12/9/2010
Generating Soil Moisture Characteristic
Curves with Vapor Pressure Methods
Outline
 Introduction
 Defining water content, water potential and Soil
Moisture Characteristic Curve (SMCC)
 Measuring water potential and water content
 Generating
g SMCC
 Sample preparation
 Data analysis
 Applications
 Automated SMCC instruments
Douglas R. Cobos, Ph.D.
Decagon Devices and Washington State
University
 Aquasorp for dry soils
 Hyprop for moist soils
Two Variables are Needed to
Describe the State of Water
Heat content
Charge
and
Water potential
Quality
Intensity
Related Measures
and
Temperature
and
Voltage
Definition: Volumetric Water Content

Vw
VT



Air
 is volumetric water
content (VWC),
Vw is the volume of water
VT is total sample volume
15%
Water
35%
Soil
50%
35% VWC
Separate into constituent parts
 Relates water content to
water potential in a soil
 Different for each soil
 Used for
 Converting
g from water
content to water potential
(and vice versa)
 Finding plant available
water
 Estimating specific surface
area of soil
 Predicting soil swelling
 Many other uses
0.5
Clay
Loam
0.4
Sand
0.3
0.2
0.1
Wate
er Content (g/g)
Water content
Quantity
Extent
Soil Moisture Characteristic Curve
(SMCC)
0
-2000
-1500
-1000
-500
0
Water Potential (kPa)
Definition: Gravimetric water content
 Gravimetric water
content (w)
w
mw
md
m – mass
w – water
d – dry solids
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12/9/2010
Definition: Water Potential
Water Potential: important points
Energy required, per quantity of water, to
transport, an infinitesimal quantity of water
from the sample to a reference pool of
pure,, free water
p
 Energy per unit mass, volume, or weight of
water
We use units of pressure (Bars, MPa, kPa,
meters H2O,)
 Differential property
A reference must be specified (pure, free water
is the reference; its water potential is zero)
 The water potential in soil is almost always
less than zero
Water Potential is influenced by:
 Binding of water to the soil matrix
 Solutes in the water
 Pressure on the water (hydrostatic or
p
pneumatic)
)
 Position of water in a gravitational field
Only the first two can affect the SMCC
Measuring water potential
 Liquid equilibrium techniques
 Work best in the wet range of water
potential
 Only measure matric potential
 Important for understanding liquid water flow
 Tensiometer (0 to - 0.085 MPa)
 Pressure plates (0 to -0.5 MPa)
Total = Sum of Components
 = m + o + g + p
m
o
g
p
matric
osmotic
gravitational
pressure
adsorption forces
solutes
position
hydrostatic or pneum.
Water Potential and Relative
Humidity
Relative humidity (hr) and water potential (Ψ)
related by the Kelvin equation:

RT
ln hr
Mw
R is universal gas constant
Mw is molecular mass of water
T is
i temperature
t
t
Condition
Water Potential (MPa)
Relative Humidity (hr)
Pure, free water
0
1.000
Field Capacity
-0.033
0.9998
Permanent wilting point
-1.5
0.989
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12/9/2010
Vapor Equilibrium Methods
Vapor Equilibrium Methods
 Only work well in the dry end of soil water
potential
 Give total water potential (matric + osmotic)
 Thermocouple psychrometer
 Dew point hygrometer
 Best for p
plant water relations or other biological
g
systems
 In non-saline soils, osmotic component is negligible,
so total water potential ≈ matric potential
Dew Point Method for Measuring Water
Potential
Chilled Mirror Dew Point Method
 Cool mirror until dew forms
 Gives total water potential
(matric + osmotic)
 Detect dew optically
 Measure mirror temperature
Optical Sensor
Mirror
Infrared Sensor
 Measure sample temperature with
IR thermometer
Sample
 Water potential is approximately
linearly related to Ts - Td
Measured Su
uction (MPa)
WP4C - Verification with standards
Fan
 Range is 0 to -300 Mpa
 Accuracy is about ±0.05
MPa
Measuring Water Content: Gravimetric (w)
Technique
400
300
200
100
measurements
1:1 line
0
0
100
200
300
400
Suction of Standard (MPa)
w = (moist soil mass – dry soil mass)/dry soil mass
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12/9/2010
Volumetric vs. Gravimetric Water
Content
Field measurements of water content
 All field methods measure Volumetric water
content

Volumetric
Water Content
(VWC)

Outline
 Introduction
 Defining water content, water potential and Soil
Moisture Characteristic Curve (SMCC)
 Measuring water potential and water content
Water volume
per unit total
volume

Soil bulk Density, b
b 
md
VT
 w
b
w
Gravimetric
Water Content
(GWC)

Water weight
per unit dry
soil weight
Preparing samples: the simple method
1. Air dry soil
2. Grind and/or sieve with 2 mm sieve (if necessary)
 Generating
g SMCC
 Sample preparation
 Data analysis
 Applications
 Automated SMCC instruments
 Aquasorp for dry soils
 Hyprop for moist soils
Preparing samples: the simple method
3. Fill 10-12 stainless steel sample cups ~1/2 full of dry soil
- Weigh out same mass of soil in each cup
- ~2-7 g depending on density
Preparing samples: the simple method
4. Add ascending amount of DI water to each
sample
- 0, 1, 2, 4, 6, 8, 10, 14, 18, 22… drops of water works well
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12/9/2010
Preparing samples: the simple method
Preparing samples: the simple method
5. Mix samples thoroughly
6. Cap samples and allow to equilibrate at least
overnight
7. Done!
Preparing samples: the precise method
 Calculate amount of
water to add
 Must use larger samples
w1.5
(g/g)
wad
(g/g)
Sand
0.008
0.003
loamy sand
0.015
0.005
sandy loam
0 045
0.045
0 015
0.015
sandy clay loam
0.143
0.048
Texture
 Must know soil texture
0.5
Loam
0.4
Sand
0.3
0.2
0.1
0
-1500
-1000
-500
Water Potential (kPa)
0
Water Content (g/g)
Clay
-2000
Preparing samples: the precise method
Loam
0.106
0.035
sandy clay
0.204
0.068
silt loam
0.098
0.033
Silt
0.075
0.025
clay loam
0.174
0.058
silty clay loam
0.166
0.055
silty clay
0.204
0.068
Clay
0.234
0.078
Measure water potential with the WP4C
Subsample into stainless steel cups
Detailed instructions:
http://www.decagon.com/assets/Uploads/AN-Determining-15-Bar-Permanent-WiltWater-Content-of-Soils-with-the-WP4C.pdf
Measure the water content
Dry in a 105 C oven
for 24 hours
Insert sample
Weigh moist samples
Seal chamber
Wait ~5 min. and
read the result
Weigh dry samples
w = (moist soil mass – dry soil mass)/dry soil mass
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12/9/2010
Data formatting
Does Sample Density Matter?
 Plot water potential vs.
water content
 Density effects negligible on samples drier than -0.1
MPa
 Water is held mainly on particle surfaces and in very
small pores not affected by packing
0.12
Water C
Content (g/g)
 Log10 water potential
vs. water content
becomes a straight line
 In the dry range, no
0.14
0.1
0.08
0.06
0.04
0.02
 X intercept is oven dry
water potential (-1000
MPa)
 In the wet range, yes
0
1
10
100
1000
 Packing can strongly affect water potential because it
affects sizes of capillaries holding the water
 WP4C pushes into this range
Water Potential (-MPa)
WP4C – improved wet end accuracy
WP4C – improved wet end accuracy
WP4C dewpoint
Soil B2
1
WP4C #1
WP4C #2
WP4C #3
WP4C #3
WP4C #4
‐5000
‐4000
‐3000
‐2000
‐1000
0
KCl solution water potential (kPa)
gravimetricc water content ((g/g)
Measurement error (kPa)
WP4C wet end accuracy
100
80
60
40
20
0
‐20
20
‐40
‐60
‐80
‐100
T5 tensiometer
0.8
0.6
0.4
0.2
0
1
10
100
1000
10000
100000
water potential (‐kPa)
http://www.decagon.com/forums/soil-water-potential/show/24#post24
WP4C – improved wet end accuracy
Soil B4
WP4C dewpoint
gravime
etric water content
(g/g)
1
T5 tensiometer
0.8
0.6
Outline
 Introduction
 Defining water content, water potential and Soil
Moisture Characteristic Curve (SMCC)
 Measuring water potential and water content
 Generating
g SMCC
0.4
 Sample preparation
 Data analysis
0.2
0
1
10
100
1000
water potential (‐kPa)
10000
100000
 Applications
 Automated SMCC instruments
 Aquasorp for dry soils
 Hyprop for moist soils
6
12/9/2010
 Estimate water potential from a measured
water content
 Estimate water content from a measured
water potential
 Estimate plant available water in soil
 Estimate specific surface of soil
 Evaluate the expansiveness of a soil
Estimate WP from WC or WC from WP
0.14
w  a  b ln( )
0.12
Water Conten
nt (g/g)
What can we do with the SWCC?
 aw

 b 
0.1
   exp
0 08
0.08
0.06
0.04
0.02
0
1
10
100
1000
Water Potential (-MPa)
Plant Available Water
 Lower limit of plant available
water is water content at -1.5
MPa (Permanent Wilting Point)
EGME Surfac
ce Area (m2/g)
250
 Drained upper limit (Field
Capacity)


Profile specific (-0.01 to -0.033
MPa)
Often 2X PWP
Can use tensiometer or pressure
plate in this range
2
y = 1231.3x + 406.15x
2
R = 0.9961
200
150
100
50
0
0
0.05
0.1
0.15
0.2
0.25
Expansive Soil Classification from
McKeen(1992)
Moisture Characteristics for a Range of
Soil Texture
3
Class
I
II
III
IV
V
Slope
> -6
-6
6 to
t -10
10
-10 to -13
-13 to -20
< -20
0.3
Slope of Semilog plot
Expansion
special case
hi h
high
medium
low
nonexpansive
Dune Sand
Bentonite
2.5
Log10 (M
MPa)

Soil Specific Surface Area from a
Moisture Characteristic
Missouri
Texas
2
New Mexico
1.5
1
0.5
0
0
0.05
0.1
0.15
0.2
0.25
Water Content (g/g)
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12/9/2010
Expansive Soil Classification from
McKeen(1992)
Coefficients for Semi-Log Fit
Soil
Dune
D
ne Sand
New Mexico
Missouri
Texas
Bentonite
Texture
sand
clay loam
silty clay loam
clay
clay
Slope
a
-224
224
-25.8
-22.3
-14.2
-5.2
Intercept
b
2 94
2.94
3.14
3.02
3.10
2.46
2
r
0 999
0.999
0.997
0.997
0.993
0.98
Outline
Class
I
II
III
IV
V
Slope
> -6
-6
6 to -10
10
-10 to -13
-13 to -20
< -20
Expansion Sample
special case Bentonite -5.4
high
medium
Clay -14.2
low
Silty clay loam -22.3
nonClay loam -25.8
expansive
Sand -224
AquaSorp: Automatic SWCC Generation
for Dry Soils
 Introduction
 Defining water content, water potential and Soil
Moisture Characteristic Curve (SMCC)
 Measuring water potential and water content
 Generating
g SMCC
 Sample preparation
 Data analysis
 Applications
 Automated SMCC instruments
 Aquasorp for dry soils
 Hyprop for moist soils
15
Water Contentt (g/100 g)
Internal Workings of AquaSorp
A Chi Plot of Dry Range Soil Water
Characteristics
Bentonite
Palouse B
New Mexico
10
Walla Walla
Royal
L S il
L-Soil
5
0
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
-ln(-ln(aw))
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12/9/2010
New Mexico Clay – WP4 and
Aquasorp Comparison
HyProp for wet end SMCC and
hydraulic conductivity function
Water Contentt (g/100 g)
14
12
10
-1
1 MPa
8
6
4
2
-100 MPa
0
-2
-1
0
1
2
3
4
5
6
-ln(-ln(a w))
HyProp for wet end SMCC and
hydraulic conductivity function
SMCC
Hydraulic conductivity
Take home
 The SWCC relates water content to water
potential for a given soil
 Each different soil has a different SWCC
 Knowledge of the SWCC gives valuable information
about a soil
 Vapor pressure methods are the only methods
available to construct SMCCs below about -0.5
MPa
 There are quick and easy methods to develop dryend SMCCs (manual and automated)
For more information on instruments,
techniques, or lab services
Thanks!
[email protected]
[email protected]
Or see http://www.decagon.com
p //
g
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