soil water reading: applied hydrology sections 4.1 and 4.2 topics –soil water properties –soil...

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  • Soil WaterReading: Applied Hydrology Sections 4.1 and 4.2TopicsSoil water propertiesSoil water measurementSoil water balance

  • Subsurface waterInfiltrationSoil moistureSubsurface flowGroundwater flow

  • Porous Medium FlowGroundwaterAll waters found beneath the ground surfaceOccupies pores (void space not occupied by solid matter)Porous media Numerous pores of small sizePores contain fluids (e.g., water and air) Pores act as conduits for flow of fluidsThe storage and flow through porous media is affected byType of rocks in a formation Number, size, and arrangement of pores Pores are generally irregular in shape because of differences in the minerals making up the rocks geologic processes experienced by them.

  • Zones of SaturationUnsaturated zoneZone between the land surface and water tablePore contains water and airAlso called as vadose zone or the zone of aerationSaturated zonepores are completely filled with waterContains water at greater than atmospheric pressureAlso called phreatic zoneWater tableSurface where the pore water pressure is atmospheric Divide between saturated and unsaturated zoneCapillary fringeZone immediately above the water table that gets saturated by capillary forces

  • Soil WaterHygroscopic waterMicroscopic film of water surrounding soil particlesStrong molecular attraction; water cannot be removed by natural forcesAdhesive forces (>31 bars and up to 10,000 bars!)Capillary waterWater held by cohesive forces between films of hygroscopic waterCan be removed by air drying or plant absorptionPlants extract capillary water until the soil capillary force is equal to the extractive force Wilting point: soil capillary force > plant extractive forceGravity waterWater that moves through the soil by the force of gravityThree categoriesField capacityAmount of water held in the soil after excess water has drained is called the field capacity of the soil.

  • Soil Sieves

  • Soil Particle Sizes(USDA Soil Classification SystemTable 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system.

    0.001 mm 0.1 mm.0.01 mm1 mm

    Name of soil separate Diameter limits (mm) Very coarse sand* 2.00 - 1.00 Coarse sand 1.00 - 0.50 Medium sand 0.50 - 0.25 Fine sand 0.25 - 0.10 Very fine sand 0.10 - 0.05 Silt 0.05 - 0.002 Clay less than 0.002


  • Soil Texture TriangleSource: USDA SoilSurvey Manual Chapter 3

  • Soil Water ContentSoil Water Content

  • Soil Water Flux, qq = Q/A

  • Soil Water Tension, yMeasures the suction head of the soil water Like p/g in fluid mechanics but its always a suction (negative head)Three key variables in soil water movementFlux, qWater content, qTension, yTotal energy head = hz=0z1z2q12

  • Neutron scattering (attenuation)Measures volumetric water content (v)Attenuation of high-energy neutrons by hydrogen nucleusAdvantages: samples a relatively large soil sphere repeatedly sample same site and several depths accurateDisadvantages: high cost instrument radioactive licensing and safety not reliable for shallow measurements near the soil surfaceDielectric constantA soils dielectric constant is dependent on soil moistureTime domain reflectometry (TDR)Frequency domain reflectometry (FDR)Primarily used for research purposes at this timeSoil Water Measurement

  • Soil Water MeasurementNeutron AttenuationMeasures Soil Water Content,

  • TensiometersMeasure soil water potential (tension)Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils)Electrical resistance blocksMeasure soil water potential (tension)Tend to work better at higher tensions (lower water contents)Thermal dissipation blocksMeasure soil water potential (tension)Require individual calibrationSoil Water Measurement

  • Tensiometer for Measuring Soil Water Potential, Porous Ceramic TipVacuum Gauge (0-100 centibar)Water ReservoirVariable Tube Length (12 in- 48 in) Based on Root Zone Depth

  • Electrical Resistance Blocks & Meters

  • Soil Water Tension, yMeasures the suction head of the soil water Like p/g in fluid mechanics but its always a suction (negative head)Three key variables in soil water movementFlux, qWater content, qTension, yTotal energy head = hz=0z1z2q12

  • Darcys LawK = hydraulic conductivityq = specific discharge V = q/n = average velocity through the area

  • DefinitionssolidPore withairPore withwaterElement of soil, V(Saturated)Element of soil, V(Unsaturated)

  • Continuity Equationyzxdydzdx

  • Continuity (Cont.)ContinuityEquation

  • Surface TensionBelow surface, forces act equally in all directionsAt surface, some forces are missing, pulls molecules down and exert tension on the surfaceIf interface is curved, higher pressure will exist on concave side Pressure increase is balanced by surface tension, s s = 0.073 N/m (@ 20oC)waterairNo net forceNet force inwardInterface

  • Richards EquationRecall Darcys LawTotal headSo Darcy becomes

    Richards eqn is:

    Soil water diffusivity


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