Methods Used to Determine Hydraulic Conductivity

1
Methods Used to Determine Hydraulic Conductivity

• By
• Josh Linard

2
Background

• Hydraulic Conductivity, K, is essential to
  understanding flow through soils.
• Darcys Law
• Richards Equation
• Advection-Dispersion-Equation
• Soil characteristics that determine K
• Particle size
• Porosity
• Bulk density

3
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.

4
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).

5
Experiment location

• Field
• Advantages
• Soil is undisturbed.
• Disadvantages
• Cant control the environment.
• Logistics.
• Laboratory
• Advantages
• Highly controlled environment.
• Disadvantages
• Sample can be aggravated during transport.
• Facilities

6
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.

7
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.

8
Constant Head Method

• Wet the column from the bottom up.
• Can be a problem depending on sample size.
• Add water until its at the desired height.
• Hydraulic gradient 1 (Figure 10.1a)
• Macropore collapse? Need a different gradient.
• (Figure 10.1b)
• Capture the outflow, when its rate becomes
  constant Ks is obtained.

9
Constant Head Apparatus
10

• 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
  Darcys Law.

11
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)

12
Falling Head Apparatus
13

• 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.

14
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.

15
Ks Method Summary

• The constant head method is used for soil with a
  high Ks (gt 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.

16
Determining Unsaturated K

• Field methods
• Ring infiltrometer.
• Laboratory methods
• Instantaneous profile method.
• Note ensure that all instruments make good
  contact with soil.

17
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 gt
  0.
• When ym is 0 a surface crust of a known potential
  can be used.

18
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.

19
Ring Infiltrometer
Water Supply
Double Ring
20
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

21
Instantaneous Profile Method
TDRs
Tensiometers
Gamma Ray Detector
Gamma Ray Emitter
22
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.

23
Conclusion/Recommendations

• Methods described allow for determining K in most
  settings.
• Its hard to account for macropore flow.
• There is no method for determining horizontal K
  in situ.
• Scales of measure are subject to criticism.