The following exercises utilize the steady-state
applets to enable users to gain understanding of water movement in soils
as described by the Darcy and Dacry-Buckingham equations.
Exercise 1. Steady-State Water Movement in Homogeneous Saturated Soils: These experiments are designed to introduce students to water movement in uniform saturated soils. Observations to be made and questions asked are designed to enable the user to do the following:
1. See that water content is uniform throughout the saturated soil system.
2. Interpret flux density in terms of volumetric flow rate.
3. Distinguish between flux density and hydraulic conductivity.
4. Understand the impact of changes in inlet and outlet potentials, column length, and column orientation upon flux density.
5. Observe form of potentials as functions of distance along the soil and verify that total potential is equal to the sum of the gravitational and matric potentials everywhere in the soil.
6. Discover conditions needed to obtain a flux density of zero.
7. Formulate an algebraic expression that
describes all of the observations made on different flow systems. Design
new experiments on which to test the expression developed.
Exercise 2. Steady-State Water Movement in Unsaturated soils: These experiments are designed to extend the knowledge gained in exercise 1 to include flow in unsaturated soils. In exercise 1, matric potentials were always greater than or equal to zero. Here they are allowed to become negative and the impact of that upon flow is observed. Here the student can discover the following:
1. Soil water content decreases as matric potential becomes more and more negative.
2. As matric potentials at ends of soil column decrease (and are less than zero), the flux density decreases, even if the driving force remains the same.
3. Hydraulic conductivity decreases with decreasing matric potential in unsaturated soils.
4. Total and matric potentials are generally not linearly related to position in the soil. Users are challenged to explain why this occurs based on discoveries made already.
5. Water does not always move from high water contents to lower water contents.
6. Water content within a soil need not
be uniform when no flow occurs.
Exercise 3. Steady-State Water Movement in Layered soils: In nature, we seldom encounter uniform soils. Hydraulic properties often change with depth. It is not uncommon to have different hydraulic conductivities in different layers or horizons within a soil. These experiments utilize the applet for layered soils to enable the user to observe impacts of soil layers upon one-dimensional steady-state flow. Experiments are designed to enable the user to observe the following:
1. The flux density through layered soils depends upon length and conductivity of each layer, not just the conductivity of the layer with the lowest conductivity.
2. For saturated soils, the driving force is constant within a layer, but greatest in the layer with lowest conductivity.
3. For saturated soils, the equivalent conductivity lies between the saturated conductivities of the two layers.
4. For saturated soils, interchanging the two layers does not affect the flux density.
5. Aspects of flow in layered unsaturated soils that are similar to and that differ from those of layered saturated soils.
6. Flow systems exist where both ends are saturated but places within may be unsaturated. The student is challenged to deduce the requirements for such a system and to test their deduction with additional experiments.
7. Flow systems exist where both ends are
unsaturated but places within may be saturated. The student is challenged
to deduce the requirements for such a system and to test their deduction
with additional experiments.
4. An Introduction to Infiltration:
Exercises 1, 2 and 3 deal with steady-state water movement where flow rates
and potentials do not change with time. In general, water movement in soils
is very dynamic and changes from time to time. Infiltration is the process
of water entering the soil surface. If the supply is not limiting as in
the case of flood irrigation, the infiltration rate (or flux density through
the soil surface) is initially high and decreases with time. This exercise
is designed to help users understand why this occurs. Thus it forms a nice
bridge from an introductory study of steady-state flow to the more complex
Written by D.L. Nofziger, July, 2000. Send comments and suggestions to firstname.lastname@example.org.