WATER-The Substance of Life Limits Kinds and Amounts of Vegetation on Earth Limits Growth of Cities...

WATER-The Substance of Life

• Limits Kinds and Amounts of Vegetation on Earth

• Limits Growth of Cities and Kinds of Industry

آب رابطهگیاه و خاكتكميلی

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Isfahan University of TechnologyIsfahan University of Technology

EARTH’S WATER SUPPLY

70% of Earth’s Surface covered by water97% of Earth’s water supply in oceansIce at Polar Caps next most abundant supply (>2%)Groundwater next most abundant supply (~0.5%); approximately 50% of ground water is > 0.5 miles deepFresh water in lakes, ponds and streams ~0.008%Soil and atmospheric water ~ 0.001%Biological water ~0.0001%Average annual rainfall on land ~ 30 inches


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Ground water

Capillary fringe zone

Portion of aquifer where pore spaces are occupied with water and air (unsaturated zone)

Precipitation

Vadose ZoneSoil-Air Interface

Soil-Water Interface

Evaporation

Applications of soil physics are crucial to sustainable use of natural resources for agricultural and other land uses


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نفوذ

نزوالت جوی

آبیاری

تعرق و تبخیر

نروابآ

در ذخیرهخاک

نفوذ عمقی جریان

عمق داخلیخاک


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Precipitation/Evaporation

• P/E>0.75= Humid (Forests)

• P/E >0.5= Sub-Humid (Mixed Forest and Grasslands)

• P/E >0.25= Semi-Arid (Mixed Grasslands and Semi-Deserts)

• P/E <0.25= Arid (Deserts)


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Soil Water RelationshipsSoil Water Relationships


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• Bulk Density (Bulk Density (bb))

bb = soil bulk density, g/cm = soil bulk density, g/cm33

– MMss = mass of dry soil, g = mass of dry soil, g– VVbb = volume of soil sample, cm = volume of soil sample, cm33

• Typical values: 1.1 - 1.6 g/cmTypical values: 1.1 - 1.6 g/cm33

• Particle Density (Particle Density (pp))

PP = soil particle density, g/cm = soil particle density, g/cm33

– MMss = mass of dry soil, g = mass of dry soil, g– VVss = volume of solids, cm = volume of solids, cm33

• Typical values: 2.6 - 2.7 g/cmTypical values: 2.6 - 2.7 g/cm33

b

sbV

M

ps

s

M

V


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• Porosity ()

• Typical values: 30 - 60%

volume of pores

volume of soil

1 100%b

p


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Water in SoilsWater in Soils

• Soil water content

– Mass water content (m)

m = mass water content (fraction)

– Mw = mass of water evaporated, g (24 hours @ 105oC)

– Ms = mass of dry soil, g

s

wm

M

M


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• Volumetric water content (v)

V = volumetric water content (fraction)– Vw = volume of water– Vb = volume of soil sample– At saturation, V = V = As m

– As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3)

– As = b numerically when units of g/cm3 are used• Equivalent depth of water (d)

– d = volume of water per unit land area = (v A L) / A = v L– d = equivalent depth of water in a soil layer– L = depth (thickness) of the soil layer

vw

b

V

V


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Volumetric Water Content & Equivalent DepthVolumetric Water Content & Equivalent Depth

(g) (g)

(cm3)

(cm3)

Equivalent DepthEquivalent Depth


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Volumetric Water Content & Volumetric Water Content & Equivalent DepthEquivalent Depth

Typical Values for Agricultural SoilsTypical Values for Agricultural Soils

1 in.1 in.

0.50 in.0.50 in.

0.15 in.0.15 in.

0.20 in.0.20 in.

0.15 in.0.15 in.

Soil Solids (Particles): 50%Soil Solids (Particles): 50%

Total Pore Total Pore Space: 50%Space: 50%

Very Large Pores: 15% Very Large Pores: 15% (Gravitational Water)(Gravitational Water)

Medium-sized Pores: 20% Medium-sized Pores: 20% (Plant Available Water)(Plant Available Water)

Very Small Pores: 15% Very Small Pores: 15% (Unavailable Water)(Unavailable Water)


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Water-Holding Capacity of SoilWater-Holding Capacity of SoilEffect of Soil TextureEffect of Soil Texture

Coarse SandCoarse Sand Silty Clay Loam Silty Clay Loam

Gravitational WaterGravitational Water

Water Holding CapacityWater Holding Capacity

Available WaterAvailable Water

Unavailable WaterUnavailable Water

Dry SoilDry Soil


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Soil Water Content Soil Moisture ContentWater that may be evaporated from soil by heating at 1050C to a constant weight

Gravimetric moisture content (w) =mass of water evaporated (g)

mass of dry soil (g)

Volumetric moisture content () =volume of water evaporated (cm3)

volume of soil (cm3)

= w *bulk density of soil

density of water g

cm

cm

g

g

g

cmgcmg

g

g

cm

cm 3

3

3

3

3

3

Bulk density of soil () =mass of dry soil (g)

volume of soil (cm3)


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Soil Moisture Content: Methods of Measurement1. Difficulties encountered for accurate moisture measurement in the

field:

2. Soils are highly variable

3. Soil moisture is highly dynamic (spatial temporal variability)

4. Plant water uptake is highly variable depending upon the stage of growth

5. State of growth is again dependent upon nutrient application, water availability, pests etc.

6. Chemicals present in the soil can make measurements unreliable

7. Costs involved


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Methods for soil water content

Direct method (Gravimetric; Thermogravimetric)

Indirect methods

Electrical properties

Radiation technique

Acoustic method

Thermal properties

Chemical methods

Electrical Conductance

Dielectric constant

-Neutron scattering- ray attenuation

- Gypsum blocks- Nylon blocks- Change in conductance

TDR

Principles underlying different methods of assessment of soil water content


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Methods of soil water content determination

DIRECT

Gravimetric: evaporating water at 1050C.

Thermogravimetric: Soil sample is weighted and saturated with alcohol and burned several times until a constant dry weight is obtained

INDIRECT

Electrical Conductance


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DIRECT

Gravimetric: evaporating water at 1050C. Feel Method: Thermogravimetric: Soil sample is weighted and saturated with alcohol and burned several times until a constant dry weight is obtained

There are many classifications for soil types and major differences within each classification

Soil management can have a major impact upon these soil properties. Compaction is the major cause of error in bulk density.

Advantages: ensures accurate measurements, not dependent on salinity and soil type, easy to calculate

Disadvantage: destructive test, time consuming, inapplicable to automatic control, must know dry bulk density to transform data to volume moisture content, inaccurate because of soil variability

http://edis.ifas.ufl.edu/


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Frequency Domain Reflectometry: radio frequency (RF) capacitance techniques

Actually measures soil capacitance

A pair of electrodes is inserted into the soil

Soil acts as the dielectric completing a capacitance circuit, which is part of a feedback loop of a high frequency transistor oscillator

As high frequency radio waves (about 150 MHz) are pulsed through the capacitance circuitry, a natural resonant frequency is established which is dependent on the soil capacitance, which is related to the dielectric constant by the geometry of the electric field established around the electrodes

Two commercially available instruments using this technique: the Troxler Sentry 200-AP probe and the Aquaterr probe


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The soil bulk dielectric constant (K) is determined by measuring the time it takes for an electromagnetic pulse (wave) to propagate along a transmission line (L) that is surrounded by the soil

Since the propagation velocity (v) is a function of K, the latter is therefore proportional to the square of the transit time (t, in seconds) down and back along the L

Time Domain Reflectometry (TDR): , 28 s

K = (c/v)2 = ((c.t)/(2.L))2

where c is the velocity of electromagnetic waves in a vacuum (3•108 m/s or 186,282 mile/s) and L is the length embedded in the soil (in m or ft)


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TDR determinations involve measuring the propagation of electromagnetic (EM) waves or signals

Propagation constants for EM waves in soil, such as velocity and attenuation, depend on soil properties, especially and EC

Disadvantage: Costly, not really independent of salt content

The propagation of electrical signals in soil is influenced by q and EC The dielectric constant, measured by TDR, provides a good measurement of this soil water content


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http://edis.ifas.ufl.edu/EDISImagePage?imageID=1199206555&dlNumber=AE266&tag=FIGURE%204&credits=

Time Domain Transmission (TDT)

This method measures the one-way time for an electromagnetic pulse to propagate along a transmission line (L). Thus, it is similar to TDR, but requires an electrical connection at the beginning and ending of the length.

Notwithstanding, the circuit is simple compared with TDR instruments.

Disadvantages: Reduced precision, because the generated pulse is distorted during transmission; soil disturbance during installation; needs to be permanently installed in the field


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NUCLEAR TECHNIQUES: Neutron Scattering, , 1 to 2 min

With this method, fast neutrons emitted from a radioactive source are thermalized or slowed down by hydrogen atoms in the soil

Since most hydrogen atoms in the soil are components of water molecules, the proportion of thermalized neutrons is related to

Advantages: can measure a large soil volume, can scan at several depths to obtain a profile of moisture distribution, nondestructive, water can be measured in any phase Disadvantages: high cost of the instrument, salinity, must calibrate for different types of soils, excess tube, radiation hazard, insensitivity near the soil surface, insensitivity to small variations in moisture content at different points within a 30 to 40 cm radius, and variation in readings due to soil density variations (error rate of up to 15 percent)


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Gamma Attenuation: volumetric water content, < 1 min

This method assumes that the scattering and absorption of gamma rays are related to the density of matter in their path

The specific gravity of a soil remains relatively constant as the wet density changes with increases or decreases in moisture

Changes in wet density are measured by the gamma transmission technique and the moisture content is determined from this density change

Advantages: can determine mean water content with depth, can be automated for automatic measurements and recording, can measure temporal changes in soil water, nondestructive measurement

Disadvantages: restricted to soil thickness of 1 inch or less, but with high resolution, affected by soil bulk density changes, costly and difficult to use, large errors possible when used in highly stratified soils


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evaporation

Soil and Water


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WATER CONTENT

LIQUID• Gravimetric

(Mass/Mass)

• Volumetric

(Volume/Volume)

• Relative

VAPOR• Concentration

(Mass/ volume air)

• Pressure

(KPa)

• Relative Humidity

USES of WATER in PLANTS

• 1. Constituent

• 2. Solvent

• 3. Reactant-product

• 4. Turgidity

• 5. Temperature Control

CONSTITUENT

• Water constitutes more than 70% of fresh weight of most plants (Seeds are exception)

• Between 60-90% of the water is contained in the plant cell providing both biological and physical functions.

• The remaining 10-40% is contained as liquid in cell walls providing a continuum between the soil supply and the living cell.

SOLVENT

• Dissolves both organic and inorganic constituents essential for life

• Dissolves gases-CO2

REACTANT-PRODUCT

• PhotosynthesisCO2+ H2O = SUGAR

• RespirationCH2O + O2 = CO2 + H2O

ENERGY BALANCE

• Evaporation dissipates heat• Condensation-precipitation releases

heat• Amount of water passing through

plants as transpiration depends on environment and speciesWheat 1000 kg H2O per kg dry matter

Cotton 5000 kg H2O per kg lint

1 mole water ~ 18 cm1 mole water ~ 18 cm3 3 contains 6.02 x 10contains 6.02 x 102323 molecules molecules 1 cm1 cm33 ~ 3.3 x 10 ~ 3.3 x 102222 ( (33 thousand billion billion) 33 thousand billion billion) Consider a Consider a beachbeach 1.6 x 10 1.6 x 1066 m (1000 miles) long m (1000 miles) long 200 m (656 feet) wide 200 m (656 feet) wide 100 m (328 feet) deep 100 m (328 feet) deep Volume beach = Volume beach = 3.2 x 103.2 x 101010 m m33 sand sand Assume that each grain = a sphere 1 mm diameter Assume that each grain = a sphere 1 mm diameter With loose packing 10With loose packing 1099 (one billion) grains in 1 m (one billion) grains in 1 m3 3

Entire beachEntire beach 3.2 x 10 3.2 x 1019 19 sand grains, sand grains, 1000 < number 1000 < number of molecules in 1 cmof molecules in 1 cm33 water water

It would take 1000 beaches to contain as many sand It would take 1000 beaches to contain as many sand grains as molecules in 1 cmgrains as molecules in 1 cm33 of water of water

Each water molecule is ~ 3 A (3 x 10Each water molecule is ~ 3 A (3 x 10-10-10 m) m) It would take 33 x 10It would take 33 x 1066 layers to form a layer layers to form a layer of water 1 cm deep.of water 1 cm deep.

و خاك آب رابطهتكميلی گیاهعباسی حاج

UNIQUE PROPERTIES of WATER

OH H

105O

-

++

H+

H+

O--= + -H2O

Hydrogen bond

Gives structural strength

Bond depends on temperature:

Higher is the temperature weaker is bond

Positive end attraction with -ve end of other water molecules


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1050

Oxygen

Hydrogen Hydrogen Electro positive

Negative

Polarity

Symmetrical

H-O : 0.97 A

H-H : 1.54 A

angstroms


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O-

H+H+

Polymer type of grouping

Cations: Na+, K+, Ca2+ : become hydrated through their attraction to the Oxygen

Anions or negatively charged clay surfaces: attract water through hydrogen


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Some definitions

Adhesion - the attraction or clinging together of unlike substances

Cohesion - the attraction of a substance for itself; the mutual attraction among molecules or particles comprising a substance tat allows it to cling together as a continuous mass.

absorption - the process by which one substance is taken into and included within another substance, as the absorption of water by soil or nutrients by plants.

adsorption - the increased concentration of molecules or ions at a surface, including exchangeable cations and anions on soil particles.

flickering clusters refers to the quasicrystalline state of water molecules while in a liquid state. The molecules associate and dissociate repeatedly in transitory or flickering polymer groups. Water molecules are attracted to one another due to the hydrogen bonding that takes place between the negatively charged end of the oxygen atom in the molecule and the positively charged ends of hydrogen atoms in adjacent water molecules.


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heat of fusion (ice) The amount of energy required to turn a liquid into a solid.

heat of vaporization The amount of energy required to turn a liquid to a vapour (to overcome the attractive forces between adjacent molecules in a liquid).

dipole moment a measure of the tendency of a polar molecule to be affected by an electrical or magnetic field (i.e., NMR - Nuclear magnetic resonance)

Volumetric heat capacity is the change in the heat content of a unit volume per unit change in temperature.

Specific heat is the change in the heat content of a unit mass per unit change in temperature.

surface tension A molecule at the surface of a liquid is not completely surrounded by other molecules of the liquid. The forces acting upon it are unbalanced, with the result that it experiences a stronger attraction into the body of the liquid (cohesion) rather than into the less dense gaseous phase. This unbalanced force draws the surface molecules inward with and results in the tendency for the surface to contract and the molecules to be slightly denser at the surface.


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sublimation the direct transition from the solid state to the vapor state

hydrophobic water repellent

capillary attraction - a liquid's movement over or retention by a solid surface due to the interaction of adhesive and cohesive forces.

capillary fringe a zone just above the water table that is maintained in an essentially saturated state by capillary forces of lift.

viscosity (centipose, cP, N s m-2 x 10-3, kg m-1 s-1). When a fluid is moved in shear (that is to say, when adjacent layers are fluid are made to slide over each other), the force required is proportional to the velocity of shear. Viscosity is the proportionality factor. It is the property of the fluid to resist the rate of shearing and can be visualized as an internal friction. Fluids of lower viscosity flow more readily. Thus oil has a higher viscosity than water.

Fluidity is the reciprocal of viscosity. Viscosity is the preferred term.


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Cohesive – Cohesive – forces of attraction between like forces of attraction between like molecules. molecules. At an air-water interfaceAt an air-water interface –– surface tension. surface tension. Adhesion – Adhesion – attraction of one substance for a attraction of one substance for a substance of another kind. substance of another kind. Tensile strength –Tensile strength – work that must be done to work that must be done to create or extend a new or create or extend a new or larger surface. larger surface. Viscosity –Viscosity – resistance to flow. resistance to flow.


If water were an ordinary compound whose molecules are subject to weak forces, its boiling and freezing point would fall below hydrogen sulfide

Strong hydrogen bonding between water molecules prevents this

Water occurs in all three states (solid, liquid, and gaseous) at prevailing temperatures on the earth’s surface

Example: Ice cubes in a glass at room temperature


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Why water wets clean glass? Surface of glass has O and unpaired electrons

Water molecules form hydrogen bond

Force stronger than gravity

Surface of grease has no O and free electrons

Water molecules cannot form hydrogen bond

Therefore, water do not stick

Why water does not stick to glass surface coated with grease?


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Forces acting on a water molecules

A

B

Consequently, there is a net downward force on the surface molecules, and result is something like a compressed film at the surface. This phenomenon is called surface tension

Air

Water

Air-water Interface

At point B:

Forces acting on water molecule are equal in all direction

At point A:

Attraction of air for water molecules is much less than that of water molecules for each other.


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The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension

The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

Surface Tension

Surface tension is typically measured in dynes/cm. The force in dynes required to break a film of length 1 cm

Equivalently, it can be stated as surface energy in ergs/cm2

Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury


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Solid Liquid Gas

Contact Angle

Liquid and gas (air) in contact with solidInterface between air and water forms a definite angle “contact angle”

Solid

AirL

sa > sw; cos = + or < 900

Angle of contact is acute in a liquid that wets the solid

Solid

Air

L

Angle of contact is obtuse (between 90 and 180) in a liquid that does not wet the solid

wa

swsa

cosYoung’s equation


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Hydrophilic Versus Hydrophobic Soils

When the adhesive forces between water molecules and an object are weaker than the cohesive forces between water molecules, the surface repels water and is said to be hydrophobic. Hydrophobic soils restrict the entry of water, which 'balls up' or sits on the soil in beads rather than infiltrating the soil.

Hydrophobic soils exhibit an obtuse (greater than or equal to 90o) wetting angle that causes capillary repulsion, so preventing water from entering soil pores

Hydrophilic or normally wettable soils display an acute (less than 90o) angle of contact with water, allowing infiltration. adhesive forces between water molecules and an object are stronger than the cohesive forces between water molecules


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By adhesion, solids hold water molecules rigidly at their soil-water surface

Gravity

Capillary

Capillary Fundamentals and Soil Water

Cohesion: Attraction of molecules for each other

Adhesion: Attraction of water molecules for solid surfaces

Together it is possible for soil solids to retain water and control it’s movement

By cohesion water molecules hold each other away from solid surfaces


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Dipolar Bonding in WaterThe dipolar interaction between water molecules represents a large amount of internal energy (the energy associated with the random, disordered motion of molecules) and is a factor in water's large specific heat (the amount of heat per unit mass required to raise the temperature by one degree Celsius).

The dipole moment of water provides a "handle" for interaction with microwave electric fields in a microwave oven.

Microwaves can add energy to the water molecules, whereas molecules with no dipole moment would be unaffected.


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http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/inteng.html#c2

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html#c1

Water rises in the capillary against the force of gravity

!!!! What happens if there is no force of gravity !!!!!

Water Water


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Capillary Mechanism

Water

Rise continues till:Weight of water in the tube (force of gravity) = Total cohesive and adhesive forces

2 r1

h1 h2

2 r2


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Force of gravity = Mass of water column * Acceleration

= (volume of water * density) * g

= ( * r2* h) *dw * g …………(A)Total cohesive and adhesive forces

= (perimeter) * surface tension

= 2 * * r * …………(B)Water

2 r

h

At equilibrium: A = B

( * r2* h) *dw * g = 2 * * r *

gdrh

w **

*2

rh

15.0

use

= 72.75 dynes/cm

dw= 0.9982 g/cm3

g = 980 cm/s2

Show


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rh

15.0

This relationship tells us that:

Capillary rise is higher in small pores

r = 0.1 cm; h = 1.5 cm

r = 1.0 cm; h = 0.15 cm

r = 10 cm; h = 0.015 cm

RadiusCa

pill

ary

Ris

e

If two principle radii r1 and r2

21

1115.0

rrh


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The inverse relationship between height of rise of water and radius of soil pores may not be always valid:

Soil pores are not straight uniform openings as a tube

Some soil pores may entrap air and slow down the capillary rise

Tortuous flow paths of water

Soil solids

Entrapped air

water


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Adsorbed water

Capillary water

Enlarged soil particles or aggregates

Two forms of water in soil

Soil solids tightly absorb water

Capillary forces hold water in capillary pores


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rh

15.0

He

igh

t (cm

)

Time (days)

Clay compacted

Loam

Sand

Brady,1984


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TEMPERATURE RELATIONS– Heat of Vaporization = 540 cal/g@100C

2.45 MJ/kg

– Heat of Fusion = 80 cal/g@0C– Heat Capacity = 1.0 cal/g– Thermal Conductivity = High


DENSITY

– 0.9998 g/cm3 at 0 oC– 1.0 g/cm3 at 4 oC– 0.9956 g/cm3 at 30 oC

Temperature range in liquid phase for H+ compounds

100

50

0

-50

-100

0 50

Molecular Weight

Te

mp

era

ture

(0C

)

100

H2O

H2S

H2Se

H2Te

Boiling point

Freezing point

Hydrogen sulfide

Hydrogen selenide

Hydrogen telluride

(2+16=18)

(2+32=34)

(80)

(130)


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Does water swell and shrink with Temperature?

1

0.998

0.996

0.994

0.992

0.990

-10 0 10 20 30 40 50

De

nsi

ty (

g c

m-3)

Temperature (0C)

40C


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SOLVENT– Small Size– Polar characteristics– High Dielectric Constant


Transparent to Visible Radiation

(400-700nm) Highly Absorbent in Infra-Red

(>1000 nm)


HIGH SURFACE TENSION– Adhesion– Cohesion– Tensile Strength

( >30MPa ~4000#/in2)


IONIZATION– Only 1 molecule out of 55 x 107 is

ionized

pH ~ - Log (H+)

CH4 NH3 H20 C2H6 C5H12 C6H16 C4H30

MP5.5 91- 130- 172- 0 78- 184- BP 251 98 36 36- 100 33- 161- MW 198 100 72 30 18 17 16

ذوب ( نقطه جوش) (MPمقايسه ملكولي ) (BPنقطه ) MWووزنماده آب با چندين


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100

75

50

25

0

-25

-50

-75

-100

H2Te (129 g/mole)

H2Se (80 g/mole)

H2S (34 g/mole)

H2O theory (18 g/mole)

H2O actual

Temperature range of waterand similar structuredmolecules (from Leopold, 1966)

Cent

igra

de (

C)

Fig. 1. Temperature range of various hydride groups.

Properties of Water


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نسبتا باوجوداينكه آب ملكولي است وزن كمپيوند مقاومت دروني ويدرهولي وساختمان ژني

باعث بصورت تا شده آن طبيعي حرارت دردرجهباشد كهعناصري مايع معني بخاربدين بصورت نه

تقريبا با ملكولي درجه شبيه وزن در آب بهشوند مي ذوب بسيارپايين ويابجوش حرارتهاي

آيند عناصرديگربايد از مي ديگر وزن طرفداشته ملكولي بتوانند تا باشند بسياربااليييا دردرجه ذوب آب ياجوش ذوب بحالت حرارتهايدرآيند درحالت جوش آب مانند يعني مايع

. كند مي عمل بسيارسنگين ملكولهاي


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COLLIGATIVE PROPERTIESof

AQUEOUS SOLUTIONS

PROPERTYPURE WATER 1.O M SOLUTION

1. Vapor Pressure 0.61 KPa@Oo Decreased according

101.3 KPa@100o to Raoult’s Law

2. Boiling Point 100oC 100.52oC

3. Freezing Point 0oC -1.86oC

4. Osmotic Potential 0 -2.27 MPa

5. Chemical Potential 0 Decreased

WATER VAPOR TEMP

oC

CONCENTRATION

G M-3

MOLE FRACTION PRESSURE

kPa

-10 2.358 0.00256 0.260

0 4.847 0.00603 0.611

10 9.399 0.01211 1.227

20 17.30 0.02307 2.337

30 30.38 0.04187 4.234

40 51.19 0.07281 7.378

50 83.06 0.1218 12.34

Vapor pressure=4.62 x10-4 vdT

DRY AIR = 1. 205 Kg m-3

SATURATED AIR = 1.194 Kg m-3

Wet bulb

Dew Point

RAUOLT’S LAW

e = eo Nw

Nw+Ns

e= Vapor Pressure of solution

eo= Vapor Pressure of pure water

Nw= moles of solvent

Ns= moles of solute

OSMOTIC PRESSUREVan’t Hoff equation

= NsRT V = osmotic pressure in Mega PascalsNs= moles of soluteR= gas constant (0.0083 L MPa /mol @273o)T= absolute temperatureV= volume of solvent in Liters(RT= 2.272 @) 0oC &2.437 @ 20oC literMPamol-1)

CHEMICAL POTENTIAL of WATER in a SOLUTION

A measure of the ability of WATER to do work

Go = partial molal Gibb’s free energyGo = -RTlnK (K = equilibrium constant)

A B G Negative = Spontaneous G Positive = Additional Energy

Required

CHEMICAL POTENTIAL of WATER

µw-µwo = RT lnNw

µw= chemical potential of water in solution (Jmol-1)

µwo= chemical potential of pure water

R= Gas constant = 8.314 J mol-1 K-1

T= Absolute temperature

Nw= mole fraction of water in solution-can

be replaced by ln e/eo

WATER POTENTIAL(PRESSURE UNITS)

w = RT ln e/eo

Vw

The water potential of a solution is decreased by those factors which reduce the vapor pressure

1. Addition of solutes (Osmotic)

2. Matric forces ( Interfacial Adhesive Forces)

3. Reduction in Temperature

4. Tension

WATER MOVEMENTFlux Jw (moles per meter2 per second)

= Concentration Gradient ( - molsDistance * Diffusion Coefficient (DW)

LIQUID STATE• Potential Gradient• Hydraulic

Conductivity

(Cells 7 x 10-13ms-1)

(Soil 10-2 – 10-10 m2 sec-1)

• VAPOR STATE• Vapor Pressure

Gradient• Diffusion Coefficient

(2.4 x 10-5 m-2s-1)

• Resistance

Forces that affect movement of water into the soil Gravity: a constant force that pulls the water downward Cohesion: attraction of water molecules for each other. It is the force that holds a droplet of water together Adhesion: attraction of water molecules to other substances. This force causes water molecules to adhere to other objects, such as soil particles

Placing a drop of water on a piece of newsprint paperForce of adhesion between the water molecules and the paper molecules is greater than the force of cohesion that holds the water molecules together The water droplet spreads out and soaks into the paper

Placing a drop of water on a piece of waxed paper Force of adhesion between the water molecules and the paper molecules is lower than the force of cohesion that holds the water molecules together The water droplet remains intact


عباسی حاج


TOTAL WATER POTENTIAL

W = p – ( + m + g)

W = Total Water Potential

p = Pressure Potential

= Osmotic Potential

m = Matric Potential

g = Gravitational Potential

WATER POTENTIAL

SOIL SYSTEM

• Matric ForcesSurface Tension

Electrostatic Association

• Osmotic

PLANT SYSTEM

• Surface Tension

• Cohesion

• Osmotic

• Pressure

DIFFUSION COEFFICIENTS(10-5 M 2 S-1)

TEMPERATURE oC

H2O VAPOR CO2

0 2.13 1.33

10 2.27 1.42

20 2.42 1.51

30 2.57 1.60

40 2.72 1.70

Physical Properties of WaterPhysical Properties of Water

Liquid phases in soil and plant are similar Liquid phases in soil and plant are similar

In both systems the liquid is a solution of In both systems the liquid is a solution of water and dissolved substances water and dissolved substances

Physical properties of waterPhysical properties of water


WATER-The Substance of Life Limits Kinds and Amounts of Vegetation on Earth Limits Growth of Cities...

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