Combining dewpoint and
Wind/Schindler methods to
create full range SMCCs
Doug Cobos, Colin Campbell, and Leo Rivera
Decagon Devices, Inc., Pullman, WA
Washington State University, Pullman, WA
Characterizing unsaturated soils
 Relationship between suction and water
content defines soil water characteristic
curve (SWCC)
Soil water characteristic curve (SWCC) is central
to the behavior of unsaturated soils (Fredlund
and Rahandjo, 1993; Barbour, 1998)
 Key in understanding unsaturated soils like
Compacted soils
Swelling clays
Low bulk density soils
Characterizing unsaturated soils
Measurements
Water content is relatively easy to
measure
Suction requires more sophisticated and
time-consuming methods
Goal
Investigate two improved methods for
obtaining SWCC
Background: Creating the soil water
characteristic curve
Soil suction
Soil suction
Soil water content
Background: Filter Paper
 Based on work by Hamblin (1981), AlKhafaf and Hanks (1974), and Deka et
al. (1995)
 Calibrated method
 Filter paper in suction equilibrium with soil
sample
 Measure water content of filter paper
 Correlated with suction through calibration
relationship (SWCC of filter paper)
 Provided suction measurements without
difficult lab setup
Background: Filter Paper
Problems
Calibrated method that relies on
repeatable filter paper SWCC
Results are affected by equilibration
time, hydraulic conductivity, paper
contact with soil, fungal growth
Slight temperature gradient has huge
effect (8 MPa/C error)
Filter paper SWCC has hysteresis
Labor and time intensive
Axis translation
 Porous plate and soil sealed in
chamber
 Outflow at atmospheric
pressure
 Chamber and soil at elevated
pressure
 Can achieve much higher ΔP
than under tension
Axis translation
 Effectiveness of axis translation at
low (dry) water potential routinely
questioned
 Recent work shows that samples
equilibrated at -1.5 MPa only
reached -0.55 MPa
 Hydraulic disconnect between plate
and soil sample
 Low Kunsat at low (dry) water potential
Axis translation
 Or and Tuller 2002, Baker and
Frydman 2009
 Soil pores don’t drain the same
way under positive pressure as
they do under tension
 SMCCs with axis translation
fundamentally different from
those developed under tension
D. Or and M. Tuller. 2002. Cavitation during
desaturation of porous media under tension.
Water Resources Research 38: (19-1) – (19-4)
“No-man’s Land” of
suction
instrumentation
New Measurement Methods
Liquid equilibrium for wet region
Tensiometer
WIND/SCHINDLER integrated tensiometer
and scale evaporation method
Vapor pressure method for dry region
Simple, fast (5 to 15 min)
Evaluate consistency between wet and
dry regions
Tensiometer: Suction in “wet” soil
 Equilibrates water under
tension with soil water
through a porous cup
 Measures pressure of
water
 Highest accuracy, but
limited range (Suction: 0
to 80 kPa)
 Must be measured in
representative sample
(compaction)
Wind/Schindler Evaporation Method
SMCC with HyProp (Wind Schindler)
 HyProp is setup with
saturated soil sample
 Measures sample weight and
tension at two different points
as sample naturally dries
 Typically takes 4 to 7 days
HyProp Output
The average water content and the
average water potential give a discrete
value of the SMCC at any time.
HyProp Output
hi1 - hi2 and Δ mass of sample give
hydraulic conductivity
Air Entry Point of Ceramic
 Can obtain one additional (drier) data point
using the air entry point of the ceramic
Suction in “Dry” range
Cool mirror until dew forms
Detect dew optically
Measure mirror temperature
Measure sample temperature
with IR thermometer
 Accuracy +/- 50 kPa or better




Optical Sensor
Mirror
Infrared Sensor
Sample
Fan
Generating SMCC with WP4C
Preparing SMCC samples
1. Air dry soil
2. Grind and/or sieve with 2 mm sieve (if necessary)
Preparing samples
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
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
5. Amount of water added depends on soil type and
range of interest
Preparing samples
6. Mix samples thoroughly
Preparing samples
7. Cap samples and allow to equilibrate overnight
8. Remove lids and allow to dry for 30-60 minutes
9. Replace lids and allow to re-equilibrate for 3-6
hours
Done with preparation!
Measure water potential with the WP4C
Insert sample
Seal chamber
Wait ~5 min. and
read the result
Measure the water content
Dry in a 105 C oven
for 24 hours
Weigh moist samples
Weigh dry samples
w = (moist soil mass – dry soil mass)/dry soil mass
Construct SMCC
moisture characteristic curve
linear scale
0.120
0.080
0.060
0.040
0.020
0.000
0
20
40
60
water potential
(-MPa)
80
100
moisture characteristic curve
semi-log plot
0.120
0.100
VWC (m3/m3)
VWC (m3/m3)
0.100
0.080
0.060
0.040
0.020
0.000
0.1
1
10
water potential
(-MPa)
100
1000
Silt loam SWCC: Tensiometer &
WP4
(MPa)
Suction (-Mpa)
Water potential
100
New WP4C: 10x better
temperature measurement:
0.001o C precision
10
1
Data Void: Original WP4
0.1
all
dry range
wet range
0.01
0.001
0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350
Water Content (g/g)
VWC (m3/m3)
Chilled mirror absolute error of wetend suction (WP4C and WP4)
Error of Original Chilled Mirror Sensor (WP4) +/- 100 kPa
Combined Tensiometer and Chilled
Mirror SWCC: Coarse Textured Soil #1
Soil B2
gravimetric water content
(g/g)
1
0.8
WP4C dewpoint
0.6
T5 tensiometer
0.4
Campbell and
Shiozawa
0.2
0
1
10
100
1000
water
potential
Suction
(-kPa)
(kPa)
10000
100000
Combined Tensiometer and Chilled
Mirror SWCC: Coarse Textured Soil #2
Soil B4
gravimetric water content
(g/g)
1
0.8
WP4C dewpoint
0.6
T5 tensiometer
Campbell and
Shiozawa
0.4
0.2
0
1
10
100
1000
water
potential
Suction
(kPa)
(-kPa)
10000
100000
Schwana loamy fine sand
Palouse silt loam
Important considerations
 Hysteresis
 Hyprop always on drying leg
 WP4C must be on drying leg too in fine textured soils
 Soil structure (fabric)
 For best measurements in wet end, WP4C should use
intact samples
 Matric vs. total suction
 Tensiometers – Matric
 WP4C – Matric + Osotic
 Correction needed in salty soil
Summary
 New techniques make determining soil water
characteristic curves easier and more accurate
 Improved measurement range
 Faster and less time consuming measurements
 New chilled mirror measurements bridge traditional
“no man’s land”
 Measurements at low suctions match nicely with
tensiometer
 WIND/SCHINDLER method allows automation of “wet”
range SWCC and unsaturated hydraulic conductivity
 Simple drying procedure
 Software fits SWCC and gives hydraulic conductivity
function
References
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Al-Khafaf, S., and Hanks, R.J. 1974. Evaluation of the filter paper method for estimation soil water
potential. Soil Sci. 117:194-199
R. Baker and S Frydman. 2009. Unsaturated soil mechanics: Critical review of physical foundations.
Engineering Geology 106: 26-39.
Barbour, S.L. 1998. Nineteen Canadian geotechnical colloquium: The soil-water characteristic cure: A
historical perspective. Canadian Geotechnical Journal. 35:873-894.
Bittelli, M. and Flury, M. 2008. Errors in Water Retention Curves Determined with Pressure Plates. Soil Sci.
Soc. Am. J. 73:1453-1460
Deka, R.N., Wairiu, M., Mtakwa, P.W., Mullins, C.E., Veenendaal, E.M., and Townsend, J. 2995. Use and
accuracy of the filter-paper technique for measurement of soil matric potential. Eur. J. Soil Sci. 46:233-238
Fredlund, D.G. and Rahardjo, H. 1993. Soil mechanics for unsaturated soils. John Wiley and Sons, Inc.:
New York.
Gardner, W.R. 1937. A method of measuring the capillary tension of soil moisture over a wide moisture
range. Soil Science. 43(4), 277-283
Gee et. al, 2002. The influence of hydraulic disequilibrium on pressure plate data. Vadose Zone Journal.
1: 172-178.
Hamblin, A.P. 1981. Filter paper method for routine measurement of field water potential. J. Hydrol.
53:355-360