Methods of Media Characterization

1. Methods of Media Characterization A challenging area of rapid advancement Williams, 2002 http://www.its.uidaho.edu/AgE558 Modified after Selker, 2000 http://bioe.orst.edu/vzp/

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 during drying, and not function • Inept users! • Poor for sites with low skill operators of units • Easy to get “garbage” data if not careful • Fine-textured soils (won’t measure <-0.8bar)

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

14. Gypsum block, continued • Problems: • The properties of the media change with time (e.g., gypsum dissolves; clay deposition on surface changes gypsum moisture curve) • Making reproducible media very difficult (need calibration per each unit) • Hysteresis makes the measurement inaccurate (more on this later)

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

16. 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

17. 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 • Selker had very poor luck with these in Willamette valley (no correlation!)

18. Dielectric • A dielectric is a substance that doesn’t conduct electricity (an insulator) • Word dielectric used when considering the effect of AC fields on the substance; usually a non-metal. • Commonly considered synonymous with insulator used when material is used to withstand a high electric field (e.g. in a capacitor)

19. 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)

20. 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

21. 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

22. 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)

23. Indirect Electrical • Other Surface and Subsurface Geophysical Methods: • DC Resistivity • Electromagnetic Induction (Emag) • Ground-penetrating radar (GPR)

24. 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

25. 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

26. 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

27. 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

28. 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 

29. 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 soil scientist into the field and thinking about water and soil • Possibly the most effective water monitoring strategy

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

31. Ways to measure Flux • Measure flux (q) because you need to know it per se, • ……or to infer K • See Hubbell presentation on student project page.

32. 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. • Interogates large area

33. Interpreting Infiltration Experiments • Horton Equation: • Rate of infiltration, i, is given by • i = if + (io - if) exp(-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

34. 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.

35. New term: Sorptivity • 1957, Sorptivity introduced by Philip • “measure of the capacity of a medium to adsorb or desorb a liquid. • Where I is the cumulative infiltration at time t, and S is the sorptivity

36. 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

37. 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 1980’s, becoming the industry standard

38. Interpreting Tension Infiltrometer Data • The data from the tension infiltrometer is typically interpreted using the results for steady infiltration from a disk source develped 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 

39. Typical Tension infiltrometer Data

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

41. Permeability: Well permeameter • Establish a fixed height of ponding • Variation on this design: BAT ™ • Measure rate of infiltration • Can estimate K(h) relationship via time rate of infiltration

42. 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 Elricks result • Where * is tabulated

43. 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

44. 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

45. 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.

46. 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]

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

48. Identify the Air and Water Entry Pressures • ha – air entry pressure • hw – water entry pressure • Typically assume that • ha = 2 hw

49. Procedure • Pound Ring in with slide hammer about 10 cm • Purge air and allow infiltration until wetting front is at 10 cm • Measure dH/dt to obtain infiltration rate • Close water supply valve • Record pressure on vacuum gauge: record minimum value

50. Water Entry Pressure • The water entry pressure will be taken as half the value of the measured air entry pressure (the minimum pressure from the vacuum gauge on the infiltrometer) • WATCH OUT: correct observed pressure for water column height in unit