Soil Water

Chapter 5

Water PotentialComponentsMatric potential (tension, suction, -pressure)Capillarity

Soil Moisture Characteristic Curve

Water FlowSaturatedUnsaturatedVapor

Plant-Available WaterTermsFactors affecting

Energy of water affected by

GravityPressureAttraction by soil solidsSolutes

Reference

Pure water at atmospheric pressure atspecified elevation

Energy per unit quantity of water

Water potential

The 2 kinds of quantities commonly used as a basis for water potential arevolume and weight (not mass). Energy per unit volume (E / V = [F x L]/ [A x L] =F / A = P) has the dimensions of pressure, whereas energy per unit weight, a force(E / W = F x L / F = L) has the dimension, length. Water potential in pressure is commonly used with biological systems, and water potential as length is used in engineering.

Gravitational Potential

Higher potential, higher in the gravitationalfield, no, so water moves downhill.

Pressure Potential

Higher

HereorHere

Obviously, down there.

P = 2 / R

The pressure differential, air > water,is given by the above expression, whereγ is surface tension and R is radius of thesphere. Notice the bubble is concave intothe water.

Matric Potential

Attraction bysoil solidsreduceswater potential

- relative toatmosphericpressure

Unsaturated soil

So, concave into water film means thepressure in the water is < pressure in air.

Osmotic Potential

Solutes reducewater potentialrelative topure waterWater enters the osmometer through asemipermeable membrane and rises inthe tube. The top of the water in the beakeris at atmospheric pressure (call it zero netpressure) but at the same level inside the osmometer, there is a standing column ofwater. The negative osmotic potential just balances the positive pressure potential.

Total Potential

TP = GP + (PP or MP) + OP

There can be only positive pressure potential (below a free water surface, or watertable, in saturated soil) or negative matric potential (unsaturated soil), not both.Matric potential is negative pressure, or tension.

In the absence of a semipermeable membrane, only gravitational potential and either pressure or matric potential affect water flow.

Attraction to surfacesdue to adhesion andcohesion

H-bonding

Dipole interaction

..

..

..

..

These properties of waterare responsible for itscohesion unto itself and itsadhesion to hydrophilic surfaces. So, when somewater molecules stick to asurface, they bring their buddies along.

Responsible for capillarity

Ptop = -2 / R

Pbot = gh

Ptop + Pbot = 0

h = 2 / ( gR)

Yes, like with capillarity. Thisis a derivation of the capillaryrise equation. Note height of

rise, h, is inversely related toradius of tube,R. ρ is densityof water andg is accelerationdue to gravity.

Pressure outside the capillary tube at thewater surface is zero and also inside the tubebecause the negative pressure in the waterat the top of the tube is just balanced by thepositive pressure of the water column.

Capillary bundlemodel forsoil water

Soil physicists have used capillary tubes of differentradii as models for soil poresto explain water movement insoil. Note that soil pores of aparticular radius are filled withwater only to the height that corresponding capillary tubesare filled.

Of course soil pores are very short, not long capillaries.

Here are twocapillary pores,one with a smallradius and onewith a larger radius.When tension is applied to thebottom of each, thelarger one emptiesfirst because it can’twithstand as greatof tension as thesmaller capillary.The same thingholds for water insoil pores. Considerthe next slide.

The decrease inwater content of a soil as tension on itincreases is due topores draining, firstthe largest and lastlythe smallest. Thereis a continuum of poresizes so this decreaseis smooth.

These dots aresupposed to bepore size distribu-tions. So, whichwould be the sandand which wouldbe the loam andclay? And what would be the effecton the soil moisturecharacteristic curve?

Shape depends on texture and structure

Clay orSand

Besides the rapid decrease in water content with increasing tension (moreand more negative matric potential), you might suspect the pink soil to be the sand also based on the lower water content at saturation (recall, generallyhigher bulk density, thus, lower total porosity, in sand than clay).

Good structure or compacted

Same idea holds here.It’s a matter of poresize distribution –compact a soil and youdecrease total porosity and reduce the

number of large pores.

How to Measure Soil Water

Content

Tension

Content

Gravimetric

Neutron Probe

TDR

You’ve done gravimetric. It’s veryaccurate but destructive. Attenuation offast neutrons by interaction with H nuclei(calibrate the instrument at different know soil water contents) can be relatedto water content. Time domain reflectometry is newer technology that relateschanges in dielectric constant to water content. Non-destructive methods.

Tension (Matric Potential)

Tensiometer

Simple device consisting of rigid tubewith a porous ceramic cup on the end.Fill with water, cap and stick in the soil.The greater the tension in the soil water,the greater the tension in the tube.The latter is read by vacuum gage or pressure transducer. Works fine atlower tensions, i.e., not dry soil.

Water Movement

Saturated

Unsaturated

Vapor Phase

Always moves from higher to lower potential

Darcy’s LawThese considerations allow you to sort of derivethis. Obviously, volumetric flow (e.g., cm3 h-1) is

directly related to crosssectional area.

Everything else thesame (soil, length ofit and area), there isgreater flow whenthere is more standingwater but there will always be some minimum flow provided you add water.Thus, flow is proportional to water depth. However, flow is inversely related to

length of flow through soil –resistance.

At this point, we’ve Q is proportional to A x (D + C) / L. At zero depth of wateron the surface of saturated soil of different lengths of flow, there is the same flow,and the only way this can be the case is if the unknown constant, C, is actually L.Thus, Q is proportional to A x (D + L) / L but it is going to vary with the soil, i.e.,with the pore size distribution, bigger pores, faster flow and conversely.

Clay

Sand

Are the conductivities, Ksat,the same?

Q = K A ([D + L] / L) Darcy’s Law

If K = 1 cm h-1,what’s the flow?

Unsaturated Flow

Wet Dry

This is the typical case, unsaturated.Here, despite no difference in gravitationalPotential, water moves from left to right.

This, of course, is due to the difference in matric potential.

Application of Darcy’sLaw is not straightforwardfor unsaturated soil. Abig issue is that conduc-tivity decreases aswater content decreases.This is because the areafor flow decreases as thesoil dries and the paththat water moves becomes longer. More importantly,flow is restricted to smallerand smaller pores, throughwhich water moves slower.

Why watergoes to roots

Like is always the caseeverywhere and every time,it goes down a potential gradient, from higher to lower.

Vapor Phase Movement

From higher to lower partial pressure

So, does water vapor spontaneously move

Hot Cold

Non-saline Salty

Wet Dry

Well, isn’t the vapor pressure of waterhigher above relatively hot water than cold water? Don’t solutes reduce watervapor pressure? In fact, the soil may become so dry that adhesion of film waterto soil solids actually reduces watervapor pressure.

If you let a saturated soil drain, it drains fast at first but slows. This isthe behavior of a clay and a sand. The early thinkers on the matter concluded that water was draining under the influence of gravity and sincedrainage was so fast, that portion of the maximum water content was notreally available to plants. So, gravitational water was plant-unavailable.

Regardless of soil type, the tension of soil water when this gravitationalwater has drained is about - 0.2 or - 0.3 or - 0.33 bar (depending on authority).

Plant-Available Water and Related

Matric Potential

-0.2 bar, Field Capacity

GravitationalWater

This is a soilmoisture characteristiccurve, no?

Plants are gonerswhen theycan’t uptakewater againstthe tension atwhich it is heldby soil solids.They permanentlywilt and the associated tension is about -15 bar.

So, plant-available wateris in between, no?

Hygroscopic water

Texture Affects Plant-Available Water

Plant availablewater dependson texture.

Field capacityand wiltingpoints aredetermined frommoisturecharacteristiccurves.

Max at aboutsilt loam.

Organic Matter Affects Plant-Available Water

Organicmatter is goodin this way, too,no?