 ### 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 4

Exercise 4. Introduction to Infiltration

Exercises 1-3 deal with steady-state water movement where flow rates and matric potentials do not change with time. In general, water movement in soil is very dynamic and changes from time to time. Infiltration is the process of water entering the soil surface. The rate at which water enters an initially dry soil depends upon the rate it is applied, the hydraulic properties of the soil, the initial water content of the soil, and the time since infiltration began. If the supply is not limiting, the infiltration rate (or flux density through the soil surface) is initially high and decreases with time. This occurs even if no soil structural changes take place during wetting. This exercise is designed to help us understand why this occurs.

Consider a soil which has water ponded on it to a depth of 5 cm. Before ponding, the soil was somewhat dry having a matric potential of -1000 cm. Let's use the applet to examine the flux of water through the upper 1 cm of soil.

1. Specify a soil with a saturated hydraulic conductivity of 10 cm/day oriented vertically with x = 0 on top (90 degree angle of inclination). The length of the soil is 1 cm.

1. Set the matric potential at x = 0 to 5 cm and that at x = 1 cm to -1000 cm.

1. Record the flux density and equivalent conductivity through the soil.

2. Is the flux density larger than the equivalent conductivity? Why?

2. The flux density recorded in step A is an estimate of the infiltration rate into the soil for the initial instant when flow begins. However, as soon as water reaches the 1 cm depth, the matric potential there increases because the water content increases. A short time later, the matric potential at the bottom will increase to -900 cm. Later it will increase to -800 cm, then -700, -600, -500, , -100, -50, -40, -30, -20, -10, -1, and 0 cm as the soil continues to wet up.

1. Record the flux density and equivalent conductivity through the soil for each of the above matric potentials at x = 1 cm.

2. Is the flux density larger than the equivalent conductivity? Why?

3. Draw a graph of equivalent conductivity as a function of matric potential at B. What do you observe?

4. Draw a graph of driving force as a function of matric potential at B. What do you observe?

5. Draw a graph of the flux density through the soil as a function of matric potential at x = 1 cm. Explain why the flux density or infiltration rate decreases with increasing matric potential at B.

6. Select other soil hydraulic properties and repeat the experiment above for different soils. What similarities and differences do you observe?

Note: This exercise plots infiltration rate as a function of matric potential at the bottom. Since the matric potential at the bottom increases with time after initiation of wetting, the curves are qualitatively similar to plots of infiltration rate versus time. To obtain precise curves of infiltration rate versus time we must solve the Richards equation with a model such as CHEMFLO-2000 (Nofziger et al., 2002).

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