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.
-
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.
-
Set the matric potential at x = 0 to 5 cm and that at x = 1 cm to -1000
cm.
-
Record the flux density and equivalent conductivity through the soil.
-
Is the flux density larger than the equivalent conductivity? Why?
-
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.
-
Record the flux density and equivalent conductivity through the soil for
each of the above matric potentials at x = 1 cm.
-
Is the flux density larger than the equivalent conductivity? Why?
-
Draw a graph of equivalent conductivity as a function of matric potential
at B. What do you observe?
-
Draw a graph of driving force as a function of matric potential at B. What
do you observe?
-
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.
-
Select other soil hydraulic properties and repeat the experiment above
for different soils. What similarities and differences do you observe?
Written by D.L. Nofziger, July, 2000. Send comments and suggestions to dln@mail.pss.okstate.edu.
