soil water - characteristics and behaviour

8/3/2019 Soil Water - Characteristics and Behaviour

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Soil Water: Characteristics and Behaviour

Water is Cool

Its the molecular structure of water that allows it to influence somany soil processes.

o It consists of one oxygen atom and two hydrogen atoms. Butit is how they bond that is so interesting these elements are

bonded covalently, with each hydrogen sharing its single

electron with the oxygen.

o But these are not arranged linearly (H-O-H). Instead, theyhydrogen atoms are aligned in a V-shape with the oxygen

atom, at a 105 angle.

o That is, they are asymmetrical.

Waters Polarity

It means that the water molecule exhibits polarity.o The charges are not evenly distributed around the molecule.o The side on which the hydrogen atoms are located tends to

be electropositive.

o The side without the hydrogen atoms tends to beelectronegative.

This explains how water molecules interact with each other.o Each water molecule does not act independently but rather is

coupled with other neighbouring molecules.


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o The hydroge3n (positive side) of the molecule attracts theoxygen (negative side) of another molecule resulting in a

chain-like (polymer) grouping.

Polarity also explains why water molecules are attracted toelectrostatically charged ions and to colloidal surfaces.

o Cations such as H+, Na+, Ca2+ all become hydrated throughtheir attraction to the oxygen (negative) side of water

molecules.

o Negatively charged clay surfaces attract water, attracting thehydrogen (positive) side to them.

o Polarity of water also encourages the dissolution of salts asthe ionic components of salts have a greater attraction for

water molecules than for each other.

Waters Hydrogen Bonding

A hydrogen atom may be shared between two electronegativeatoms such as O and N, forming a relatively low-energy link.

o This is called hydrogen bonding.o This is the process through which neighbouring water

molecules are attracted.

o It also accounts for the high boiling point, specific heat andviscosity of water.

o It is responsible for the structural rigidity of some claycrystals and for the structure of some organic compounds,

such as proteins.

Hydrogen bonding accounts for two basic forces responsible forwater retention and movement in the soil: cohesion and

adhesion.

o Cohesion describes the attraction of water molecules for eachother.

o Adhesion describes the attraction of water molecules to solidsurfaces.


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o Together the forces of cohesion and adhesion make it possiblefor soil solids to retain water and control its movement in the

soil and use.

o It also explains the plasticity of clays.

Cohesion

Adhesion


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Waters Surface Tension

Water has a higher surface tension compared to most other liquids.o Consider watching a water strider on the surface of water.o This is important when examining how waster moves and is

retained in soil through the process of capillarity.

Two forces cause capillarity:o The attraction of water to a solid (adhesion or absorption)o The surface tension of water, which is largely due to the

attraction of water molecules to each other (cohesion)

Capillarity in Soils

Capillarity can be illustrated by putting one end of a fine, cleanglass tube in water.

o The water will rise in the tube.o The smaller the diameter of the tube, the higher it will rise

above the water level.

o This is due to the water molecules being attracted (throughadhesion) to the sides of the tube.

o The calculation is:h = 0.15/r

where r = radius of the tube

h = height of capillary rise


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The same is true with water in soils.o The water will rise in the soil and is based on the pore size.o However, the rise in height is less than that expected based

on the soil pore size alone.

o This is because soil pores are not aligned in a straight line orare of uniform diameter and this slows down the movement

of water through the pore spaces.

o Also, some pore spaces become filled with trapped air,blocking the movement of the water.

Capillarity differs from one soil type to another.o In general, the eventual height of water rise in a soil through

capillarity above the water level is higher in finely-textured

soils than in sandier soils.

o The rate of capillarity action is generally slower in finelytextured soils than in coarser soils because of the friction in

the tinier pores.

o For example, if water rises by capillarity to a height of 37 cmabove a free-water surface in a soil, then it can be calculated

by rearranging the capillarity equation (to r = 0.15/h) that

the smallest pores must have a radius of about 0.004 cm.

This provides you with an approximation of the minimum

effective pore radius in a soil.o Keep in mind that capillary action is not always up/down or

vertical, but also explains water movement horizontally too.

Soil Water Energy

The movement and retention of water in soils, and its uptake byplants, is all about energy.

o Specifically, it is about potential energy of water.o The stored energy of position in an object is referred to as

potential energy. For example, a drawn bow is able to store a

lot of energy as the result of its position. When assuming its

usual position (i.e. when not drawn), there is no energy

stored in the bow. Yet when its position is altered from its


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usual equilibrium position, the bow is able to store energy by

virtue of its position. This stored energy of position is referred

to as potential energy. Potential energy is the stored energy

of position possessed by an object.

o All substances tend to move or change from a higher to lowerstate of energy.

o Water potential integrates a variety of different potentialdrivers of water movement, which may operate in the same

or different directions.

Water potential quantifies the tendency of water to move from onearea to another due to osmosis, gravity, mechanical pressure, and

matrix effects such as surface tension.

o Water potential has proved especially useful in understandingwater movement within plants, animals, and soil.

o Water potential is typically expressed in potential energy perunit volume and very often is represented by the Greek letter

.

o The three most important forces that impact on potentialpressure are:

Matric force, responsible for absorption/adhesion andcapillarity, that tend to reduce energy state of water

near particle surfaces Osmosis, that tends to reduce the energy state of

water in the soil solution

Gravity, which always pulls the water downward. Many different factors may affect the total water potential, and the

sum of these potentials determines the overall water potential and

the direction of water flow:

= 0 + n + p + s + v + m

Where:

0 is the reference correction

n is the solute potential

p is the pressure component

s is the gravimetric component

v is the potential due to humidity


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m is the potential due to matrix effects (e.g. fluid cohesion

and surface tension)

When water is in contact with solid particles in soil, adhesiveintermolecular forces binding the water and the solid can be large

and important. This attraction of water to solid surfaces gives riseto the matric potential.

o The forces between the water molecules and the solidparticles in combination with attraction among water

molecules promote surface tension and the formation of

menisci (a surface tension) within the solid matrix.

o Force is then required to break these menisci. The magnitudeof matric potential depends on the distances between solid

particles the width of the menisci and the chemical

composition of the solid matrix. In many cases, matrix

potential can be quite large and comparable to the other

components of water potential discussed above.

o It is always a negative value since it reduces the energypotential of water.

The osmotic potential is the product of solutes in the soil solution.o These can be organic compounds or inorganic salts.o The greater the concentration of solutes, the lower the

osmotic potential of water. Gravity acts on water the same way it acts on any body pulling it

downwards.

o The gravitation potential of soil water is:s = gh

Where:

g = acceleration due to gravity

h = the height of the soil water above a reference

elevation

Water Flows Through Soil

Water moves through soil in various ways.


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o Three types of generally recognized: saturated flow;unsaturated flow; and vapor movement.

o Saturated flow takes place when the soil pores are completelyfilled (or saturated) with water.

o Unsaturated flow occurs when the larger pores in the soil arefilled with air, leaving only the smaller pores to hold and

transmit water.

o Vapor flow occurs as vapor pressure differences develop inrelatively dry soils. Vapor migrates from an area of high vapor

pressure to an area of low vapor pressure.

Hydraulic conductivity is a soil property that describes the easewith which the soil pores permit water (not vapor) movement.

o It depends on the type of soil, porosity, and the configurationof the soil pores. In saturated soils, the hydraulic conductivity

is represented as Ksat and in unsaturated soils, the hydraulic

conductivity is represented as K.

o The hydraulic conductivity depends on the soil grain size, thestructure of the soil matrix, the type of soil fluid, and the

relative amount of soil fluid (saturation) present in the soil

matrix.

o The important properties relevant to the solid matrix of thesoil include pore size distribution, pore shape, specificsurface, and porosity. In relation to the soil fluid, the

important properties include fluid density, and fluid viscosity.

o Generally sandy soils have higher saturated conductivitiesthan fine-textured soils because they have more macropore

spaces.

Flow through an unsaturated soil is more complicated than flowthrough continuously saturated pore spaces.

o Macropores are filled with air, leaving only finer pores toaccommodate water movement.

o The movement of water in unsaturated soils is dictated bydifferences in matric potential, not gravity.

o The matric potential gradient is the difference in the matricpotential of the moist soil areas (high matric potential) and


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nearby drier areas (low matric potential) and nearby drier

areas (low matric potential) into which the water is moving.

Infiltration & Percolation

Onto every field or pasture a little rain falls and gets absorbed intothe soil.

o The absorption of water from the atmosphere into the soilconsists of two processes: infiltration and percolation.

o Infiltration refers to the process by which water enters thesoil pore spaces and becomes soil water.

o Percolation describes the processes through which infiltratedwater moves downwards into the profile of a soil.

The rate at which water can enter the soil is called the infiltrationcapacity (I):

I = Q / (A * t)

Where:

Q = the volume quantity of water (m3) infiltrating

A = the area of soil surface exposed to infiltration

t = time

o Generally expressed in cm/h The infiltration rate is not constant over time.

o It generally decreases during an irrigation or rainfall episode.o This is because, when a soil is dry, the macropores open to

the surface rapidly accept the water and conduct the water

into the soil. So the infiltration rate at the start is very high.

o But then these macropores fill up, reducing the infiltrationrate, quickly at first, but then it levels.

Infiltration rates are also influenced by soil types, specifically itstexture and structure.

o It is also influenced by the presence of any horizons that mayblock the downward movement of water.

Both saturated and unsaturated flow are involved in percolation ofwater into a soils profile.

o Rate of percolation is related to a soils hydraulic conductivity.


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o Percolation can be observed as wet soil is dark in colour. Andthere is a distinctive front to this area called the wetting front

between the dry soil and wet soil.

o During an intense rain or heavy irrigation, water movementnear the soil surface is affected primarily by gravity.

o At the wetting front, however, water is moving in response tomatric potential gradients as well as gravity. So it is not just

imply moving downwards, but outwards as well.

o In a light rain, both infiltration and percolation may occurmainly by unsaturated flow as water is drawn by matric forces

into the fine entrapped pores without accumulating at the soil

surface or in macropores.

The movement of water by percolation can also be affected by thestructure of differing horizons.

o In fields, many soil profiles contain subsurface horizons withpore sizes that contrast markedly with adjacent layers in the

profile.

o You can also come across relatively impervious horizons fragipans, claypans, coarse layers that impact on the

movement of water in a soil.

o The presence of such conditions results in the downwardmovement of the water being impeded.

This is true even when the subsurface layer consists of coarse orsandier texture and structure which seems to go against what one

might think.

o One might expect the sand layer to speed up, rather thanimpede, percolation due to the larger, more numerous

macropores.

o The sands macropores offer less attraction (adhesion) towater so the water doesnt get pulled into the sand layer

easily.

The impact of a layer of coarse materials is important in theconstruction industry.

o It can also inhibit the rise of water from moist subsoil layersto the surface soil.

o Have you ever poured a concrete slab? What do you do first?


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o The larger pores in the coarse layer are not able to supportcapillary movement up from the smaller pores in the finer

layer. Consequently, water rise by capillary action up to the

coarse-textured layer, but it can go no further.

o This principle allows a layer of gravel to act as a barrier tocapillary action under a concrete slab foundation to prevent

water from soaking up from the soil and through the

concrete.

soil water - characteristics and behaviour

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