1
Unsaturated zone / Soil Moisture
Onverzadigde zone / bodemvocht

This lecture
• Discharge from unpaved land.
The process of rainfall , infiltration and
recharge of ground water
• Storage of water in soil? (Unsaturated zone)
1

Example Hydrological cycle
evapotranspiration
Transpiration
275 mm
Evaporation
75 mm
350 mm
Atmosphere
Precipitation
800 mm
Ground level
Surface
runoff
25 mm 250 mm
425 mm
Sea
700 mm
275 mm
Open surface
water system
Soil Moisture or
Unsaturated zone
crops
Capillary rise
50 mm
Absorption
by roots
Water table or
Groundwater level
Percolation or
recharge
475 mm
Precipitation
Surface level
Unsaturated zone or
Soil Moisture
Saturated zone1

Hydrological cycle (surface)
• Precipitation
• Interception (leaves), delay and evaporation
• Infiltration to soil (mm/day)
• If net precipitation > infiltration
Storage in depression (Indication 10 mm)
Surface runoff is possible
• No paved area
2

Infiltration
2
Grondsoort Soil
Indication
Infiltration
capacity
(m/day)
matig grof zand moderate coarse sand 3
fijn zand met enige humus fine sand with some humus 0.3
Fijn zandige leem fine sandy loam 0.2
lichte zavel silt 0.3
matig zware klei clay 0.02
zandig veen sandy peat 0.1
veen peat 0.06

Dutch (Delta) situation
• Low intensity precipitation (100 mm 24
hour, = 4 mm / hour)
• High infiltration capacity
• Land is horizontal and flat
• So little surface runoff
2

Hydrological cycle (moisture)
(bodemvocht) unsaturated zone
• Infiltration from surface
• Percolation / Recharge ground water
• Transpiration (roots)
• Storage in pores
• Forces on moisture (bodemvocht)
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Forces on Moisture
• adsorption, osmotic and capillary forces.
• Adsorption= H2O bond to negative soil parts
• Osmotic=Cation’s ( pos zoutdeeltjes) cause a high
concentration of molucules at the soil part. This
attracks H20 molucules.
• Adsorption and Osmotic bond water not
available for plants
3

Capillary action
• Cohesion ; intermolecular attraction
between like molecules.
• Adhesion; attraction between unlike
molecules
• Capillary action; Adhesion > Cohesion
3

Capillary action
• Smaller pores have a stronger capillary force
as bigger pores
• Equilibrium with gravitation
3

Remarks?
• In the unsaturated zone there are forces
which can slow down percolation
(small pores are filled first)
• How many pores in the unsaturated zone are
filled with water. How much is left for
storage of water.
3

Moisture curve
Wilting point
Water not available for plants
Volume water
pressure
sand
peat
clay
clay / sand mixture
light clay / sand mixture
Field capacity
Water available for plants
4
(Verwelkingspunt)
pF cm
101,0 10
101,5 32
101,7 50
102,0 100
102,3 200
102,7 501
103,0 1000
103,4 2512
104,2 15849

Staring series
Volume water
Pressure (cm)
5
pF cm
101,0 10
101,5 32
101,7 50
102,0 100
102,3 200
102,7 501
103,0 1000
103,4 2512
104,2 15849

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Staring series, soil
Nr. Bodemhorizonten Soils (translation is just an indication)
1 uiterst humusarm, matig fijn, leemarm zand little humus, little loam, moderately fine sand
2 zeer humusarm, matig fijn, leemarm zand much humus, little loam, moderately fine sand
3 matig humusarm, matig fijn, leemarm zand moderate humus, little loam, moderately fine sand
4 matig humeus, matig fijn, leemarm zand moderate humus, little loam, moderately fine sand
5 zeer humeus, matig fijn, leemarm zand much humus, little loam, moderately fine sand
6 humusrijk, matig fijn, leemarm zand much humus, little loam, moderately fine sand
7 zandig veen sandy peat
8 veen peat
9
uiterst humusarm, matig fijn tot zeer fijn, zwak
lemig zand little humus, little loam, fine sand
10 uiterst humusarm, zeer fijn, sterk lemig zand little humus, much loam, fine sand
11 uiterst humusarm, zeer fijn, zeer sterk lemig zand little humus, much loam, fine sand
12 uiterst humusarme, zandige leem little humus, sandy loam
13 uiterst humusarm, uiterst fijn, leemarm zand little humus, little loam, fine sand
14 uiterst humusarm, matig grof, leemarm zand little humus, little loam, coarse sand
15 zeer humusarme, lichte zavel Silt
16 zeer humusarme, lichte zavel Silt
17 zeer humusarme, lichte zavel Silt
18 zeer humusarme, zware zavel Heavy silt loam
19 zeer humusarme, lichte klei Silty clay loam
20 zeer humusarme, zware klei Clay
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How to use the Staring series
• Relation between pressure and moisture content
• Gives the potential storage in the unsaturated zone
• Storage coefficient (bergingscoefficient)
= Volume available storage (m3) / Volume soil (m3)
• Example storage coefficient = 0.02
Each m3 soil can store max 0.02 m3 of water5