Methods of Media Characterization

1. Methods of Media Characterization A challenging area of rapid advancement

2. Topics • Measurement of pressure potential • The tensiometer • The psychrometer • Measurement of Water Content • TDR (dielectric) • Neutron probe (thermalization) • Gamma probe (radiation attenuation) • Gypsum block (energy of heating) • Measurement of Permeability • Tension infiltrometer • Well permeameter

3. Physical Indicators of Moisture • All methods measure some physical quantity What can be measured? • weight of soil • pressure of water in soil • humidity of air in soil • scattering of radiation that enters soil • dielectric of soil • resistance to electricity of soil • texture of soil • temperature/heat capacity of soil • Each method takes advantage of one indicator

4. Methods: Direct versus indirect • Direct methods measures the amount of water that is in a soil • Indirect methods estimates water content by a calibrated relationship with some other measurable quantity (e.g. pressure) • We will see that the vast majority of tools available are “indirect” • The key to assessing indirect methods is the quality/stability/consistency of calibration

5. Methods: direct • Gravimetric • Dig some soil; Weigh it wet; Dry it; Weigh it dry • Volumetric • Take a soil core (“undisturbed”); Weigh wet, dry Pro’s Con’s - Accurate (+/- 1%) - Can’t repeat in spot - Cheap - Slow - 2 days equipment - free - Time consuming per sample - free

6. Methods: Indirect via pressure • Tensiometers • Psychrometers • Indirect2: Surrogate media Gypsum blocks (includes WaterMark etc.)

7. Communicating with soil: Porous solids • The tensiometer employs a rigid porous cup to allow measurement of the pressure in the soil water. • Water can move freely across the cup, so pressure inside is that of soil

8. Reservoir Gauge Body Removable Cup Pressure measurement: The tensiometer • Can be made in many shapes, sizes. • Require maintenance to keep device full of water • Useful to -0.8 bar • Employed since 1940’s • Need replicates to be reliable (>4)

9. Pressure measurement: The tensiometer • Can be made in many shapes, sizes.

10. Pressure measurement: The tensiometer • Thumbnail: Watch out for: • Swelling soils • tensiometer will loose contact, and not function • Inept users! • Poor for sites with low skill operators of units • Easy to get “garbage” data if not careful • Fine texture soils (won’t measure <-0.8bar) • Most useful in situations where you need to know pressure (engineered waste etc.)

11. Pressure potential: The psychrometer • A device which allows determination of the relative humidity of the subsurface through measurement of the temperature of the dew point Pressure Relative humidity Gas constant Temperature

12. Pressure potential: The psychrometer • Thumbnail: most likely not your 1st choice... • Great for sites where the typical conditions are very dry. In fact, drier than most plants prefer. • Low accuracy in wet range (0 to -1 bar) • Need soil characteristic curves to translate pressures to moisture contents - problem in variable soils • Great for many arid zone research projects

13. W Indirect pressure: Gypsum block, Watermark et al. • Using a media of known moisture content/pressure relationship • Energy of heating a strong function of  • Resistance embedded plates also f(). • Measure energy of heating, or resistance; infer pressure • Problems: • The properties of the media change with time (e.g., gypsum dissolves; clay deposition)! • Making reproducible media very difficult (need calibration per unit) • Hysteresis makes the measurement inaccurate (more on this later)

14. Example: Watermark $260 for meter $27 for probes

15. Indirect Pressure: Gypsum block, Watermark et al. • Idea of indirect pressure measurements: • Measure water content of surrogate media, infer pressure, then infer water content in soil Surrogate Media Soil Pressure Pressure Water content Water content We want a value for water content in our soil We measure water content in the surrogate media

16. Indirect Pressure: Gypsum block, Watermark et al. • Thumbnail: • Generally a low cost option • Calibration typically problematic in time and between units • Poor in swelling soils • Poor in highly variable soils • Sometimes adequate for yes/no decisions

17. Indirect electrical: the nature of soil dielectric • Soils generally have a dielectric of about 2 to 4 at high frequency. • Water has a dielectric of about 80. • If we can figure a way to measure the soil dielectric, it shows water content. • WATCH OUT: the soil dielectric is a function of the frequency of the measurement! For it to be low, need to use high frequency method (>200 mHz)

18. Indirect electrical: Capacitance (dielectric, low frequency) • Stick an unprotected capacitor into the soil and measure the capacitance. • Higher if there is lots of dielectric (i.e., water) • Need to calibrate capacitance vs volumetric water content per soil PROBLEM: • soils have very different dielectrics at low frequency $70 $500

19. Indirect electrical: TDR (dielectric) • Observe the time of travel of a signal down a pair of wires in the soil. • Signal slower if there is lots of dielectric (i.e., water) • Calibrate time of travel vs volumetric water content • Since high frequency, can use “universal” calibration

20. Indirect electrical: TDR (dielectric) • Lots of excitement surrounding TDR now. Why? • non-nuclear • universal calibration • measures volumetric water content directly • wide variety of configurations possible • Long probes (up to 10 feet on market) • Short probes (less than an inch) • Automated with many measuring points • Electronics coming down in price (soon <$500) • Potentially accurate (+/- 2% or better)

21. Indirect radiation: interactions between soil & radiation • When passing through, radiation can either: • be adsorbed by the stuff • change color (loose energy) • pass through unobstructed • Which of these options occurs is a function of the energy of the radiation • Each of these features is used in soil water measurement

22. Indirect radiation:Neutron probe (thermalization) • Send out high energy neutrons • When they hit things that have same mass as a neutron (hydrogen best), they are slowed. • Return of slow neutrons calibrated to water content (lots of hydrogen) • Single hole method • Quite accurate (simply wait for lots of counts) • Lots of soil constituentscan effect calibration

23. Indirect radiation:Neutron probe (thermalization) • Pro’s • Potentially Accurate • Widely available • Inexpensive per location • Flexible (e.g., can go very deep) • Cons • Needs soil specific calibration (lots of work) • Working with radiation • Expensive to buy • Expensive to dispose • Slow to use • can’t be automated

24. Indirect radiation: Gamma probe • Radiation attenuation • Source & detector separated by soil. • Water content determines adsorption of beam energy. • Must calibrate for each soil. • Same used in neutron and x-ray attenuation. • Can use various frequencies to determine fluid content of various fluids (e.g., Oils) • Not used in commercial agriculture

25. Gamma Attenuation • Attenuation follows Beer’s law: each frequency attenuated at different rate; each material having a different attenuation rate. • I= incident radiation • I= transmitted radiation • xi=thickness of medium i • ai=attenuation coefficient for material i at frequency 

26. Indirect via feel:getting to know your soil • A reasonable soil water status may be obtained by checking the feel of the soil • Does It make a ribbon? • Does it stick to your hand? • Does it crumble? • Although crude, the information is immediate, and gets the farmer into the field and thinking about water and her soil • Possibly the most effective water monitoring strategy

27. Directions in the future • Much lower cost TDR • Much more flexible systems • radio telemetry for cheap • auto-logging systems • computer based tracking • Much less water to work with • Much more call for precise and frequent water monitoring

28. Permeability: Double ring infiltrometer • Establishes 1-d flow by having concentric sources of water • Measure rate of infiltration in central ring • Easy, but requires lots of water, and very susceptible to cracks, worm holes, etc. • Interrogates large area

29. Interpreting Infiltration Experiments • Horton Equation: • Rate of infiltration, i, is given by • i = if + (io - if) e(-t) • where if is the infiltration rate after long time, io is the initial infiltration rate and  is and empirical soil parameter. Integrating this with time yields the cumulative infiltration

30. The Brutsaert Model • The Brutsaert Model • S = sorptivity • 0<<1 pore size distribution parameter. wide pore size distributions  = 1; other soils  = 2/3 • The Brutsaert cumulative infiltration is • from which you can determine Ksat and S.

31. Interpreting Infiltration Experiments, cont. • The two term Philip model suggests fitting the rate of infiltration to • i = 0.5 S t -1/2 + A • and the cumulative infiltration as • I = S t1/2 + At

32. Interpreting Infiltration Experiments, cont. • The Green and Ampt Model (constant head) • L = depth of wetting front • n = porosity • d = depth of ponding • hf = water entry pressure • The cumulative infiltration is simply I = nL. • To use this equation you must find the values of Ksat and hf which give the best fit to the data.

33. Fitting the Models to Real Data

34. Chilean Typical Site, Green & Ampt Model Note that the Double Ring Method can Not determine Both Ks and hf , but rather the product of the two.

35. Chilean Data: low K site; Green and Ampt

36. Are the values Lognormally distributed?Pretty close.

37. Range of Values, and Contrast with Depth

38. Permeability: Tension infiltrometer • Applies water at set tension via Marriotte bottle • Using at sequence of pressures can get K(h) curve • Read flux using pressure sensors • Introduced in 1980s, due to being fragile, not widely used

39. Interpreting Tension Infiltrometer Data • The data from the tension infiltrometer is typically interpreted using the results for steady infiltration from a disk source developed by Wooding in 1968 for a Gardner conductivity function K=Ksexp(-t) • r is the disk radius. Using either multiple tensions or multiple radii, you can solve for Ks and 

40. Typical Tension infiltrometer Data

41. Interpretation requires fitting a straight line to the “steady-state” data. • Note: noise increases as flow decreases

42. Permeability: Well permeameter • Establishes a fixed height of ponding • Measure rate of infiltration • Can estimate K(h) relationship via time rate of infiltration

43. Making sense of Well Permeameter data • Interpretation of well permeameter data typically employs the result of Glover (as found in Zanger, 1953) for steady infiltration from a source of radius a and ponding height H • The geometric factor c is given, for H/a<2 by • For H/a>2, error can be reduced by using Reynolds and Elrick’s result • Where * is tabulated

44. Values of alpha-star for Well Permeameter

45. Guelph Permeameter Data From Chile

46. dV/dt ; Rate of infiltration for guelph

47. Ks - Lab methods: constant head • Basically reproduces Darcy’s experiment • Important to measure head loss in the media • Typically use “Tempe Cells” for holding cores, which are widely available

48. Ks - Lab methods: falling head • Better for low permeability samples. • Need to account for head loss through instrument • Measure time rate of falling head and fit to analytical solution radius r Core radius R

49. Measuring Green and Ampt Parameters • The Green and Ampt infiltration model requires a wetting front potential and saturated conductivity. The Bouwer infiltrometer provides these parameters • [WRR 4(2):729-738, 1966]

50. The Device • Key Parts: • Reservoir • Pressure Gauge • Infiltration Ring