Soil Physics 2010

Outline

• Announcements

• Aeration

• Permeability

• Relative permeability

• Air permeability

Soil Physics 2010

• Review session Friday, 11:00 – 1:00

• Homework 5 due now

• Exam II on Friday, April 2

• Remember I’ll be gone most of next week

Announcements

Soil Physics 2010

And now Delise,

on the topic of

Aeration

Soil Physics 2010

Aeration mechanisms

• Advection (also called convection, mass movement, mass flow)

• Diffusion

We’ll look at advection today, and start diffusion on Friday.

Soil Physics 2010

Advection

Darcy’s law is about advection (or mass flow):

dx

dKQ s

Mass moved (flux

density)Proportionalit

ycoefficient

Potentialgradient

Soil Physics 2010

Now replace the water with molasses(keep the same soil, same potential gradient)

dx

dKQ s

Same flux?

Same proportionality

coefficient?

Same potentialgradient

Ks has 2 parts:• Soil properties• Fluid properties

Soil Physics 2010

Parts of Darcy’s Ks

dx

dKQ s

Soil property: Permeability k L2

Fluid property: Fluidity f L-1 T-1

Fluidity is often given as g/: fluid densityg gravitational acceleration fluid viscosity

dx

dgk

Soil Physics 2010

What’s with the L2?

Permeability k L2

dx

dgkQ

The actual length L that is squared for a soil’s permeability is quite small.

It is usually interpreted as the soil’s effective pore diameter.

Soil Physics 2010

Back to water, molasses, etc.

dx

dgkQ

Soil permeability fluidity

The soil’s permeability does not depend on the fluid’s properties.

So permeability can also describe the flow of air

Soil Physics 2010

Relative permeability kr

Water permeability of saturated soil

Air permeability of dry soil

=

kr

0

1

Sw0 1Sa1 0

Little pores

Big pores

Soil Physics 2010

Relative permeability

kr

0

1

Sw0 1Sa1 0

Petroleum engineers, and some geologists,

use these “relperm” plots

air entry value

Soil Physics 2010

Air permeability ka

Sw0 1

ka

0

1

Sa1 0

h

0

No air flow until soil has dried to the air entry value.

This is like r for hydraulic conductivity: K(r) = 0

Soil Physics 2010

Measuring air permeability