Soil Water Movement and Retention

Functions of Soil

Medium for plant growth Regulator of water supplies Recycler of raw materials Habitat for soil organisms Engineering medium

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

Water Movement

Two Forces Responsible for Water Movement in Soils

Gravity Capillarity

Gravity

Capillarity

Spontaneous movement of water into and through pore spaces in soil without the aid of gravity.

Adhesion and Cohesion

Cohesion

Adhesion and Cohesion

adhesion cohesion H oxygen H H S U R F A C E oxygen H

Adhesion and Cohesion

droplet adhesion Cohesion (H-bonding)

Adhesion and Cohesion Strong adhesion Weak adhesion

Weak Adhesion

Adhesion to Soil Particles Strong Adhesive Forces

Soil Pores

Adhesion and Cohesion capillarity Adhesion to the tube or pore wall Cohesion between water molecules

Capillarity

Tube/Pore wall

h = 0.15

r

adhesion cohesion Force down

Capillarity

h h = 0.15

r Small pores

Capillarity

Soil Pores and Pore Size Distribution Texture Density Structure

Texture

Particle Size Pore Size Capillarity Large/coarse

Sand Loamy Sand Sandy Loam Silt

Medium

Sandy clay Loam Silty clay Loam Silt Loam Loam

Fine/Small

Clay Loam Sandy Clay Silty Clay Clay

Large/Macro Weak Meso/ Medium Moderate Micro/Small Strong

Soil Pores

Sandy Silty

Clayey

Capillarity Dominated Gravity Dominated

Density

Depth in Profile Arrangement of Particles Compaction

Structure Macropores Micropores

Examples

Water

Sand Clay

Initial Saturation

Sand Clay

Initial Saturation Sandy Loam

Uncompacted Compacted

Aggregates

Same Texture and Density

Wet Moist

Relevance transpiration water

Quantification: Soil Water Energy

Potential Energy

Energy waiting to be used or exploited

Gravitational Potential Energy Water moves in response to differences in potential energy, from high potential energy to low potential energy.

High potential Energy The greater the difference in height The greater the difference in Gravitational potential energy.

Low potential Energy

Gravitational Potential ψ

g The potential energy of a unit quantity of water.

Unit quantities: volume mass weight

= mg

Ψ

g

Ψ

g mg

= mgh = h

(cm) The greater the height, the greater the potential energy.

Height (cm) 100

Gravitational Potential

Independent of soil properties

a

50 40

b

ψ ga = 100 cm ψ gb = 40 cm

soil

Ψ g = 0 Reference level Difference in energy determines movement

Height (cm) 100 40 0

Gravitational Potential

a b Ψ ga = 60 cm Ψ gb = 0 cm Reference level (Ψ g = 0) Ψ ga – Ψ gb 60 - 0 = 60 cm

Gravitational Potential

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 - strong force - compared to free water h = 0.15

r Small particles, small pores Applies to

unsaturated

soils

Primary Factors in Matric Potential

Texture, Density, Aggregation Moisture Content Pore Size Distribution 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

Capillary Potential

Porous block 100 cm Suction (capillarity) Ψm = -100 cm (suction) Dry soil

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

Sandy Soil Soil Texture

Porous block suction 1000 cm ψ m = -1000cm (suction) Dry soil Vertical distance between the surface of the water and the porous cup .

Soil Texture

Fine-textured soil suction 10,000 cm Ψ m = -10,000 cm (suction) Dry soil Vertical distance between the surface of the water and the porous cup.

Soil Texture

suction suction Clay Sand

Unsaturated soils have negative matric potential energy

Submergence Potential

Submergence Potential ( ψ s ) 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 (4 o C) 1 KPa = 10 cm water 1 bar = 100 kPa

Total Potential Energy is the sum of the gravitational, submergence, and matric potential energies.

Ψ g + ψ m + ψ s = ψ T

Gravitational Potential + Matric Potential = Total Potential

Height (cm) 50 a Ψm = -65 cm Ψg = 50 cm Ψ T = -15 cm 20 10 Ψ g = 0 Reference level

Gravitational Potential + Matric Potential = Total Potential

Height (cm) 50 a Ψm = -65 cm Ψg = 50 cm Ψ T = -15 cm 20 10 Ψ g = 0 b Ψm = -5 cm Ψg = 10 cm Ψ T = 5 cm Reference level

Energy Differences Height (cm) 50 a Ψ Ta = -15 cm 20 10 Ψ g = 0 b Ψ Tb = 5 cm Reference level Ψ Ta –

Ψ Tb = (-15cm) - 5cm = -20 cm

Which way will water move?

Height (cm) 50 a Ψ Ta = -15 cm 20 10 Ψ g = 0 b Ψ Tb = 5 cm Reference level Ψ Ta –

Ψ Tb = (-15cm) - 5cm = -20 cm

Determining the Direction of Water Flow 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 4. Point A – Point B 5. Water moves from high to low energy Positive Negative Point

A

to Point

B

Point

B

to Point

A

Next: Characterizing Water Status