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Methods Used to Determine Hydraulic Conductivity. By Josh Linard. Background. Hydraulic Conductivity, K, is essential to understanding flow through soils. Darcy’s Law Richards’ Equation Advection-Dispersion-Equation Soil characteristics that determine K Particle size Porosity

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Presentation Transcript
background
Background
  • Hydraulic Conductivity, K, is essential to understanding flow through soils.
    • Darcy’s Law
    • Richards’ Equation
    • Advection-Dispersion-Equation
  • Soil characteristics that determine K
    • Particle size
    • Porosity
    • Bulk density
more about k
More about K
  • K is a function of pressure or moisture content
    • low matric potential = high moisture content = high K
  • Want to know either
    • Saturated hydraulic conductivity, Ks, or
    • Unsaturated hydraulic conductivity, K.
other considerations
Other considerations
  • What should the sample size be?
  • Where to conduct experiment?
  • How is the water applied?
  • Sample size
    • Contemporary soil core devices.
    • Representative Elementary Volume (REV).
experiment location
Experiment location
  • Field
    • Advantages
      • Soil is undisturbed.
    • Disadvantages
      • Can’t control the environment.
      • Logistics.
  • Laboratory
    • Advantages
      • Highly controlled environment.
    • Disadvantages
      • Sample can be aggravated during transport.
      • Facilities
water application
Water Application
  • Ideally, the soil should be wetted from the bottom up.
  • Should use a deaerated 0.005 M CaSO4 solution to limit air retention.
  • What volume of water is required and what volume is available.
determining k s
Determining Ks
  • Laboratory Methods
    • Constant head
    • Falling head
  • Field Methods
    • Test basins
  • Note: for each method….
    • good contact must be made at the lateral boundaries of the core.
    • Evaporation must be measured.
constant head method
Constant Head Method
  • Wet the column from the bottom up.
    • Can be a problem depending on sample size.
  • Add water until it’s at the desired height.
    • Hydraulic gradient = 1 (Figure 10.1a)
    • Macropore collapse? Need a different gradient.
    • (Figure 10.1b)
  • Capture the outflow, when it’s rate becomes constant Ks is obtained.
slide10
L is length through the soil
  • y is the height of ponded water
  • x is the height of water required to lower the gradient so that y can be maintained.
  • Note: if the gradient is 1 then Ks = q as per Darcy’s Law.
falling head method
Falling Head Method
  • Wet the column from the bottom up.
  • Fill a burette to above the height of the soil column and allow it to drain.
  • Drain until the rate of head loss is constant.
  • (Figure 11.1)
slide13
a is the cross-sectional area of the burette
  • A is the cross-sectional area of the soil column
  • t2 – t1 is the time required for the head to drop from H1 to H2.
test basin method
Test Basin Method
  • Isolate a column of soil
    • Usually much larger than a core to be used in the laboratory.
  • Seal the lateral faces of the column
  • Ensure the column is saturated
  • Apply a constant head of water at rate P.
  • Obtain Ks using a mass balance approach: I = P - E where, Ks is equal to I since the soil is saturated.
k s method summary
Ks Method Summary
  • The constant head method is used for soil with a high Ks (> 0.001 cm/s).
  • The falling head method is used for soils with lower Ks (10-3 - 10-6 cm/s).
  • Laboratory experiments can obtain Ks in each dimension.
determining unsaturated k
Determining Unsaturated K
  • Field methods
    • Ring infiltrometer.
  • Laboratory methods
    • Instantaneous profile method.
  • Note: ensure that all instruments make good contact with soil.
ring infiltrometer
Ring Infiltrometer
  • Used either in the field or laboratory.
  • Can use either one or two rings.
    • Scale dependent on ring size.
    • 2 rings allows vertical K to be isolated.
  • Can measure K when the matric potential, ym, is >0.
    • When ym is 0 a surface crust of a known potential can be used.
ring infiltrometer method
Ring Infiltrometer Method
  • Isolate soil column as in other methods.
  • Place the infiltrometer on the soil, ensuring good contact.
  • Water is ponded on the soil and the infiltration rate recorded.
  • Unsaturated K is determined using the Richards’ equation.
ring infiltrometer19
Ring Infiltrometer

Water Supply

Double Ring

instantaneous profile method
Instantaneous Profile Method
  • Uses tensiometers and gamma ray absorption to measure matric potential, f, and moisture content, q, respectively.
  • Pond water until the outflow is constant and then start the experiment when the last of the water has entered the soil.
  • K is obtained using
instantaneous profile method21
Instantaneous Profile Method

Tensiometers

TDR’s

Gamma Ray Detector

Gamma Ray Emitter

unsaturated k method summary
Unsaturated K Method Summary
  • Ring infiltrometer
    • Different sample sizes require different rings and sometimes infiltrometers.
    • Water can be hard to provide depending on the sample size.
    • Have to ensure good contact with soil.
  • Instantaneous profile method
    • Expensive to operate and hard to set up.
    • Have to ensure good contact with the soil.
conclusion recommendations
Conclusion/Recommendations
  • Methods described allow for determining K in most settings.
  • It’s hard to account for macropore flow.
  • There is no method for determining horizontal K in situ.
  • Scales of measure are subject to criticism.