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 Bulk density

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 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 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

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 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.

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.

Constant Head Apparatus

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 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)

Falling Head Apparatus

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 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.

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

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 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 Infiltrometer Water Supply Double Ring

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 Method Tensiometers TDR’s Gamma Ray Detector Gamma Ray Emitter

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 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.