Hello my name is Leo Rivera I’m the Director of Scientific Outreach here at METER Group, and in this course, we are going to talk about methods for measuring hydraulic conductivity.

One of the first things I think we really want to think about when we’re measuring hydraulic conductivity is that there are a lot of tools available to make this measurement, and there are a few things you want to think about when choosing the tool for your measurements. The first thing that you want to think about is understanding the difference between lab and field methods.

In this example, we have two instruments on the left that are great for making measurements of hydraulic conductivity in the lab, and on the right we have a device that is a great tool for measuring hydraulic conductivity in the field. But, there are a few differences that you need to understand.

Lab measurements—When we’re making measurements in the lab, we’re making measurements of a point sample. So for our lab devices, we’re typically taking a core sample from the field, and that is what will take our measurements from. This is a single point within the soil profile.

Field measurements—When we’re making measurements in the field, we’re typically taking a measurement from the surface. We infiltrate water down through the surface, through a ring like this that’s driven into the soil.

Differences between methods

When it comes to making those measurements in the lab versus the field, sometimes we see differences in the data:

• Profile sample—Typically, the field measurements represent water movement through a profile. We’re integrating all of the soil in that profile where that wetting front is moving through.
• Point sample—Lab measurements are very select. They measure water moving through the single point of where we’re taking the measurement.
• Pore structure— When you take samples, you wind up sometimes with open ended pores that can impact your measurement.

So now we’re going to dive into some of the different methods for making these measurements, and some of the nuances and things you want to think about when it comes to the measurements and the tools you use.

Lab measurements

So first we’re going to focus on the laboratory methods and talk about some of the different measurements that you’re going to try and make, and the first thing you want to think about is, what am I trying to measure?

So typically, we’re either looking at saturated hydraulic conductivity in the field using a device like this, where we have a that core sample that’s been saturated, and then we’re measuring how fast the water infiltrates into the core or through the sample using either a falling head or a constant head method. You can do both with a tool like the KSAT.

We do some corrections to that measurement based on the pressure heads, as well as geometry factors that we need to correct for after the measurement’s done. We can use a tool like the HYPROP, with tensiometers positioned at two different points within the core. We measure as that sample is evaporating, and we’re measuring the unsaturated hydraulic conductivity with a tool like this. So you get two different measurements.

And these are the tools that we would use when we’re trying to generate that hydraulic conductivity curve and fully characterize the soils. These are great tools, because, again, we can be selective in what layer of soil we’re characterizing, and take a sample from that layer and then fully characterize that sample, getting both the saturated and unsaturated hydraulic properties of that sample.

Field measurements

We also have our field methods for measuring hydraulic conductivity, and there are a lot of tools out there for making these measurements. There are double ring infiltrometers, single ring infiltrometers, and bore hole methods.

In this lesson we’re going to focus on a ring infiltrometer like the SATURO. It is a dual head infiltrometer, and it’s a single ring infiltrometer. We drive this ring into the ground, typically to a 5- or a 10-centimeter insertion depth, and we measure the rate of flow into the infiltrometer as the water infiltrates.

The SATURO uses the dual head method, which allows us to also easily correct for the lateral flow that we see from a single ring infiltrometer. You don’t have to make corrections for three dimensional flow in the lab, but you do have to do that in the field. And, sometimes that can complicate things. If you’re not using a tool that helps correct for those things, that can create a little more error in your measurements. Tools like the SATURO, with its dual head approach, simplify that correction for us, making it easier for us to go from infiltration data that to the field saturated hydraulic conductivity.

I’ve presented a few different tools here and a few things for you to think about when it comes to making these measurements. And you have a lot of options to choose from, and I just want to give you some things to think about when it comes to which sensor to use for your application.

As a guideline, if you’re trying to characterize water flux in the field and how water infiltrates through the soil, it’s typically better to go with a field method like this, because what you’re doing is actually characterizing how water infiltrates from the surface and getting that measurement and that characteristic of the soil.

If you’re trying to delineate the different layers and better understand at different points in the soil, what do those hydraulic characteristics look like, then I would recommend using laboratory methods like this, because again, you can get down to those individual layers and characterize them, and then this also allows you to more easily fully characterize the hydraulic properties of a specific soil layer or specific soil type, which can sometimes be a little more challenging in the field.

We do also have tools for making field measurements that allow us to do unsaturated hydraulic conductivity, like tension infiltrometers that we can use to try and generate that same hydraulic conductivity function in the field, but it can be a little more challenging to do that and so sometimes it’s a little easier to do that with laboratory measurements.