soil soil properties,

AT761 Land-Atmosphere Interaction Soil Physics: Heat and Water Storage

Bonan Chapters 9-10 CSU ATS Scott Denning 1

Soil Properties, Heat and Water

Transport Please read Bonan,

Chapters 9-10

Soil •  Minerals, water, gases, organic debris, and myriad organisms that sustain plants

•  Interfaces to atmosphere, lithosphere, hydrosphere, biosphere

•  All three phases: Solid, liquid, and gas

•  Accumulates on top, but grows downward by weathering

Rock Weathering

Igneous

Sedimentary

Metamorphic

Rock

Very slowly weathered minerals (e.g., quartz, muscovite)

Slowly weathered minerals (e.g., feldspars, biotite)

Easily weathered minerals (e.g., augite, hornblende, olivine)

Chemical Weathering Iron and aluminum

oxide clays

Silicate clays

Soil solution (K+, Na+, Ca2+ , Mg2+, Fe2+, SO4

2-)

Resistant primary minerals (e.g., quartz)

Continued Disintegration

End-Products

Chemical Weathering

Physical Weathering

Pathways of Weathering

•  physical weathering of resistant quartz makes sand

•  chemical weathering of other minerals makes clay

•  dissolved minerals make sea salt

“The Rock Cycle:”

From Rock to Soil

2000 4000 6000 8000 Well-developed Mollisol

Unweathered loess

Organic matter

CaCO3, CaSO4 accumulation

Time (years)

!!!

! ! ! ! ! !! ! !

! ! !! ! !

100 1000 10 000 100 000 Well-developed Ultisol

Unweathered bedrock

Maximum leaching

Disintegrated weathered soil material

Clay accumulation

Organic matter

Time (years)

! ! !

•  Soil development depends on parent material, rainfall, temperature, and time

•  Wet forest soils vs dry grassland soils



Weathering Water carries organic acids and dissolved CO2 downward through soil column, reacts w/minerals

300 400 500 600 700 800 900 1000

Annual Precipitation (mm)

0

20

40

60

80

100

Dep

th T

o M

axim

um C

lay

(cm

)

Bedrock

C Horizon

B Horizon

A Horizon

O Horizon

E Horizon

Organic Matter

Clays Oxides

Carbonates

Salts

Soil Layer Leaching

Litter Humus

Clay minerals form and are deposited below surface

Clay in Soils •  Warm moist

climates are good for making clay minerals

•  Molecular structure is flat plates of hexagonally-arranged atoms with OH- groups sticking out

10 11 12 13 14 15 16 17 18

Annual Mean Temperature (°C)

0

10

20

30

40

50

5

15

25

35

45

Soi

l Cla

y C

onte

nt (%

)

Soil Texture

•  Coarse (sandy) soils conduct heat better, but have less heat capacity

•  Fine-grained (clay) soils “hold onto” water

Conservation Laws

a.k.a “Budgets” Stuff In minus Stuff Out = Change in Stuff

•  We will spend the first half of the semester constructing budgets – Heat budget

– Water budget – Energy budget – Momentum budget



Heat Flow through Soil Vertical heat flux in soil or rock is down-gradient “diffusion”

Formulate change in storage as a flux divergence:

Combining heat flux with change in storage

5 mm A 41°C

B 37°C

o oW 41 C 37 C1.5 o 0.01 mm C

2600 W/m

F

F

! "−$ %= − × $ %' (

= −

Heat Flow Example

Diurnal Variations of Soil Temperature •  Huge range near surface

–  25 K diurnal cycle at 0.5 cm

–  Max T around 2 PM

•  Damped and delayed with depth –  Only 6 K diurnal range at 10 cm’

–  Max T about 6 PM

–  Negligible diurnal cycle at 50 cm

•  Similar phenomena on seasonal time scales

Soil Temperature Cycles Assume periodic forcing of period p (e.g., diurnal or seasonal cycles, ice ages, whatever). Response of T(z) is also periodic, but damped and delayed with depth relative to surface forcing “Penetration depth” (e-folding) of temperature oscillations forced by surface periodicity depends on period of forcing and physical properties of material

p= 1 day D ~ 10 cm p= 1 yr D ~ 1.5 m p= 10,000 yr D ~ 150 m



Effects of Freezing

•  Latent heat of fusion “smooths” temperature changes around 273 K

•  Big differences in timing in spring and fall

Water

Positively-charged hydrogen ions at 105° angles are attracted to negatively-charged OH- ions on surfaces of clay minerals

water molecule ice

Adhesion •  Very strong attraction

of positive (H+) end of polar water molecules to negative (O and OH-) groups on polar clay molecules

•  Water “sticks” very tightly to clay!

Newton

•  Objects stay put or move uniformly in the same direction unless acted on by a force

•  Acceleration is a result of the sum (net) of forces, in the vector sense

!F∑ = m!a



Balanced Forces Newton’s Second Law Σ!F = m!a

!Fup

!Fdown

m

Suppose that !Fdown =

!Fup

Σ!F =!Fdown +

!Fup

=!Fdown −

!Fdown = 0

!a = Σ

!F m = 0

No acceleration … nothing moves!

What Holds the Air Up? A balance between gravity and the pressure gradient force.

ΔP/ Δz = ρg

A “pressure gradient force”pushes from high to low pressure.

ρg

ΔP/ Δz

Hydrostatic balance

Water in Soil Column Air

Soil + Water + Air

Soil + Water

Solid Bedrock

•  Force of gravity pulls water downward through soil pores, where it is ultimately supported by impermeable rock

•  Upper part of soil (“vadose zone”) supports water against gravity!

Soil Suction •  Molecular attraction

between water and soil materials can be very strong

•  Balance of forces between soil suction and gravity analogous to hydrostatic balance in the air

•  Measure soil suction as a negative pressure (Pa) or as force equivalent to pressure of a column of water (mm)

!Fup

!Fdown

m



Water in Soil Pore space completely filled with water

After gravitational drainage

No further water can be removed

Water in Soil •  Water potential has

two components:

– Gravitational potential

– Matric potential (a.k.a. “suction”)

•  When gravitational potential > matric potential, soil drains

•  If root potential weaker than matric potential, plant wilts Not all water is “available” to plants

Hydraulic Properties

•  Soil physical properties depend on texture (% sand, % silt, % clay)

•  Also very strongly dependent on water content of soil

Soil Water Content and “Suction”

•  “Matric potential” of soil is a very steep function of soil water content!

•  Note log scales!

•  As soil dries, matric potential increases exponentially

wilt point

field capacity



Darcy’s Law for Soil Water Flow

•  Direct analogy to downgradient heat flux and heat storage

•  Must account for both components of water potential

Water flows down potential gradient

flux

hydraulic conductivity

suction gravity

chain rule

water content

Δy

Δz

Δx

( ) 1

( ) 1 1

zF k kz zzF k k kz z z

ψ ψ

ψ ψ θ ψθ

Δ + Δ$ % & '= − = − +) *+ ,- Δ . / Δ 0

∂ + ∂ ∂ ∂$ % & ' & '= − = − + = − +) * ) *+ ,∂ ∂ ∂ ∂- . / 0 / 0

Fin

Fout

change in storage = flux in - flux out

( )in outx y z F F x ytθΔ# $

Δ Δ Δ = − − Δ Δ& 'Δ( )

1kt z zθ θ ψ

θ

# $∂ ∂ ∂ ∂& '= +( )* +∂ ∂ ∂ ∂, -. /

Δ Δ Δ Δθ / /t F z= −

50 mm A

550 mm B

mm ( 478 mm 500 mm) ( 843 mm 0 mm)2hr 500 mm

3.46 mm/hr

F

F

− + − − +" #= − × % &' (

= −

ψ=-478 mm z=500 mm

ψ=-843 mm z=0 mm

Darcy’s Law Example

Changes in Water Storage

Difficult to solve numerically because of strong nonlinearity of ψ(θ) and also k(θ)

Darcy’s Law

Richards Equation

Hydraulic Conductivity

Soil Drainage •  Gravitational drainage from initial saturation

•  Most drainage occurs right away

•  As water content decreases, matric potential becomes more negative and conductivity decreases

•  Clay still 90% sat’d after 24 hours

soil soil properties,

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