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SOIL WATERSOIL WATER
GES175, GES175, Science of SoilsScience of Soils
Water Movement- Surface water moves due to gravitational force
Does water always flow downward?
Energy ConceptEnergy Concept
Water moves from Water moves from high to low free energyhigh to low free energy
wet soil wet soil →→ dry soildry soilHH22O movementO movement
HH22O freeO free HH22O freeO freeenergy isenergy is energy isenergy ishigherhigher lowerlower
-- difference in free energy causes Hdifference in free energy causes H22O to moveO to move
Large Pores Small Pores
Soil Energy ComponentsSoil Energy Components(Potential Energy Terms)(Potential Energy Terms)
Gravimetric Potential, Gravimetric Potential, ΨΨggPressure PotentialPressure Potential–– Submergence Potential,Submergence Potential, ΨΨsubsub–– MatricMatric Potential, Potential, ΨΨmm (capillary and (capillary and
adsorptive forces)adsorptive forces)
Osmotic Potential, Osmotic Potential, ΨΨoo
Submergence Potential
Forces Creating Forces Creating Matric Matric PotentialPotential
CapillarityCapillarityWater moves into capillary soil poresWater moves into capillary soil pores
-- due to adhesive and cohesive forcesdue to adhesive and cohesive forces
Properties of WaterProperties of WaterHydrogen bondingHydrogen bonding
Adhesion Cohesion
25 KJ/mol
-
-
-
-
-
-
-
-
SiO
O
Capillary ForcesCapillary Forces
-- smaller pores hold water with greater energy smaller pores hold water with greater energy -- nondirectionalnondirectional
h = 0.15/rh = 0.15/r
2T cos αr ρw g
h =
T = surface tensionα = wetting angler = radius of poreρw = density of waterg = gravity
3.7
Water Movement by Capillary Action 3.8
Furrow irrigation
-
SiO
O
Adsorptive ForcesAdsorptive Forces- water binds strongly to mineral surfaces- results in thin water film
OH
HO
H
H
OH
H OH
H
OH
H
OH
H
OH
H
OH
H
K+
Cl-
OH
H
Osmotic Force
Water movement across semi-permeable membrane
3.9
Units (pressure/suction)Units (pressure/suction)
cm Hcm H22OO barsbars kPakPa300300 --0.30.3 --3030
1,0001,000 --11 --10010010,00010,000 --1010 --1000100015,00015,000 --1515 --15001500
see Table 5.1 in textsee Table 5.1 in text
3.11
1 atm = 760 mm Hg = 1020 cm H2O = 1 bar = 100 KPa
3.12
Sandy Soil
Clayey Soil
Water Content (%)
Pres
sure
(bar
s)
0
-20
Water Retention Curves
Aggregated Soil
Water Content (%)
Pres
sure
(bar
s)
0
-100
Compacted Soil
Water Retention Curves 3.13
Hysteresis
drying
wetting
θ
Suction (bars)
2r
2R
drying
wetting
0 -100
0.5
3.14
Ψm ∝ 1/r
Soil Water:Soil Water: Classification and AvailabilityClassification and Availabilityphysical biological
0
-30
-15
-0.3
gravitational
capillary
film/vapor
values are approximatedvalues are approximated
unavailable (drainage)
available
unavailable
Ψ (bars)
FC
PWP
3.16
clayey
sandy
H2O
Concepts of Movement
sandy
clayey
H2O
clayey
sandy
H2O
3.17
Hpi
Hgi
Hinlet= Hpi + Hgi
Reference level
soilHpo
Hgo
Ho = Hpo + Hgo∆H
∆H= Hinlet - Houtlet= Hpi - Hpo
Saturated Flow- Horizontal Column
L
3.18
H1
L
inlet
Reference plainoutlet
Hi = H1 + L
Ho = 0 + 0
Saturated Flow- Vertical Column
∆H = H1 + L
Unsaturated FlowUnsaturated Flow
q = -k(θ) H
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