).
Vertically or horizontally inserted.
Avoid rod to soil air gaps(rods diameters as large as possible).
Parallelism is important forconductivity measurements.
32Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR)
-
7/30/2019 MS 03 Soil Water Content
9/13
ETSEA
MATERIAL & METHOS Cable tester (Tectronyc 1502C)
Wave Form Analysis: beginning and end of thetransmission line (manual or automatic)
33Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR) ETSEA
WARNINGS Signal attenuation in saline soils by ionic conduction
Insulation of the central rod.
Bulk Electrical Conductivity measurement:
34Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR)
app
f
f
u
s
RZ
KmS
+
=
1
1)/(
0
0
V
VVff
= (final reflection coefficient)
Zu = 50 for Tektronix 1502B cable tester.Rapp measured by sorting the sensor rods.Ks sensor constant obtained by calibration
with saline solutions.(Temperature correction)
ETSEA
WARNINGSThe method gives and average value of water content
over the length of the transmission line.
Volume of influence: cylinder of = rod distance
35Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR) ETSEA CALIBRATION
Universal calibration or Topp equation
Works better for sandy soils. Good for relative values. Low bulk densities,
specific mineralogical properties,clays and soil structure modifies K().=> Soil-specific calibration.
Field calibration Only a dry and wet point and adjust
the Topp equation.The complete relationship.
43210)043.05,5292530(
++= KKK
36Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR)
-
7/30/2019 MS 03 Soil Water Content
10/13
ETSEA
Commercial equipment TRASE (Soilmoisture)
37Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR) ETSEA Commercial equipment
Hydrosense 3% (Campbell)
TDR100 (Campbell) TRIME 3% (SDEC)
38Soil Water Content Department of Agricultural and Forestry Engineering
Time Domain Reflectometry (TDR)
ETSEACapacitance Method
PRINCIPLE Measurement of the soil capacitance K
With a bridge at a frequency of 100-150 MHz
With a LC oscillator circuit, the frequency of oscillation is a directmeasure of the capacitance.
MATERIALS & METHODS Access Tube
Plastic (metal are not suitable, it acts as a barrier to the electric field)
The gap between access tube and soil < 0.5 mm
SAKC 108542,8 12=
39Soil Water Content Department of Agricultural and Forestry Engineering
LCF
2
1=
ETSEA
MATERIALS & METHODS Prove
Parallel electrodes
Cylindrical
40Soil Water Content Department of Agricultural and Forestry Engineering
Capacitance Method
-
7/30/2019 MS 03 Soil Water Content
11/13
ETSEA
WARNINGS Distance between electrodes. Compromise between:
Radius of influence (3-4 cm)
Depth resolution (2 cm)
Measurement dominated by the soil around theelectrodes: small-scales lateral heterogeneity, air gapsand channelling of water may interfere.
Requires soil-specific calibration.
41Soil Water Content Department of Agricultural and Forestry Engineering
Capacitance Method ETSEAWARNINGS
No linear relationship between K and
42Soil Water Content Department of Agricultural and Forestry Engineering
Capacitance Method
ETSEAFrequency Domain Reflectometry (FDR) PRINCIPLE
A low frequency (100 MHz) sinusoidal signal is propagatedalong the transmission line to the soil probe and reflects back
(change in K) causing a voltage standing wave to be set up onthe transmission line. The amplitude difference between the beginning and at the end
of the transmission line is empirically related to the complexrelative dielectric constant:
K = K - j K K is a function of . K is related to the bulk electrical conductivity.
43Soil Water Content Department of Agricultural and Forestry Engineering
ETSEA
ADVANTAGES The sensor is factory calibrated with a reference
impedance. This improves accuracy by eliminating
influences of cable lengths and quality, connectors andswitches. Cheaper sensors. They measure also temperature to make corrections. K more sensitive to changes than K potentially more
accurate than TDR. DISADVANTAGES
Due to the low frequency the relationship between K a is more influenced by the soil calibration
44Soil Water Content Department of Agricultural and Forestry Engineering
Frequency Domain Reflectometry (FDR)
-
7/30/2019 MS 03 Soil Water Content
12/13
ETSEA
SOIL-SPECIFIC CALIBRACIN
45Soil Water Content Department of Agricultural and Forestry Engineering
Frequency Domain Reflectometry (FDR) ETSEA
Commercial proves: Diviner2000 (Sentek)
Enviroscan (Sentek)
46Soil Water Content Department of Agricultural and Forestry Engineering
Frequency Domain Reflectometry (FDR)
ETSEA
Comercial probes: Thetaprove (Delta-T)
ML2x 1%
SM200 3%
Profile probe (Delta-T)
47Soil Water Content Department of Agricultural and Forestry Engineering
Frequency Domain Reflectometry (FDR) ETSEA
Commercial probes: ECH2O 3% (Decagon)
48Soil Water Content Department of Agricultural and Forestry Engineering
Frequency Domain Reflectometry (FDR)
-
7/30/2019 MS 03 Soil Water Content
13/13
ETSEAGround Penetrating Radar (GPR)
PRINCIPLE
Uses short pulses of high frequency electromagnetic waves (50-1500 MHz)
ADVANTAGES Noninvasive
Fast
Suitable forlarge areas
49Soil Water Content Department of Agricultural and Forestry Engineering
ETSEAFromFrom SoilSoil WaterWater PotentialPotential
PRINCIPLE
Estimate soil water content () measuring soil water potential(h).
DRAWBACKS: We need to know the (h)
Hysteresis of the (h) curve
We need to measure soil waterpotential
50Soil Water Content Department of Agricultural and Forestry Engineering