1. 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

2. 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

3. 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

4. Background: Creating the soil water characteristic curve Soil water content Soil suction

5. Background: Filter Paper Based on work by Hamblin (1981), Al- Khafaf 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

6. 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

7. 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

8. 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 K unsat at low (dry) water potential

9. 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)

10. “No-man’s Land” of suction instrumentation

11. 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

12. 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)

13. Wind/Schindler Evaporation Method

14. 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

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

16. h i 1 - h i 2 and Δ mass of sample give hydraulic conductivity HyProp Output

17. Can obtain one additional (drier) data point using the air entry point of the ceramic Air Entry Point of Ceramic

18. 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 Sensor Mirror Optical Sensor Fan Sample

19. Generating SMCC with WP4C

20. Preparing SMCC samples 1. Air dry soil 2. Grind and/or sieve with 2 mm sieve (if necessary)

21. 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

22. 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

23. Preparing samples 6. Mix samples thoroughly

24. 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!

25. Measure water potential with the WP4C Insert sample Seal chamber Wait ~5 min. and read the result

26. 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

27. Construct SMCC

28. Silt loam SWCC: Tensiometer & WP4 Data Void: Original WP4 Suction (MPa) Water Content (g/g) New WP4C: 10x better temperature measurement: 0.001 o C precision

29. Chilled mirror absolute error of wet- end suction (WP4C and WP4) Error of Original Chilled Mirror Sensor (WP4) +/- 100 kPa

30. Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #1 Suction (kPa)

31. Combined Tensiometer and Chilled Mirror SWCC: Coarse Textured Soil #2 Suction (kPa)

32. Schwana loamy fine sand

33. Palouse silt loam

34. 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

35. 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

36. 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