Lecture34Lecture on Soil Physics, part 2Lecture 34 Lecture on Soil Physics, part 2• Theory, Moisture Transfery– Water Potential, revisited– Moisture transfer, Darcy’s Law and the Richard’s Equation–Soil Release curve.• Observations, Moisture profiles– Seasonal patternsInfluence of soil texture–Influence of soil texture• Soil Evaporation– measurementsdl l lti–model calculationsESPM 129 BiometeorologyFogWater BalancePrecipitation/EvaporationEvaporation/TranspirationFogInterception/StemFlowsnow/iceEvaporation/SublimationpCondensationSnow MeltSurface WaterWetting FrontPerculation/saturated flowSnow MeltHydraulic Liftingett g o tunsaturatedflowygby rootsWater TableCapillary RiseESPM 129 BiometeorologySoil Water PotentialSoil Water Potential• Pressure•Osmotic• GravitationalMti•MatricpogmUnits: Energy/Volume =Pressure (Pa: kg m-1s-2)ESPM 129 BiometeorologyGravitational potentialGravitational potential•The force of gravity exerted on a water column•The force of gravity exerted on a water column produces the gravitation potential. gzPa = kg m-1s-2glgzPa = kg m1s2|hd|gheadlzgmESPM 129 BiometeorologyWhat is the Maximum Height a Suction Pump can Lift water at Sea Level??|10130010.31000 9 8gheadzg meters1000 9.8lgESPM 129 BiometeorologyTurgor or pressure potentialTurgor or pressure potential•Water potential exerted by the pressureWater potential exerted by the pressure, P, or weight of waterpPPSometimes expressed per unit density of waterpwPm2s-2ESPM 129 BiometeorologyMatric potentialMatric potential•It is water potential due to attractionIt is water potential due to attraction between water and soils. These interactions reduce the potential of–These interactions reduce the potential of water, giving it a negative sign.bmbaw~w is relative water contenta and b are coefficientsESPM 129 BiometeorologyOsmotic potentialOsmotic potential• Osmotic potential arises from the dilution of solutes in pwater, eg salts, sugars etc. – For the osmotic potential to drive water flow, a semi-permeable membrane must separate two bodies of water such as cells andmembrane must separate two bodies of water, such as cells, and pools of water. ocvRTc is the concentration is the osmotic coefficient is the number of ions per mole ESPM 129 BiometeorologySaturated FlowSaturated Flowpg00pgpg;Dominated by Pressure and GravityESPM 129 BiometeorologyRT eln()Vapor potentialvwsVeln()ppMPa)-4-20Water Potential (M-10-8-6W-16-14-12relative Humidity0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.02R is the universal gas constant (8.314) V i th l l l f t (18 l l1)ESPM 129 BiometeorologyV is the molal volume of water (18 l mol-1)Unsaturated FlowgmDominated by Gravitation and Matrix ForcesESPM 129 BiometeorologySoil moisture release curveSoil moisture release curve 140160Data from dew point D f DANRl (-bar)100120Data from DANRer potential6080Wate2040Gravimetric soil water content (%)0 102030400ESPM 129 BiometeorologyGravimetric soil water content (%)Soil Water Retention Curve, f(volumetric water content)10MPa)1ater Potential (- 0.1Wa00010.01sand loam clay volumetric water content (m3/m3)0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.550.001ESPM 129 BiometeorologyPedo Transfer FunctionsSoil Water Retention Curve, f(relative water content)satsb10000otential (m),1001000(8Soil Water Po110 sand loam clayVolumetric Soil Moisture Content0.0 0.2 0.4 0.6 0.8 1.00.1clay ESPM 129 BiometeorologyPedoTransfer Function1000d (cm)100silt loamVan GenuchtenHead100.0 0.1 0.2 0.3 0.4 0.5 0.61Volumetric Water Contentresidsat residnm[()]1ESPM 129 Biometeorology[()]1How Much Water is Available to Plants?Clapp Hornberger10MPa)1sandwilting pointer Potential (- M0.1sand loam clay Field capacityWate0.01volumetric water content (m3/m3)0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.550.001ESPM 129 BiometeorologyPeat Water Retention Curve, Campbell Model, Data of Letts0.0MPa-0.5ter Potential, M-1.0Wat-1.5Volumetric Water Content0.2 0.4 0.6 0.8 1.0 1.2-2.0ESPM 129 BiometeorologyPeat is either really Wet or Really Dry (in a Thermodynamic Sense)Available Water and Soil Texture1.0nt (m3 m-3)0.8field capacityric water conten0.40.6field capacity permanent wilting pointVolumetr0.2Sandloamy sandsandy loamsandy clay loamLoamclay loamsilt loamsandy claysilty clay loamsilty clayclaypeat0.0ESPM 129 Biometeorology1000Organic Soils(m)100Water Potential 10W10Volumetric Water Content (cm3/cm3)0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.551ESPM 129 BiometeorologyWater Availability of PeatESPM 129 BiometeorologySchwartzel et al. 2002. J Plant Nutrition and Soil SciDarcy’s LawHenry DarcyHenry DarcyThe volume of water, V, passing through a bed of sand per unit time is a functionpg g pof the cross-sectional area, A, the thickness of the bed, L, the depth of water on top of the bed, h, and the hydraulic conductivity of the sand, K:VtKAhLtJVKhJAtKLESPM 129 Biometeorologysoils.usda.gov/.../note6fig2_lowres.jpgPhilip JR (1995) Desperately Seeking Darcy in Dijon. SilSiS A J 59 319324ESPM 129 BiometeorologySoil SciSoc Am J 59:319-324Darcy’s Law: Conditions of UseDarcy s Law: Conditions of Use•valid forvalid for– low Reynolds numbers, where flow is laminar –viscous forces dominate (Re <1). ()• invalid for:– Conditions where K is a function of head• e.g. unsaturated soils, karst limestone and dolomite soils.–dense clay soils, which have low permeability.ESPM 129 BiometeorologyKVLHydraulic Conductivity is a Function of Soil MoistureAthy, kg s m-31101001000 sand loam clay KKmseb() ()/23ic Conductivity0.0010.010.1100010203040506Hydrauli0.0000010.000010.0001volumetric water content0.00.10.20.30.40.50.6ESPM 129 BiometeorologyMass Flux of Water and Its Budget Equation(/) ()water wmmassmVFD Dzz kg m-2s-1wwhFDtz zz ESPM 129 BiometeorologyRichards Equation for Unsaturated SoilsDescribes the time rate of change of soil moisture in a soil column Classic Form, in terms of Head, hLNMOQPtzKhhz()( )1ESPM 129 BiometeorologyIn Terms of Water Potential()wFDzthe flux density of Energy is a