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VADOSE ZONE

Medium for plant growth

Regulator of water supplies

Recycler of raw materials

Habitat for soil organisms

Engineering medium

Functions of Soil

Functions of Soil

Medium for plant growth

Physical Support

Gas exchange

Water

Temperature

Nutrient source

Functions of Soil

Regulator of water supplies

Infiltration

Run-off

Storage/Movement

Distribution

Purification

Integral to hydrologic cycle

Hydrologic cycle: WHERE IS VADOSE ZONE?

The vadose zone

Vadose Zone Hydrology – Profile Scale

Water Budget

http://wwwcimis.water.ca.gov/cimis/infoIrrBudget.jsp

Ap May June July Aug. Sept Oct

Recharge Recharge Runoff

Evapotranspiration

Precipitation

Soil moisture

utilization

Actual ET

Potential ET

Wate

r am

ou

nt

ET > Precip = Soil moisture utilization

Precip > ET = Recharge, surplus, and runoff

Water Balance Diagram

What happens to the water in the diagram below?

Water

Soil A horizon - Air Dry

Answer

• The water moves sideways and downward at the same rate. This is because of adhesion and cohesion.

• Would the movement be different if the soil was saturated? – Yes. The movement would

mainly be downward due to gravity.

WATER

Water Movement

Water

Loam

Sand

Water Movement

• Water front does not move into sand until loam is saturated

Water

Loam

Sand

t 1

t 2

t3

t4

Water Movement

• Water front moves into clay upon contact with clay, but because it moves slow water builds up above the clay layer.

Water

loam

clay

Summary Points of Water Movement

1. Pore size is one of the most important fundamental properties affecting how water moves through soil. Larger pores as in sand conduct water more rapidly than smaller pores in clay.

2. The two forces that allow water to move through soil are gravitational forces and capillary forces. Capillary forces are greater in small pores than in large pores.

3. Gravitational and capillary forces act simultaneously in soils. Capillary action involves two types of attractions, adhesion and cohesion. Adhesion is attraction of water molecules to solid surfaces; cohesion is the attraction of water molecules to each other. Gravity pulls water downward when the water is not held by capillary action. Thus gravity influences water in saturated soils.

4. Factors that affect water movement through soil include texture, structure, organic matter and bulk density. Any condition that affects soil pore size and shape will affect water movement. Examples include compaction, tillage, decayed root channels and worm holes.

5. The rate and direction of water moving through soil is also affected by soil layers of different material. Abrupt changes in pore size from one layer to the next affect water movement. When fine soil overlies coarse soil, downward water movement will temporally stop at the fine coarse interface until the fine layer above the interface is nearly saturation.

6. When a coarse soil is above a fine soil, the rapid water movement in the coarse soil is greater than through the clay and water will build up above the fine layer as the water front comes in contact with the fine layer. This can result in a build up of a perched water table if water continues to enter the coarse layer.

Two Forces Responsible for

Water Movement in Soils

Gravity

Capillarity

Source: Dept of Agriculture Bulletin 462, 1960

Water movement

Capillarity

Spontaneous movement of water

into and through pore spaces in soil

without the aid of gravity.

Adhesion and Cohesion

Cohesion


oxygen

H

H

S

U

R

F

A

C

E

adhesion

oxygen

H

H

cohesion


adhesion

Cohesion

(H-bonding)

Surfa

ce

droplet


Strong adhesion Weak adhesion

Weak Adhesion

Adhesion to Soil Particles

Strong Adhesive Forces

Soil Pores


Adhesion to the tube or pore wall

Cohesion between water molecules

capillarity

Capillarity

Tube/Pore wall

}

Force down

h = 0.15

r

adhesion

cohesion

Capillarity h = 0.15

r

h

Small pores

Capillarity

Capillary fringe Capillary pores in the zone of aeration draw up water

from the zone of saturation beneath the water table.

In very fine-grained soils, this capillary fringe can

saturate the soil above the water table

Tensiometer reading is negative

Capillary fringe is a part of vadose zone

Vadose Zone : the upper layer of the earth that contain

a three-phase system of solid, liquid, and gaseous

material.

Also called the zone of aeration or unsaturated zone.

Soil Pores and Pore Size Distribution

Texture

Density

Structure

Particle Size Large/coarse Medium Fine/Small

Sand

Loamy Sand

Sandy Loam

Silt

Sandy clay Loam

Silty clay Loam

Silt Loam

Loam

Clay Loam

Sandy Clay

Silty Clay

Clay

Pore Size Large/Macro Meso/Medium Micro/Small

Capillarity Weak Moderate Strong

Texture

Soil Pores

Sandy Silty Clayey

Gravity

Dominated Capillarity

Dominated

Density

Depth in Profile

Arrangement of Particles

Compaction

Structure

Micropores

Macropores

MOISTURE CONTENT

Micropores

Macropores

Examples

Sand

Clay

Water

Sand Clay

Initial Saturation

Uncompacted Compacted

Initial Saturation

Sandy Loam

Wet Moist

Same Texture and Density

Quantification: Soil Water Energy

High potential Energy

Low potential Energy

Water moves in response to

differences in potential energy,

from high potential energy to

low potential energy.

Gravitational Potential Energy

The greater the difference in height

The greater the difference in

Gravitational potential energy.

Gravitational Potential

Ψg = mgh

The greater the height, the greater the potential energy.

The potential energy of a unit quantity of water.

Unit quantities: volume

mass

weight

ψg

= mg

Ψg = h (cm) mg


Reference level Ψg = 0

Height (cm)

100

50

a

b

ψga = 100 cm

ψgb = 40 cm 40

soil

Difference in energy determines movement

Independent of soil

properties


Reference level

(Ψg = 0)

Height (cm)

100 a

b

Ψga = 60 cm

Ψgb = 0 cm

Ψga – Ψgb

60 - 0 = 60 cm

40

0


1. Gravitational potential energy is

due only to the height of an object

(water) above some reference point.

2. Gravitational potential energy is

independent of soil properties.

Capillary Potential Energy

(Matric Potential Energy)

Matric Potential

“suction” potential - capillarity

Narrow capillary tube – high capillary rise h = 0.15

- strong force r

- compared to free water

Small particles, small pores

Applies to unsaturated soils

Primary Factors in Matric Potential

Texture, Density, Aggregation

Pore Size Distribution

Moisture Content

Which Pores are Filled

Capillarity and Soil Texture

Small pores

Strong suction

Strong capillarity

Large pores

Weak suction

Weak capillarity

Capillary Potential Energy

water

Dry soil

Suction potential energy

Matric potential energy

Porous block

Suction (capillarity)

Capillary Potential

100 cm

Dry soil

Ψm = -100 cm

(suction)

Vertical distance between the surface of the water and the porous cup.

suction

Soil Texture

1000 cm

Dry soil

ψm = -1000cm

(suction)


Sandy Soil Porous block

suction

Soil Texture

10,000 cm

Dry soil

Ψm = -10,000 cm

(suction)


Fine-textured soil

suction suction

Clay Sand

Soil Texture

Unsaturated soils have negative matric potential energy

Submergence Potential

Submergence Potential (ψs)

Equal to the distance below a free water surface

Water Table

10 cm

Units of Potential

Centimeters of water

Bars

Pascals

1 bar = 1020 cm water (4oC)

1 KPa = 10 cm water

1 bar = 100 kPa

Total Potential Energy is the sum

of the gravitational, submergence,

and matric potential energies.

Ψg + ψm + ψs = ψT

Gravitational Potential + Matric Potential = Total Potential


Height (cm)

50

20

a

10

Ψm = -65 cm Ψg = 50 cm

ΨT = -15 cm



Height (cm)

50

20

a

b 10

Ψm = -65 cm

Ψm = -5 cm Ψg = 10 cm

Ψg = 50 cm

ΨT = -15 cm

ΨT = 5 cm


Height (cm)

50

20

a

b 10

ΨTa = -15 cm

ΨTb = 5 cm

ΨTa – ΨTb = (-15cm) - 5cm = -20 cm

Energy Differences


Height (cm)

50

20

a

b 10

ΨTa = -15 cm

ΨTb = 5 cm

ΨTa – ΨTb = (-15cm) - 5cm = -20 cm

Which way will water move?

Determining the Direction of Water Flow

4. Point A – Point B

5. Water moves from high to low energy

Positive Point A to Point B

Negative Point B to Point A

1. Sum the individual potentials at each point

2. Determine if there is a difference in potential

3. Water will move from the higher to the lower energy

Tensiometer

Quantifying Water Movement

Gradient

The difference in potential divided by the

Distance between the two points considered

total potential at point A – total potential at point B

distance between points A and B

The driving force for water flow.

The stronger the gradient,

the greater the driving force

for water movement.


Height (cm)

50

20

a

b 10

ΨTa = -20 cm

ΨTb =-100 cm

Difference in total potential = 80 cm = 2

Distance between the points 40 cm =

Gradient

Difference in potential energy = -20 cm – (-100 cm) = 80 cm

Gradient =

Distance between points A and B = 40 cm

Distance (cm) 0

Height (cm)

50

20

a b

10

Difference in total potential -100 - (-200) = 100 cm = 5

Distance between the points 20 cm 20 cm =

5 25

Ψma = -100 cm

Ψga = 0 cm

Ψmb = -200 cm

Ψgb = 0 cm Ref.



Height (cm)

50

20

a

10

Ψm = -95 cm Ψg = 50 cm

ΨT = -45 cm



Height (cm)

50

20

a

b 10

Ψm = -95 cm

Ψm = -25 cm Ψg = 10 cm

Ψg = 50 cm

ΨT = -45 cm

ΨT = -15 cm

ΨTa – ΨTb = (- 45cm) - (-15cm) = -30 cm

Quantifying Water Movement

Gradient

The difference in potential divided by the

Distance between the two points considered

total potential at point A – total potential at point B

distance between points A and B

The driving force for water flow.

The stronger the gradient,

the greater the driving force

for water movement.


Height (cm)

50

20

a

b 10

ΨTa = -20 cm

ΨTb =-100 cm



Gradient


Gradient =


Distance (cm) 0

Height (cm)

50

20

a b

10



5 25

Ψma = -100 cm

Ψga = 0 cm

Ψmb = -200 cm

Ψgb = 0 cm Ref.

Characterizing Soil Moisture Status

Water Content Based

Water Content Based

Soil Water Content

Water content by weight

Moist weight – Dry weight

Dry soil weight =

Water weight

Dry soil weight

Multiply by 100 to yield % water by weight

V = Πr2h

Water content by Volume

Volume Water

Volume Soil

Multiply by 100 to yield % water by volume

Example:

You collect a 200 cm3 soil sample. Its moist weight is

150 g. After drying, the dry weight is 100 g.

Gravimetric water content:

Moist weight – Dry weight

Dry weight =

Water weight

Dry weight

150 g - 100g

100g =

50 g = 0.5 or 50%

100g

Example:

You collect a 200 cm3 soil sample. Its moist weight is

150 g. After drying the dry weight is 100 g.

Volumetric water content:

150 g - 100g

200 cm3 = = 50 cm3 water = 0.25 or 25%

200 cm3 soil

Volume Water

Volume Soil Density of water

1 g/cm3

50 g

200 cm3

Energy-Based

Characterizing Soil Moisture Status

Relating water content and matric potential (suction)

suction

porous plate

Soil Core

Characterizing Soil Water

Characterizing Soil Water

Suction applied in

discrete increments.

Water

Remaining

In soil

Suction applied (cm) 0 10,000

One soil

saturated

*

Soil Core

Moisture Release Curve

Texture, Density

Water

Remaining

In soil


saturated

*

A

B

Two Soils

coarser

finer

Pore Size Distribution

Water

Remaining

In soil

Suction applied (cm) 10,000

saturated

*

Soil Moisture Status

Soil Moisture Status

Field Capacity: Water content of soil after drainage from saturation by gravity

Suction equivalent: -0.33 bars (or –0.10 bars)

- 33 KPa

- 330 cm water

Permanent: Water can no longer be accessed by plants

Wilting point Suction equivalent: -15 bars

-1500 KPa

- 15,000 cm water

Saturation: Water content of soil when all pores are filled

Suction equivalent: 0 bars

0 KPa

0 cm water

Plant Available water: Field Capacity - PWP

Texture Field

Capacity

Perm. Wilting

Point

Sandy Loam 17 9

Loam 24 11

Clay 36 20

Heavy Clay 57 28

Energy and Texture

Smaller particles and pores

Water Content (%) at

Practical Measures

Water

Remaining

In soil


saturated

*

Direct Methods

Soil Resistance Blocks

Time Domain Reflectometry

The Rate of Water Movement

Hydraulic Conductivity

Strongly responsible for water distribution within the soil volume. Determines the rate of water movement in soil.

Texture Density Structure Water content

The ease with which water moves through soils

Coarse

uncompacted

Fine

compacted

Hydraulic Conductivity

h

L

A

Volume time

h * A L W

A T E R

Determining Saturated Hydraulic Conductivity

Volume time

= h * A L

K

K = V * L

h * A * t

Soil

Approximate Ksat and Uses

Ksat (cm/h) Comments

36 Beach sand/Golf Course Greens

18 Very sandy soils, cannot filter

pollutants

1.8 Suitable for most agricultural,

recreational, and urban uses

0.18 Too slow for most uses

<3.6 x 10-5 Extremely slow; good if compacted

material is needed

Saturated hydraulic conductivity

Determining Saturated Flow

Determining Saturated Flow

Darcy’s Equation

Volume flow

Area * time = Q

A

= Ksat * gradient


Height (cm)

50

20

a

b 10

ΨTa = -20 cm

ΨTb =-100 cm



Gradient


Gradient =


Darcy’s Equation

Volume flow

Area * time = Q = Ksat * gradient

(Q) = 5 cm/hr * 2

= 10 cm/hr


Distance between the points 40 cm = Gradient =

Distance (cm) 0

Height (cm)

50

20

a b

10



5 25

Ψma = -100 cm

Ψga = 0 cm

Ψmb = -200 cm

Ψgb = 0 cm Ref.

If Ksat = 5 cm/hr, then the flow (Q) = 5 cm/hr * 5 = 25 cm/hr

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