1. Methods Used to Determine Hydraulic ConductivityBy
Josh Linard

2. 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

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 Laboratory Advantages Disadvantages
Soil is undisturbed.
Disadvantages
Can’t control the environment.
Logistics.
Laboratory
Highly controlled environment.
Sample can be aggravated during transport.
Facilities

6. Water Application
Ideally, the soil should be wetted from the bottom up.
Should use a deaerated M CaSO4 solution to limit air retention.
What volume of water is required and what volume is available.

7. Determining Ks Laboratory Methods Field 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 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.

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 Darcy’s 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 (> cm/s).
The falling head method is used for soils with lower Ks ( 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 > 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
Tensiometers
TDR’s
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.
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.