NRES 201 Lectures 16-17 (Fall 2014): Soil Water11NRES 201Soil Water2Today’s lecture topics- Properties of water- Capillarity and soil water- Energetics of soil water- Classification of soil waterSoil WaterNRES 201 Lectures 16-17 (Fall 2014): Soil Water23Properties of waterNatural occurrence- The only chemical compound that occurs naturally on Earth as a solid, liquid, and gas- The liquid phase is by far the most abundant- The only inorganic compound that occurs naturally on Earth as a liquidSource: http://earthobservatory.nasa.govYellowstone National Park4Polarity- H2O forms by covalent bonding- V-shaped configuration due to greater electron affinity by O than H- The result is a polar molecule consisting of:- An electronegative O atom- Two electropositive H atomsSource: Brady and Weil (2008)++ NRES 201 Lectures 16-17 (Fall 2014): Soil Water35Hydrogen bonding- Arises when a H atom of one water molecule isattracted to the O end of another- The reason why water polymerizes and is unusually high in:- Boiling point- Specific heat- ViscositySource: http://apbrwww5.apsu.edu6Hydration- Polar water molecules areattracted to ions or clay particles- Charge-structured orientation- H end toward negative ions orclay particles- O end toward positive ions- Due to polarity, water is the“universal solvent”- Better than any otherliquidSource: https://www.ionways.comSource: http://www.terragis.bees.unsw.edu.auNRES 201 Lectures 16-17 (Fall 2014): Soil Water47Cohesion versus adhesion- Two forces are responsiblefor water retention andmovement in soils:- Cohesion, the attractionof water molecules foreach other- Adhesion, the attractionof water molecules forsolid surfaces- Both are due to H-bonding- Also causes “flickering” H2O clusters to form and dissociate in liquid waterSource: Brady and Weil (2008)8Surface tension- A property of liquids in contact with air- The liquid molecules are attracted more tothemselves (cohesion) than to air (adhesion)- Unusually high for water due to H-bonding- Important for capillarity in soils- Common examples:Source: http://anniesbiozone.blogspot.comSource: http://www.krassycandoit.comWater striderNRES 201 Lectures 16-17 (Fall 2014): Soil Water59Capillary action- Water rises in a tube when:- Some of the molecules climb upthe tube wall by adhesion- And they pull others up bycohesion- A curved surface (meniscus) formswhere water meets air- Simple example:- Dye movement up a celery stalkCapillarity and soil waterSource: http://discovermagazine.comSource: http://springtolife.weebly.com10Height of capillary rise- Determined by the upward-acting force of attraction tothe tube wall- Which effectively reducesthe downward-acting forceof gravity for water in thetube, relative to free wateroutside the tube- Increases with decrease intube radius- Because the wall is in contact with a greater proportion of water in the tubeSource: http://www.daviddarlling.infoFree waterNRES 201 Lectures 16-17 (Fall 2014): Soil Water611- Also increases- With lower liquid density- And with• Greater surface tension• And greater adhesiveattraction to the tube surface- Calculated as:h (cm) = 0.15 (cm2)r (cm)Source: Brady and Weil (2008)(r)12Capillary rise in soils- Occurs in all moist soils- But limited by:- Tortuous nature ofsoil pores- Entrapment of air- Height of rise: clays > loams > sands- Rate of rise: sands > loams > claysSource: Brady and Weil (2008)Source: http://www.groasis.comNRES 201 Lectures 16-17 (Fall 2014): Soil Water713Some fundamentals- Water runs downhill- From a place of higherpotential energy- To a place of lowerpotential energy- The difference in energylevels determines thedirection and rate ofwater flow- Soil water varies in energy- Low energy at high water content- High energy at low water contentEnergetics of soil waterSource: http://trekearth.comStarved Rock State ParkHigh potential energyLow potential energy14Soil water potentials- Matric potential (m)- Water is attracted to soil solids (soil matrix)- This attraction (adhesion) lowers the “free” energy of the water- mrepresents the amount of work that must be expended to extract water that has bonded to soil solidsSource: http://www.tankonyvtar.huNRES 201 Lectures 16-17 (Fall 2014): Soil Water815- mranges:• From 0 kPa for water in thick films• To –106kPa for thin filmsWhat is kPa?- A unit of pressure (force per unit area)- kPa stands for kilopascal- Analogous to pounds per square inch (psi)- Standard atmospheric pressure is 101.325 kPa- So 1 kPa is approximately 1% of atmosphericpressure16- Osmotic potential (o)- Due to its polarity, wateris attracted to ions andother solutes- These attractions lowerthe “free” energy of water- orepresents the amount of work needed to remove water from a solution with a given concentration of solutes- oranges:• From 0 kPa for pure water• To become increasingly negative for highersolute concentrations- Can be ignored if water and solutes movetogetherSource: http://www.advancedaquarist.comNRES 201 Lectures 16-17 (Fall 2014): Soil Water917- Gravitational potential (g)- The amount of work requiredto move a given mass of wateragainst gravity- Positive value when above thereference elevation• Usually chosen within or atthe lower boundary of thesoil profile- Important for removing excesswater from the upper profile Source: http://www.bbc.co.uk18- Hydrostatic potential (h)- Positive pressure of waterin the saturated zonebelow the water table- Causes wet basements- hranges:• From 0 kPa forunsaturated soilabove the water table• To positive values thatincrease with depth below the water tableSource: http://shayshomeimprovements.comNRES 201 Lectures 16-17 (Fall 2014): Soil Water1019- Total soil water potential (t)- Represents the work that must be expended to remove water from the soil- The sum of all the individual potentials:t= m+ o+ g+ h- Simplifies to t= mwhen:• Water and solutes move together in nonsaline soil (o 0 kPa)• Elevation differences are small for water uptake by crops (g 0 kPa)• The soil is unsaturated (h= 0 kPa)20Dynamics of soil drying- As an initially saturated soil is dried, water is increasingly confined to smaller pores and thinner films- The decrease in water content brings a corresponding increase in the energy of water retention- Four stages are characterized along the continuum of soil wetnessClassification of soil waterNRES 201 Lectures