Soil Water

Soil WaterSoil Water

Chapter 5Chapter 5

Chapter 5 OutlineChapter 5 Outline

I. General Properties of WaterI. General Properties of Water

II. Capillary ActionII. Capillary Action

III. Energy ConceptsIII. Energy Concepts

IV. Flow of WaterIV. Flow of Water

V. Specific ExamplesV. Specific Examples

Michael J. Singer and Donald N. MunnsMichael J. Singer and Donald N. MunnsSoils: An Introduction, 6eSoils: An Introduction, 6e

What makes H2O so amazing?

I. General Properties of WaterI. General Properties of Water

A. PolarityA. Polarity

B. Hydrogen BondingB. Hydrogen Bonding

C. Cohesion/AdhesionC. Cohesion/Adhesion

D. Surface TensionD. Surface Tension

I. General Properties of WaterI. General Properties of Water

II. Capillary ActionII. Capillary Action

A. EquationA. Equation

B. Comparison of texturesB. Comparison of textures

III. Energy ConceptsIII. Energy Concepts

IV. Flow of WaterIV. Flow of Water

V. Specific ExamplesV. Specific Examples

Capillary Action h=0.15/r

In moist soils, the pores act In moist soils, the pores act like thin tubes, and water is like thin tubes, and water is held by capillarity. Pores are held by capillarity. Pores are not straight and smooth like not straight and smooth like the tubes in the figure, and so the tubes in the figure, and so water does not reach the same water does not reach the same height in pores as it would in height in pores as it would in tubes of the same diameter. A tubes of the same diameter. A soil with many small pores soil with many small pores holds more water by capillarity holds more water by capillarity than does a soil with few large than does a soil with few large pores. In addition, because of pores. In addition, because of capillarity, water rises higher capillarity, water rises higher from a water table in a clay soil from a water table in a clay soil than in a sandy soil.than in a sandy soil.

Figure 5.6Figure 5.6

I. General Properties of WaterI. General Properties of WaterII. Capillary ActionII. Capillary Action

III. Energy ConceptsIII. Energy ConceptsA. Energy reviewA. Energy reviewB. PotentialB. PotentialC. General Equation and termsC. General Equation and terms

1. Gravitational potential1. Gravitational potential2. Osmotic potential2. Osmotic potential3. Pressure potential (matric and hydrostatic)3. Pressure potential (matric and hydrostatic)

D. Measuring water content (Ө) and potential (Ψ)D. Measuring water content (Ө) and potential (Ψ)

IV. Flow of WaterIV. Flow of WaterV. Specific ExamplesV. Specific Examples

A. Energy ReviewA. Energy Review1. kinetic vs potential energy1. kinetic vs potential energy2. movement from high to low energy2. movement from high to low energy

B. Potential B. Potential 1. difference in energy level of water from one site to another1. difference in energy level of water from one site to another2. wet soil: most water far from soil particle surface (loosely held, 2. wet soil: most water far from soil particle surface (loosely held, high high ΨΨ))3. dry soil: most water located in small pores (tightly held, low 3. dry soil: most water located in small pores (tightly held, low ΨΨ))

Water potential is a measure of the water's availability, or escaping tendency, or tendency to move. It is measured relative to water at a standard pressure and temperature at some reference elevation.

High water potential means that water can easily move. It is High water potential means that water can easily move. It is loosely held. loosely held.

Water potential is Water potential is highhigh when it is when it is zerozero and and lowlow when it is when it is -1500 kPa-1500 kPa. .

More negative is lower potential.More negative is lower potential.

ΨΨtt = = ΨΨgg + + ΨΨmm + + ΨΨoo + + ΨΨss + …. + ….

ΨΨgg=gravitational=pulls water downward=gravitational=pulls water downward

always positivealways positive

ΨΨmm=matric=attraction to solids (suction, tension)=matric=attraction to solids (suction, tension)

adhesion and capillary (movement moist [high] to dry [low])adhesion and capillary (movement moist [high] to dry [low])

ΨΨoo=osmotic= attraction to ions (lower potential energy)=osmotic= attraction to ions (lower potential energy)

>concentration = < potential >concentration = < potential (impt for water uptake by (impt for water uptake by plant)plant)

ΨΨss=submergence=hydrostatic (saturated zones only)=submergence=hydrostatic (saturated zones only)

What are the major components of water potential? What What are the major components of water potential? What factors most influence water potential in soil? factors most influence water potential in soil?

The major The major components of water potentialcomponents of water potential are the are the gravitational, osmotic, and matric potentials. gravitational, osmotic, and matric potentials.

Dry SoilsDry Soils In unsaturated soils, matric and osmostic potentials are In unsaturated soils, matric and osmostic potentials are

dominant and decline respectively with decreasing water dominant and decline respectively with decreasing water content or increasing salinity. content or increasing salinity.

Wet SoilsWet Soils In saturated soils and aquifers, gravitational potentials are In saturated soils and aquifers, gravitational potentials are

dominant. dominant.

Table 5.1Table 5.1

D. Measuring water content (Ө) and potential (Ψ)D. Measuring water content (Ө) and potential (Ψ)

How would you measure the water content and the water How would you measure the water content and the water potential of a soil?potential of a soil?

Water contentWater content (Ө) of soil is most easily measured by (Ө) of soil is most easily measured by weighing a sample wet and again after drying (oven to weighing a sample wet and again after drying (oven to constant weight). Water content = weight loss/dry weight. constant weight). Water content = weight loss/dry weight.

Water potentialsWater potentials (Ψ) in soil materials can be measured with (Ψ) in soil materials can be measured with tensiometers. tensiometers.

Water content calculationWater content calculation