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Tyson Ochsner

Professor

Plant and Soil Sciences

Oklahoma State University

371 Ag Hall

Stillwater, OK 74078

Phone: (405)-744-3627

tyson.ochsner@okstate.edu

 

Experiment 2

STEADY-STATE FLOW EXPERIMENTS

Exercise  2. Water Movement in Unsaturated Soils

In exercises 1 we used the applet to examine flow in saturated soils. Most soils in the real world are not saturated. Instead their pores contain both water and air. In this exercise we will examine water movement in unsaturated soils. A soil system is usually considered to be unsaturated if any portion of it is unsaturated or if the matric potential is less than zero anywhere in the soil.

To begin we will consider a horizontal soil with a saturated hydraulic conductivity of 12 cm/day and a length of 20 cm. The soil has a matric potential of 50 cm at x = 0 and zero at x = 20 cm. 

  1. Is the soil saturated? What is the water content at x = 0? At x = 20 cm? What is the flux density?

  2. Decrease the matric potentials at x = 0 and x = 20 cm by 25 cm. (the values will now be 25 cm and -25 cm, respectively)

    1. What is the water content at x = 0? At x = 20 cm?

    2. What is the flux density?

  3. Decrease the matric potentials at x = 0 and x = 20 cm by 25 cm. (the values will now be 0 cm and -50 cm, respectively)

    1. What is the water content at x = 0? At x = 20 cm?

    2. What is the flux density?

  4. Compare the flux densities for cases 1, 2, and 3. Observe the decrease in flux density even though all three soil systems have the same length and the same difference in total potential. This can be explained by recognizing that the hydraulic conductivity of the soil decreases as the soil gets dryer. The pores are no longer filled with water. Water moving through the soil must move closer to the soil surfaces. As a result of these changes in the physical properties of the soil, the equation used to describe flow in unsaturated soils must be modified to reflect this change.

  5. Examine graphs of conductivity, water content, and matric potential vs distance for the three flow systems used in parts 2, 3, and 4 and answer the following questions for each system.

    1. Does the water content change with position?

    2. Does the matric potential change with position?

    3. Does the conductivity change with position? If so, does it increase or decrease as the matric potential increases? Does it increase or decrease as the water content increases?

  6. Set the matric potential at x = 0 to 0 cm and that at x = 20 cm to -50 cm. Examine the graph of total potential vs distance. Is the graph a straight line or is it curvilinear? Explain why it has this shape. The following exercise may help you.

    1. Set the saturated conductivity of the soil to 10 cm/day and the matric potential at x = 20 cm to 0 cm. Determine the total potential at x = 0 required to obtain a flux density of 5 cm/day. Calculate the difference between the total potential at x = 0 and the total potential at x = 20 cm.

    2. Repeat the process carried out in step a. above for saturated hydraulic conductivities of 8, 6, 4, 2, and 1 cm/day.

    3. Examine the data above. What can be said about the difference in total head required to obtain a flux of 5 cm/day as the conductivity decreases from 10 to 1 cm/day?

    4. Reset the matric potential at A to 0 and that at B to -50 cm. Does the conductivity change with position? Where is it largest? What is the difference in total head between depths of 0 and 4 cm. What is the difference in total heads between 16 and 20 cm?

    5. Now explain the graph of total potential vs distance.
Written by D.L. Nofziger, July, 2000. Send comments and suggestions to dln@mail.pss.okstate.edu.
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Division of Agricultural Sciences and Natural Resources