Soil Properties and Soil Water Movement Environmental Hydrology Lecture 4.

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  • Soil Properties andSoil Water MovementEnvironmental HydrologyLecture 4

  • ObjectivesUnderstand soil properties affecting soil water retention and movement

    Understand physics governing soil water movement in unsaturated zone

    Learn methods for measuring soil physical properties and infiltration rates

    Explore effects of landuse and disturbance on infiltration into soils

  • Photo credit: Dept. of Land Resources & Environmental Science, Montana State University

  • Soil textureWard & Trimble, Fig 3.1

  • Soil textureSoil textural triangle from Christopherson, 2001. Elemental Geosystems. Prentice Hall

  • Soil matrix Vt = Va + Vw + Vs

    or

    Vt = Vv + VsWard & Trimble, Fig 3.3

  • PorosityVt = Va + Vw + Vs

    Vt = Vv + VsMeasure of void spaces in a soil, expressed as fraction or percentage by substitution

    porosity (n) = 1 Vs / Vt

  • Soil water contentVolumetric water content (qv) = Vw / Vt

    Gravimetric water content (qg) = mw / ms

  • Bulk densityBulk density (rb) = mt / Vt

    Dry bulk density (rdry) = ms / Vt well derive some of these for soils sampled in lab!

  • Soil water movementImage credit: Michigan State University Extension

  • What controls soil water movement?Ward & Trimble, Figure 3.4.

  • Surface tensions for various interfaces(from CRC Handbook of Chemistry & Physics, 68th ed.)

    SubstanceFormulaIn contact withTemp (oC)t (dynes/cm)WaterH2OAir20 72.75AcetoneC3H6OAir20 23.70BenzeneC6H6Air20 28.85Water20 35.00MercuryHgWater20 375

  • Soil water movementgravitational forces = surface tension forces*

    (pr12) h rw g = (2pr1) t cos a* Under equilibrium conditions. see Ward & Trimble eq 3.12 for definition of termsTo move water through the soil, gravitational forces must exceed surface tension forces

  • Soil Moisture Characteristic CurvesWard & Trimble, Figure 3.5. Pore geometry controls relationship between moisture content and pressure (or soil suction).hysteresis

  • Shape of characteristic curve depends on soil textureWard & Trimble, Figure 3.6

  • Ward & Trimble, Figure 3.9.Water available to plants depends upon soil texture (because of pore size effects on filling/drainage!)

  • Flow through porous mediaQ = K dh/dLwhere K = hydraulic conductivity.

    But, unsaturated hydraulic conductivity is a function of moisture content and soil suction!

  • Measuring water movement in the unsaturated zoneDouble-ring infiltrometer (infiltration rate)Guelph permeameter (hydraulic conductivity)Tensiometers (soil tension)Time domain reflectometry (soil moisture)

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