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/

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

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

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

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

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

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

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)

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

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)

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

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

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

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)

Example: Watermark $260 for meter $27 for probes

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

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!)

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)

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)

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

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

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)

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

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

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

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)

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

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 

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

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

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.

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

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

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.

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

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

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

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 

Typical Tension infiltrometer Data

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

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

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

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

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

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.

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]

The Device Key Parts: Reservoir Pressure Gauge Infiltration Ring

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

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

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

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 56:139-147 Aldabagh and Beer, 1971 TASAE 14:29-31

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