1. Methods of Media Characterization
A challenging area of rapid advancement
Williams,
Modified after Selker,

2. Topics Measurement of pressure potential Measurement of Water Content
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 Volumetric Pro’s Con’s
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. Pressure measurement: The tensiometer
Cup
Gauge
Reservoir
Body
Removable
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. 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 W

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 measure water content
in the surrogate media
We want a value for water
content in our soil

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 constituents can effect calibration

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

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 I= incident radiation I= transmitted radiation
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): , 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

51. Limitations on Bouwer Method
All parameters are “operational” rather than fundamental
Conductivity is less than K found in labs due to trapped air
Rocks and cracks can render measured value of hw incorrect.
For more details on method see:
Topp and Binns 1976 Can. J. Soil Sci 56:
Aldabagh and Beer, 1971 TASAE 14:29-31

52. Employ falling head method for Ks
Recall standard falling head result from lab methods:
Remember that Kfs is about 0.5 Ks