soil physics 2010

Soil Physics 2010

Outline

• Announcements

• Where were we?

• Water retention curve

• Hysteresis

Soil Physics 2010

Announcements

• Homework 3 is due now

• Slides or Blackboard? (blackboard was preferred)

• A brief run through problems 2 & 4

Soil Physics 2010

Homework problem 2

15 cm soil

5 cm water

A

D

B

C

10 cm soil

Pressure + Elevation = Total potential

A 0 30 30

B 5 25 30

C 15

D 0 0 0

Soil Physics 2010

Homework problem 2

Pressure + Elevation = Total potential

A 0 30 30

B 5 25 30

C 3 15 18

D 0 0 0

15 cm soil

5 cm water

A

D

B

C

10 cm soil

Soil Physics 2010

Homework problem 4a

Mass in air: 1.1 kg

Density of iron: 5.5 kg / L

Volume of iron = 0.2 L

Volume

MassDensity

This is not thevolume of the chunk!

Soil Physics 2010

Homework problem 4

airiron ironVolkg 1.1

waterchunkchunkVolkg 8.0

ironchunk VolVol ?

MassL 2.0 wateriron

gk 0.9L

kg1.0

L

kg5.5L 0.2

L 2.0kg 1.1

Vol iron

airiron

ironchunk VolVol

Soil Physics 2010

The hard way (part 1)

waterchunk chunkVolkg 8.0

Find Volchunk

L

kg 0.1

Vol

kg 1.1Volkg 8.0

chunkchunk

L

kg 0.1Volkg 1.1kg 8.0 chunk

L 3.0L

kg 0.1kg 8.0kg 1.1Vol chunk

Soil Physics 2010

The hard way (part 2)

L 3.0Vol chunk

L 2.0Vol iron

= 1/3 or 33.3%

p

b

1

3

21

L2.0kg 1.1

L3.0kg 1.1

1

Soil Physics 2010

The easy way

L

kg1.0

L

kg5.5L 0.2kg 0.9

ironVol

waterchunkchunkVolkg 8.0

Buoyancy for iron only ( = 0):1.1 kg – 0.9 kg = 0.2 kg

Actual buoyancy:1.1 kg – 0.8 kg = 0.3 kg

Volchunk = 1.5 x Voliron

so = 1/3

Soil Physics 2010

Where were we?Water retention curve

Basic idea:

If the soil were a bunch of capillary tubes, we could figure out everything about how water and air move in it…

…if we also knew the size distribution of those capillary tubes.

The water retention curve is our best estimate of the soil’s pore size distribution.

Soil Physics 2010

With that warning, let’s look at water retention

Start with a soil core that’s saturated:

Known height

Atmospheric pressure

So we know the water’s potential everywhere

Known dry mass

Known porosity

=

Atmospheric pressure 5

Soil Physics 2010

Known height L

So we know the water’s potential everywhere

0

L

(0)

At saturation:h = 0

If it can drain out the bottom, then

, and

mean h0 = L/2

Soil Physics 2010

Then I talked about sponges

Soil Physics 2010

We pull lightly on the water

h1

L/2

2 new points:h1 = h1 + L/21 = –

(water drained/ Vol)

Soil Physics 2010

Repeat with a bigger h

h2 > h1

L/2

2 new points:h2 = h2 (+ L/2) 2= –

(water drained/ Vol)

Soil Physics 2010

Plot the points

Water contentWetness, , etc

Su

ctio

nP

oten

tial

, h, t

ensi

on, e

tc

Why use this one?


Su

ctio

nP

oten

tial

, h, t

ensi

on, e

tc

Soil Physics 2010

Hei

ght

Soil Physics 2010

Different regions


Su

ctio

nP

oten

tial

, h, t

ensi

on, e

tc

Wet

Middle

Dry

Soil Physics 2010

Wet region

h

Wet

Pore only drains if:

Big enough

Not isolated

Air can get to it

g hr

aw

cos2

Air entryAir accessStructural pores

A model porous medium being drained

Drainage allowed:

Poreradius:

Big

Small

Soil Physics 2010

Poreradius:

Big

Small

Drainage allowed:

Soil Physics 2010

A model porous medium being drained

Soil Physics 2010

Wet region

h

Wet

g hr

aw

cos2

Air entryAir accessStructural pores

When wetting, air entrapment limits the final <

Pore only drains if:

Big enough

Not isolated

Air can get to it

Soil Physics 2010

Middle region

h

Middle

Air and water are both continuous

Reasonable reflection of pore size distribution

Mixed textural & structural pores at wetter part

Textural pores at drier part

Hysteresis

Soil Physics 2010

Dry region

h

Dry

Most water is in films sorbed to solid surface

Water retention mostly determined by surface area

Little or no hysteresis (if at equilibrium)

Water flow in films is very slow

→ 0 as h → ∞ (for example, drying at

105° for 24 hrs)

Soil Physics 2010

Hysteresis

• Thermostats• Speedboats• “Ink bottle” pores

History-dependent or direction-depedent

soil physics 2010

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