### Contact Info

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 3

Exercise 3. 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. How do differences in conductivity impact flow? For example, if I have one layer with a conductivity of 10 cm/day and another with a conductivity of 2 cm/day, how rapidly will the water move through the soil? The applet for steady-state water flow in layered soils can help us answer this question.

1. Imagine a soil composed of a layer of sandy loam with thickness 20 cm overlaying a layer of sandy clay loam with thickness 10 cm. The saturated conductivity of the sandy loam is 15 cm/day and that of the sandy clay loam is 4 cm/day. The soil column is oriented vertically with the sand layer on top. A matric potential of 32 cm is imposed on the top of the sand. A matric potential of 2 cm is maintained at the bottom of the sandy clay loam.

1. What is the flux density of water through the soil?

2. What is the overall driving force causing water move through the column?

3. What would be the flux density if the sandy clay loam soil filled the entire column?

4. Is the flux density uniform throughout the soil? Explain.

5. Is the driving force uniform throughout the soil? If not, why not.

2. Modify the system studied in part 1 so that the sandy loam layer is now 28 cm long and the sandy clay loam is only 2 cm long. What is the flux density for this system. Is it different than in part 1? Why or why not?

3. Modify the system studied in part 1 so that the sandy loam layer is only 2 cm thick and the sandy clay loam layer is 28 cm thick. What is the flux density for this system. Is it different than in part 1? Why or why not?

4. Does the flux density of water passing through the soil system depend upon which layer is on the top? In other words, will the flux density found for the system in part 1 be different if the 10 cm sandy clay loam layer overlays the 20 cm sandy loam layer instead of the arrangement used in part 1?

5. The problems above deal with flow in saturated soils. What differences exist if the matric potential at the top is 0 cm and that at the bottom is -30 cm?

6. Construct a soil system that is unsaturated at each end, but has a region within the column that is saturated? How can this occur?

7. Construct a soil system that is saturated at each end, but has a region within the column that is unsaturated? How can this occur?

Written by D.L. Nofziger, July, 2000. Send comments and suggestions to dln@mail.pss.okstate.edu.