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. • 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 • Advantages • Soil is undisturbed. • Disadvantages • Can’t 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 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 (> 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 >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.