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.