jarmanc
Posts: 4
Joined: Wed Oct 10, 2007 10:45 am

### Water flow in soil. Help?

I am doing an experiment testing how soil compactness effects how quikcly water flows. Although, are infiltration rate and hydraulic conductivity the same? I know that Darcy's Law determines hydraulic conductivity, but should I be using infiltration rate?

ChrisG
Former Expert
Posts: 1019
Joined: Fri Oct 28, 2005 11:43 am
Occupation: Research Hydrologist
Hi jarmanc,
I'm glad to see you are doing a hydrology experiment. Infiltration rate and hydraulic conductivity are not the same. "Infiltration" is usually used to refer to the amount of water entering a saturated or unsaturated soil. Hydraulic conductivity is a value approximating the ability of a porous substance to transmit (or "conduct") water. Strictly speaking, Darcy's equation does not "determine" hydraulic conductivity, but it can be used to estimate a value of hydraulic conductivity. As you probably already know, Darcy's equation predicts the relationship between the hydraulic gradient, the conductivity, and the rate of infiltration (or flow).

What sort of experimental setup are you using?

I'm looking forward to hearing more about your project.

Chris

jarmanc
Posts: 4
Joined: Wed Oct 10, 2007 10:45 am

### darcy's law

So, Darcy's Law estimates how well water conducts water or the hydraulic conductivity? Is that the same as the relationship between the hydraulic gradient, the conductivity, and the rate of infiltration like you said in your last message? I will also be conducting my experiment by taking a clear plastic aquarium and putting soils (clay, sand, silt) on one top layer and put a drainer on that layer so the water can go through. I will then try to compute Darcy's Law and see how much water is absorbed as well.

jarmanc
Posts: 4
Joined: Wed Oct 10, 2007 10:45 am
Also, I am confused on what the hydraulic gradient is. This experiment is also for my eighth grade science fair.

Craig_Bridge
Former Expert
Posts: 1297
Joined: Mon Oct 16, 2006 11:47 am
What grade are you in? What math classes have you taken?

Here is a something that explains it http://en.wikipedia.org/wiki/Head_(hydraulic) reasonably well if you have had and really understood "gradients" in math. Typically gradients are studied as part of a calculus course and until you actually use them enough and apply them to real world problems in order to really understand them, they can be quite confusing.

We can probably help you understand a hydrolic gradient model in simpler terms once we know what levels of math we need to break it down into. The basic principles aren't that hard, we just need figure out a grade appropriate languge or vocabulary to explain them with.
-Craig

ChrisG
Former Expert
Posts: 1019
Joined: Fri Oct 28, 2005 11:43 am
Occupation: Research Hydrologist

### Re: darcy's law

jarmanc wrote:So, Darcy's Law estimates how well water conducts water or the hydraulic conductivity? Is that the same as the relationship between the hydraulic gradient, the conductivity, and the rate of infiltration like you said in your last message? I will also be conducting my experiment by taking a clear plastic aquarium and putting soils (clay, sand, silt) on one top layer and put a drainer on that layer so the water can go through. I will then try to compute Darcy's Law and see how much water is absorbed as well.

For an eighth grader, there will be some challenging concepts in this project, but I think you can do it. The link that Craig gave should be helpful. A good way to answer some of your questions is to look at the equation itself. A basic form of Darcy's equation is this:

Q=KA((H2-H1)/L))

Q is the rate of flow of water. For example, in your aquarium, it would be equal to the rate of flow of water coming out of the drain, which you can measure with a graduated cylinder and a clock.

K is the hydraulic conductivity. This is very difficult to measure directly and is often calculated using Darcy's equation or other hydraulic equations. I am guessing that you will need to calculate this value from Darcy's equation.

A is the area through which the water is flowing. In your experiment, if water flows from one end of the aquarium to the other, then this would be the average height of the water times the width of the aquarium.

H2 and H1 are measurements of "hydraulic head". This can be a challenging concept. For the sake of simplicity, let's say that in your experiment this will be the height of the water surface above the base of the terrarium (assuming the base terrarium is leveled) at the location of the inlet where you add the water (H2) and at the outlet, where water exits the terrarium (H1). It can also get more complicated.

L is the distance between the points where H1 and H2 are measured. So, in your case, if the water is added at one end of the aquarium and exits at the other end, it would be the total width of the aquarium.

This part of the equation (H2-H1)/L is sometimes called the "hydraulic gradient". For your experiment, this will probably be equal to the slope of the surface of the water from the inlet to the outlet. For example, if the water surface is 20 cm above the base at the inlet at one end of the aquarium where water enters and 15 cm above the base at the outlet at the other end of the aquarium where water exits, and the distance between the inlet and the outlet is 50 cm, then your gradient would be (20-15)/50 = 0.1

Hopefully that will clarify some things. These concepts are often easier to understand by seeing them in action than by reading about them, so if you can work directly with someone who knows a little hydrology, that would be a big help.

When you say "I will then try to compute Darcy's law" what sort of calculation do you mean? Another question I have is what you mean when you say that you will measure how much water is "absorbed". I probably do not understand all the details of your experimental setup.

One tip I have for you is to add water to your aquarium sediments very slowly, and preferably by adding water at an inlet near the bottom of the aquarium. If you fill it quickly from the top, you will get trapped bubbles of air in the sediment. Entrapped bubbles move around, restrict flow, and can mess up your results.

jarmanc
Posts: 4
Joined: Wed Oct 10, 2007 10:45 am

### thanks

Thank you all very much, I feel a lot clearer on my experiment and how to conduct it. A lot of sites that I went to made it not understandable. Thanks!

ChrisG
Former Expert
Posts: 1019
Joined: Fri Oct 28, 2005 11:43 am
Occupation: Research Hydrologist
Glad to help. Please let us know how your experiment goes. Return to “Grades 6-8: Life, Earth, and Social Sciences”