Hi, my name is Leo Rivera, I’m a soil scientist and the Director of Scientific Outreach here at METER Group. And in this course, we are going to talk about hydraulic conductivity and the things you need to know to understand the measurement. And for this first part, we’re going to focus on some of the fundamentals. In later lessons, we’ll talk about the measurement and the tools that you would use to make this measurement.

One of the first things to address is the definition of conductivity and the definition of infiltration. We use these words interchangeably, but it’s important to know that they don’t mean the same thing.

Infiltration

So here, what you see on the left is a graph that shows a typical infiltration curve over time. What we’re measuring on the y-axis of this graph is the infiltration rate, how fast water is actually infiltrating into the soil. The infiltration rate can vary depending on a lot of factors. What you see over time is that the rate decreases and becomes more steady. Eventually, we reach what we call a quasi-steady state in the measurement.

Hydraulic conductivity

We measure the infiltration rate to understand the hydraulic conductivity of the soil. We define hydraulic conductivity as a measure of the ability of a porous medium to transmit water. So, in this case, the porous medium is soil. We use this infiltration curve and these equations to calculate hydraulic conductivity.

Typically, we use the hydraulic conductivity to understand and model how water moves through soil. It is a consistent value that we can use because it’s independent of all the other factors that impact the ability of soil to transmit water or to infiltrate water.

Factors impacting hydraulic conductivity

There are many factors that actually determine the value of hydraulic conductivity.

• Soil texture impacts hydraulic conductivity
• Soil structure is a critical piece we sometimes forget about
• Biopores, like worm holes or decaying root channels
• Compaction of the soil or sample
• Bulk density of the soil or sample
• Initial water content of the soil or sample
• Water potential of the soil or sample, especially in expansive soils

Why measure hydraulic conductivity

Hydraulic conductivity impacts almost everything that soil is used for.

Hydraulic conductivity impacts crop production, especially when it comes to things like irrigation and drainage decisions. One of our goals in crop production is to infiltrate water and have it stored in the root zone for the plants to be able to use it. Hydraulic conductivity plays a big role when it comes to irrigation and drainage versus water running off from the soil.

Hydraulic conductivity also impacts hydrology, both in native and urban environments. When we’re trying to design systems to infiltrate water, we really need to understand this value.

Landfill performance is another critical area. In some cases we’re trying to limit the ability of water to move into the soil, and in other cases, we’re trying to store as much water into the cover as we can.

In storm water system design, we measure the native soils and how they behave, then measure the soils that we’re designing to infiltrate that water. Ultimately, our goal is to infiltrate more water and not have as much runoff.As we improve soil health, we know that that improves things like the soil’s ability to infiltrate and move water deeper into the profile.

So those are some things that you need to understand when looking at hydraulic conductivity and making those measurements.

Saturated and unsaturated hydraulic conductivity

We typically divide hydraulic conductivity into saturated hydraulic conductivity and unsaturated hydraulic conductivity. We have an example in this animation of two soils: one is infiltrating water under purely saturated conditions. That’s why we have a psi (Ψ) of zero. It means the soil is saturated in unsaturated conditions: there’s more trapped air, the soil isn’t pulling on the water as much, and so there are actually fewer channels in the soil that are contributing to the flow. Typically, this means that our hydraulic conductivity in unsaturated conditions is lower than the unsaturated conditions.

The hydraulic conductivity curve

One of the tools that we use is the hydraulic conductivity curve, or hydraulic conductivity function. And here’s a good example of what that looks like. I want to show this for a few different soil types to really help you understand some of the things that we’ve just discussed.

So here you see the infiltration curves for a poorly structured clay soil and a well structured clay soil, and you can see how that how that curve changes as we get closer to saturation. We have another example of a structuralist sandy soil, and what that infiltration curve looks like. And ultimately, what we’re trying to show here is, as conditions become unsaturated, how that changes the hydraulic conductivity for the soil at different water potentials. And then as we get closer to saturation, the structure really starts to play a bigger role in the soil’s ability to infiltrate water. And here you see that the clay, well structured clay soil is actually able to infiltrate more water in saturated conditions than the structure of the sandy soil.

And so the point I want you to get across there is that soil texture is not the primary driver when it comes to saturated conditions. It’s things like structure. It’s things like the pores that are existing in the soil, and that’s what this hydraulic connectivity function helps us understand these soils.