Combining dewpoint and Wind/Schindler methods to create full

range SMCCs

Doug Cobos, Colin Campbell, and Leo Rivera

Decagon Devices, Inc., Pullman, WAWashington 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 likeCompacted soilsSwelling claysLow bulk density soils

Characterizing unsaturated soils

MeasurementsWater content is relatively easy to

measureSuction requires more sophisticated

and time-consuming methodsGoal

Investigate two improved methods for obtaining SWCC

Background: Creating the soil water characteristic curve

Soil water content

Soil suction Soil suction

Background: Filter Paper

Based on work by Hamblin (1981), Al-Khafaf and Hanks (1974), and Deka et al. (1995)

Calibrated methodFilter paper in suction equilibrium with soil

sampleMeasure water content of filter paperCorrelated with suction through calibration

relationship (SWCC of filter paper) Provided suction measurements without

difficult lab setup

Background: Filter Paper

ProblemsCalibrated method that relies on

repeatable filter paper SWCCResults 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 hysteresisLabor 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 regionTensiometerWIND/SCHINDLER integrated

tensiometer and scale evaporation method

Vapor pressure method for dry regionSimple, 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

The average water content and the average water potential give a discrete value of the SMCC at any time.

HyProp Output

hi1 - hi

2 and Δ mass of sample give hydraulic conductivity

HyProp Output

Can obtain one additional (drier) data point using the air entry point of the ceramic

Air Entry Point of 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

Infrared SensorMirror

Optical Sensor

Fan

Sample

Generating SMCC with WP4C

Preparing SMCC samples

1. Air dry soil2. 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. andread 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

0 10 20 30 40 50 60 70 80 90 1000.000

0.020

0.040

0.060

0.080

0.100

0.120

moisture characteristic curvelinear scale

water potential (-MPa)

VW

C (

m3

/m3

)

0.1 1 10 100 10000.000

0.020

0.040

0.060

0.080

0.100

0.120

moisture characteristic curvesemi-log plot

water potential (-MPa)

VW

C (

m3

/m3

)

0.001

0.01

0.1

1

10

100

0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350

Wat

er p

oten

tial

(-M

pa)

VWC (m3/m3)

all

dry range

wet range

Silt loam SWCC: Tensiometer & WP4

Data Void: Original WP4Su

ctio

n(M

Pa)

Water Content (g/g)

New WP4C: 10x better temperature

measurement: 0.001o C precision

Chilled mirror absolute error of wet-end suction (WP4C and WP4)

Error of Original Chilled Mirror Sensor (WP4) +/- 100 kPa

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #1

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000

grav

imet

ric w

ater

cont

ent

(g/g

)

water potential (-kPa)

Soil B2

WP4C dewpoint

T5 tensiometer

Campbell and Shiozawa

Suction(kPa)

Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #2

0

0.2

0.4

0.6

0.8

1

1 10 100 1000 10000 100000

grav

imet

ric w

ater

cont

ent

(g/g

)

water potential (-kPa)

Soil B4

WP4C dewpoint

T5 tensiometer

Campbell and Shiozawa

Suction(kPa)

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 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