1.72, Groundwater Hydrology Prof. Charles Harvey Lecture Packet #12: Soil Moisture
Pressure State
Usual range of
water content
θpwp < θ < φ
Atmospheric
0
p=ψ<0
p>0
z’
Root
Zone
Intermediate
Zone
Typical
Pressure
Profile
θfc < θ < φ
A
ψ = ψae
Tension-saturated zone
(capillary fringe)
θ=φ
z’0
Water table
Groundwater
p = p0
θ=φ
B
Impermeable Layer
Symbol
θpwp
θfc
φ
ψae
p0
θ=φ
Terms and Interpretations
Definition
Permanent wilting point water content
Field capacity water content
Porosity
Pressure at which air enters the system (top of the capillary fringe)
Atmospheric pressure; underground, the definition of water table
Saturation point
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 1 of 6
Unsaturated Flow: Movement of Soil Moisture
Water molecules attract each other so that at the surface there is a net downward
pull on the molecule. The net effect is surface tension.
Air
Water
Molecules
γ = 72.7 dyn/cm for water and air [F/L]
γ = 29 dyn/cm for benzene
γ = 430 dyn/cm for mercury
Energy/L2 – the energy required to increase the area. Unlike a membrane, the
surface tension doesn’t change with expansion.
Surface tension depends on:
The substances
Any solutes
Temperature
Gas Pressure
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 2 of 6
Consider a bubble of air in water:
Air
Water
∆P = 2γ/R
Blowing air into the bubble decreases the pressure.
γLG
Water
γSL
Air
γGS
Solid
γ SL = γ GS + γ LS Cos(α )
γ − γ GS
Cos(α ) = SL
γ LG
GS = gas-solid
SL = solid-liquid
LG = liquid-gas
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 3 of 6
Capillary Rise
∆P =
2γCos(α )
r
When the height of the
meniscus is at steady-state,
then the hydrostatic tension
must balance the effect of
surface tension.
hgρ =
2γCos(α )
r
Retention Curves or Soil Moisture Characteristic Curves
A bundle of capillaries (hydrophyllic) all of the same length with pressure adjusted at
the bottom. The bundle contains a range of radii. Measure average water content
as the suction is gradually increased at the bottom. Plot suction versus water
content.
ψ = suction
θω = water content
ψ
θω
ψ
θω
ψ
θω
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 4 of 6
ψ
θω
ψ
θω
ψ
θω
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 5 of 6
Pore space in rocks and soil is much more
complex geometrically but analogous
phenomena give rise to characteristic
retention curves for a given sample
material.
ψ
θω
The shape of the retention curve for a given porous material is influenced by:
1. Texture and Structure
a. Particle-size distribution
b. Pore-size distribution
c. Particle shape
d. Specific surface
2. History of wetting and drying – Hysteresis
a. Non-wetting phase entrapment
b. Swelling and shrinking
c. Ink-bottle effect – Haines Jump
ψ
θω
r
R
Drainage
Filling
1.72, Groundwater Hydrology
Prof. Charles Harvey
Lecture Packet 12
Page 6 of 6