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Characterizing Soil WaterCharacterizing Soil Water

Three Potential Energies:

Gravitational PotentialCapillary or Matric PotentialSubmergence Potential

Gravitational PotentialGravitational 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.

We will use gravitational potentialenergy per unit weight of water (cm).

Porous block

Suction (capillarity)

Matric or Capillary PotentialMatric or Capillary Potential

100 cm

Dry soil

Ψm = -100 cm(suction)

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

Submergence Potential (ψs)

Equal to the distance below a free water surface

Water Table

10 cm

Sand

Clay

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

Ψg + ψm + ψs = ψT

Gravitational Potential + Matric Potential = Total Potential

Reference levelΨg = 0

Height (cm)

50

20

a

10

Ψm = -95 cm Ψg = 50 cm

ΨT = -45 cm

Gravitational Potential + Matric Potential = Total Potential

Reference levelΨg = 0

Height (cm)

50

20

a

b10

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 MovementQuantifying 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.

Reference levelΨg = 0

Height (cm)

50

20

a

b10

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 cmRef.

The stronger the gradient,the greater the driving force

for water movement.

Characterizing Soil Moisture Status

Water Content Based

Soil Water ContentSoil 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 is150 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 is150 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 SoilDensity of water

1 g/cm3

50 g200 cm3

Energy-Based

Characterizing Soil Moisture Status

Relating water content and matric potential (suction)

suction

porous plate

Soil Core

Characterizing Soil WaterCharacterizing Soil Water

Characterizing Soil WaterCharacterizing Soil Water

Suction applied in discrete increments.

Water RemainingIn soil

Suction applied (cm)0 10,000

One soil

saturated

*

Soil Core

Moisture Release Curve

Texture, DensityTexture, Density

Water RemainingIn soil

Suction applied (cm)0 10,000

saturated

*A

B

Two Soils

coarser

finer

Pore Size Distribution

Water RemainingIn soil

Suction applied (cm) 10,000

saturated

*

Soil Moisture Status

Soil Moisture Status

Field Capacity: Water content of soil after drainage from saturation by gravitySuction equivalent: -0.33 bars (or –0.10 bars)

- 33 KPa - 330 cm water

Permanent: Water can no longer be accessed by plantsWilting point Suction equivalent: -15 bars

-1500 KPa - 15,000 cm water

Saturation: Water content of soil when all pores are filledSuction equivalent: 0 bars

0 KPa 0 cm water

Plant Available water: Field Capacity - PWP

TextureTexture Field Field

CapacityCapacityPerm. Wilting Perm. Wilting

PointPoint

Sandy Loam Sandy Loam 1717 99

Loam Loam 2424 1111

Clay Clay 3636 2020

Heavy Clay Heavy Clay 5757 2828

Energy and Texture

Smallerparticlesand pores

Water Content (%) at

Practical Measures

Water RemainingIn soil

Suction applied (cm)0 10,000

saturated

*

Direct Methods

Soil Resistance Blocks

Time Domain Reflectometry

The Rate of Water Movement

Hydraulic ConductivityHydraulic Conductivity

Strongly responsible for water distributionwithin the soil volume.

Determines the rate of water movement in soil.

TextureDensityStructureWater content

The ease with which water moves through soils

Coarseuncompacted

Finecompacted

Hydraulic ConductivityHydraulic Conductivity

h

L

A

Volume time

' h * ALW

ATER

Determining Determining SaturatedSaturated Hydraulic Conductivity Hydraulic Conductivity

Volume time

= h * A L

K

K = V * L h * A * t

Soil

Approximate Ksat and UsesApproximate Ksat and Uses

Ksat (cm/h)Ksat (cm/h) CommentsComments

3636 Beach sand/Golf Course GreensBeach sand/Golf Course Greens

1818 Very sandy soils, cannot filter Very sandy soils, cannot filter pollutantspollutants

1.81.8 Suitable for most agricultural, Suitable for most agricultural, recreational, and urban usesrecreational, and urban uses

0.180.18 Too slow for most usesToo slow for most uses

<3.6 x 10<3.6 x 10-5-5 Extremely slow; good if compacted Extremely slow; good if compacted material is neededmaterial is needed

Saturated hydraulic conductivity

Determining Saturated Flow

Determining Saturated Flow

Darcy’s Equation

Volume flowArea * time

= Q

A

= Ksat * gradient

Reference levelΨg = 0

Height (cm)

50

20

a

b10

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

Darcy’s Equation

Volume flowArea * time

= Q = Ksat * gradient

(Q) = 5 cm/hr * 2

= 10 cm/hr

Difference in total potential = 80 cm = 2 Distance between the points 40 cm=Gradient =

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 cmRef.

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

Exam is Friday, May 22 in class

Review session: Thursday

Study Guide: Wednesday