soil 2forstudentin universitygeology.ppt

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1. SOIL WATER SOIL WATER

2. Water PotentialWater Potential
Water potential reflects the energy status of soil water.Water potential reflects the energy status of soil water.

Total potential comprise the following:Total potential comprise the following:

Capillary potentialCapillary potential

Gravity potentialGravity potential

Matric potentialMatric potential

Osmotic potentialOsmotic potential

Pressure potentialPressure potential
Active surfaces play a big role in determining Active surfaces play a big role in determining water water
holding capacityholding capacity
Soil suctionSoil suction reflects the power to suck water into the soil reflects the power to suck water into the soil
Plant activity can aid suctionPlant activity can aid suction

3. Water movement and retentionWater movement and retention

CAPILLARY RISECAPILLARY RISE

Water movement in unsaturated conditions, where water moves Water movement in unsaturated conditions, where water moves
through pore spaces that act like capillary tubes, causes the through pore spaces that act like capillary tubes, causes the
water to move upwardswater to move upwards

ADHESIONADHESION

The attraction of two different moleculesThe attraction of two different molecules

Charged H+ positive ions attracted to negatively charged ions Charged H+ positive ions attracted to negatively charged ions
like O within water molecules.like O within water molecules.

H from water is drawn to O of the surfaces of soil mineralsH from water is drawn to O of the surfaces of soil minerals

COHESIONCOHESION

The attraction of two similar molecules as in water-water The attraction of two similar molecules as in water-water
attraction attraction

The combination of adhesive and cohesive forces causes the The combination of adhesive and cohesive forces causes the
water to retained on the surfaces of soil particles to be stronger. water to retained on the surfaces of soil particles to be stronger.

4. Units for water retentionUnits for water retention

Atmospheric (atm), suction/pressure ( –ve Atmospheric (atm), suction/pressure ( –ve
pressure value): pF: log cm water column, pressure value): pF: log cm water column,
capacity for water to work : barcapacity for water to work : bar

pF 1 =- 1 kPa = 1J/kg = 0.01 bar = 0.01 pF 1 =- 1 kPa = 1J/kg = 0.01 bar = 0.01
atm = 10 cm air = 10000 dynes/cm3 (pF2 atm = 10 cm air = 10000 dynes/cm3 (pF2
= -10kPa)= -10kPa)

5. CONDITION OF WATER IN SOILCONDITION OF WATER IN SOIL
Soil water occupying micropores and a major part of Soil water occupying micropores and a major part of
macropores may occur in several phasesmacropores may occur in several phases

GRAVITY WATER: water not under the influence of GRAVITY WATER: water not under the influence of
adhesive and cohesive forces will not be retained by the adhesive and cohesive forces will not be retained by the
soil and will flow (percolate) when subjected to soil and will flow (percolate) when subjected to
gravitational pull. gravitational pull.

FIELD CAPACITY: water is held at less than 1/3 bar (-FIELD CAPACITY: water is held at less than 1/3 bar (-
33kPa). It is the water content attained after all of the 33kPa). It is the water content attained after all of the
gravity water exits; usually after 1-2 days after initial gravity water exits; usually after 1-2 days after initial
point of saturation = pF2.0 point of saturation = pF2.0

WILTING POINT: water held at 15 bar or more. The WILTING POINT: water held at 15 bar or more. The
water cannot be absorbed by plants and the plants wilt.= water cannot be absorbed by plants and the plants wilt.=
pF4.2pF4.2
WWater-holding capacityater-holding capacity: : Amount of water held Amount of water held
between field capacity and wilting pointbetween field capacity and wilting point

7. SOIL MOISTURE SOIL MOISTURE
CHARACTERISTICSCHARACTERISTICS
The soil moisture characteristics of a soil can be The soil moisture characteristics of a soil can be
determined by using a pressure plate apparatusdetermined by using a pressure plate apparatus
Soil water energy is measured based on the suction Soil water energy is measured based on the suction
capacity: water held by soil in h cm can be absorbed by capacity: water held by soil in h cm can be absorbed by
plants with a suction > h cm.plants with a suction > h cm.
Soil water energy concept a) water from wet soils are Soil water energy concept a) water from wet soils are
easily absorbed by plants and are held at low energy easily absorbed by plants and are held at low energy
levels b) water in dry soils are not easily absorbed due to levels b) water in dry soils are not easily absorbed due to
the high energy holding the water.the high energy holding the water.
AVAILABLE WATER:AVAILABLE WATER: Water obtained by plants, Water obtained by plants,
AW = Soil moisture – wilting pointAW = Soil moisture – wilting point

8. UNSATURATED FLOWUNSATURATED FLOW

Flowing water which is held with a water Flowing water which is held with a water
potential lower than 0.33 bar (ie higher potential lower than 0.33 bar (ie higher
negative values)negative values)

Water will flow towards areas of lower Water will flow towards areas of lower
potential, that is it will flow from wet soil to potential, that is it will flow from wet soil to
dry soil, and in all directions.dry soil, and in all directions.

The higher the gradient the high the rate of The higher the gradient the high the rate of
flowflow

9. SATURATED FLOWSATURATED FLOW

Flow due to gravitational pullFlow due to gravitational pull

Beginning with infiltration – the entrance of Beginning with infiltration – the entrance of
rain water or irrigation water into the soil rain water or irrigation water into the soil
surface.surface.

When the soil is sufficiently wet: “water flows When the soil is sufficiently wet: “water flows
through wet soil” or through wet soil” or percolationpercolation occurs occurs

10. INFILTRATION INFILTRATION
Factors affecting water infiltration into the soilFactors affecting water infiltration into the soil

% sand, silt and clay% sand, silt and clay

soil structuresoil structure

amount of organic matteramount of organic matter

depth to impermeable layer/ rocky layerdepth to impermeable layer/ rocky layer

amount of water in the soilamount of water in the soil

soil temperature (evaporation)soil temperature (evaporation)

soil compactionsoil compaction

Infiltration is driven mainly by the attraction of water to dry Infiltration is driven mainly by the attraction of water to dry
pores and surfaces (high matric potential of free water driven to pores and surfaces (high matric potential of free water driven to
low matric potential of pore water)low matric potential of pore water)

Gravity only aids when infiltration is downward.Gravity only aids when infiltration is downward.

A wetting front develops; behind the front it is nearly saturated, A wetting front develops; behind the front it is nearly saturated,
ahead of the front it is dry.ahead of the front it is dry.

The depth to which a quantity of water wets the soil depends on The depth to which a quantity of water wets the soil depends on
the soil’s water capacity and its initial water content.the soil’s water capacity and its initial water content.

11. SHRINK-SWELLSHRINK-SWELL

Shrink-swell characteristizes certain clayey soils high in smectites Shrink-swell characteristizes certain clayey soils high in smectites

Periodicity and magnitude of precipitation, evaporation and conditions for the Periodicity and magnitude of precipitation, evaporation and conditions for the
concentration of moisture in specific parts of the soil (poor drainage or concentration of moisture in specific parts of the soil (poor drainage or
structural seepage) control the pattern of interaction between shrink and swellstructural seepage) control the pattern of interaction between shrink and swell

The potential change in volume depends on the initial moisture content, initial The potential change in volume depends on the initial moisture content, initial
density, void ratio, microstructure, vertical stress, type and content of clay density, void ratio, microstructure, vertical stress, type and content of clay

Anisotropy can result in varying movementsAnisotropy can result in varying movements

At wilting point (pF4.2), At wilting point (pF4.2),
Water content (clay soils > Water content (soils with less clay)Water content (clay soils > Water content (soils with less clay)

Shrinkage limit is reached when evaporation does not change volume anymore Shrinkage limit is reached when evaporation does not change volume anymore

Microstructure plays an important role in determining the shrinkage process Microstructure plays an important role in determining the shrinkage process
and rate and rate

Cracks start to occur at pF4.6 Cracks start to occur at pF4.6

Wetting and drying of clay soils will result in aggregation of particles and Wetting and drying of clay soils will result in aggregation of particles and
cementation by Ca, Mg, Al atau Fe. This increases permeability and resistance cementation by Ca, Mg, Al atau Fe. This increases permeability and resistance
to compaction to compaction

The bond resulting from the drying occurring between particles increases soil The bond resulting from the drying occurring between particles increases soil
strengthstrength