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Soil Water Content PowerPoint PPT Presentation


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Soil Water Content. Soil Moisture Content. Water that may be evaporated from soil by heating at 105 0 C to a constant weight. mass of water evaporated (g). Gravimetric moisture content (w) =. mass of dry soil (g). volume of water evaporated (cm 3 ). Volumetric moisture content ( q ) =.

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Soil Water Content

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Soil Water Content

Soil Moisture Content

Water that may be evaporated from soil by heating at 1050C to a constant weight

mass of water evaporated (g)

Gravimetric moisture content (w) =

mass of dry soil (g)

volume of water evaporated (cm3)

Volumetric moisture content (q) =

volume of soil (cm3)

bulk density of soil

q = w *

density of water

mass of dry soil (g)

Bulk density of soil (r) =

volume of soil (cm3)


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Example: A soil is sampled by a cylinder measuring 7.6 cm in diameter and 7.6 cm length. Calculate gravimetric and volumetric water contents, and wet and dry bulk densities using the following data:

  • Weight of empty cylinder = 300 g

  • Weight of cylinder + wet soil = 1000 g

  • Weight of cylinder + oven dry (1050C) soil = 860 g

Volume of cylinder = p*r2*h = 3.14*(7.6/2)2*7.6 = 345 cm3

Weight of wet soil = 1000 – 300 = 700 g

Weight of dry soil = 860 – 300 = 560 g

Dry bulk density = 560/345 = 1.62 g cm-3

Gravimetric moisture content = (700-560)/560 = 0.25 or 25%

Volumetric moisture content = r *w = 1.62*0.25 = 0.41 or 41%

Know how to do these calculations for

quiz on Friday


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Calculating dry soil weight basis of samples for analysis

Weigh drying pan, moist soil subsample + pan,

Oven dry the subsample at 105C for 24 hr,

Weigh the dried soil + pan.

Calculate the moisture content (w):

w = (g moist soil – g dry soil)/(g dry soil – pan)

Rearrange the eqn to solve for dry soil wt.

Dry soil wt = g moist soil / (1 + w)


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Methods for measuring soil water content

Direct method

(Gravimetric)

Indirect methods

(need to calibrate)

Electrical properties

Acoustic method

Thermal properties

Chemical methods

Radiation technique

-Neutron scattering

-g- ray attenuation

Electrical Conductance

Dielectric constant

TDR

- Gypsum blocks

- Nylon blocks

- Change in conductance

Principles underlying different methods of

assessment of soil water content


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Water Content

Direct

Gravimetric: evaporating water at 1050C (be able to do the calc’ns)

Indirect

Neutron scattering:

Thermalization

Time domain reflectrometry:

Dielectric constant


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Soil Water (matric) Potential

In-direct:

Watermark (granular matrix sensor), gypsum block

Direct: Tensiometer


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Calibrating field instruments

http://www.bae.ncsu.edu/programs/extension/evans/ag452-3.html


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Gently tap a tube into the soil to take an undisturbed sample from the center of the effective root zone.


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Trim the soil at each end of the tube to the tube length so that the soil occupies the exact tube volume.


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Calibration for moisture content

  • Measure and weigh the tube

  • Weigh the field moist soil + tube

  • Oven dry the soil from the tube

  • Calculate:

    W = g water/g dry soil = (wet – dry) / dry soil

    Db = g dry soil / cm3 volume soil

    Θ = (W x Db) / Dw

  • Compare lab moisture content to field measurements

  • For water potential, compare water retention curves derived in lab using pressure plates.


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Water retention curves: Water content vs pressure or tension

Note: clay holds more water at a specific water potential than sand or loam;

Water is held tighter at a given water content in clay than in sand.

Structure is predominant at low potentials; as soil dries out, texture is more important


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Effect of structure on water flow

www.soils.umn.edu/.../soil2125/doc/s7chp3.htm


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The flow of water in soil

Saturated and unsaturated

flow


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Saturated flow

Ksat = Q/A x L/(Ψ1 - Ψ2)

where Q is volume of water in time (t)

A is area of cross section

Ksat is saturated hydraulic conductivity of soil (how fast water moves)

L is length of column

Ψ is the water potential at points 1 and 2


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Flux can be thought of as water flowing from a hose. The flux is the rate of water discharged by the hose, divided by the cross-sectional area of the hose.

http://soils.usda.gov/technical/technotes/note6fig1.jpg


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Saturated flow in soils

  • The pores are full of water and matric potential is considered to be negligible

    because at least some of the water is a long distance from solid surfaces

  • Under these conditions, flow is:Rapid - moving through large poresDriven by gravity and sometimesHydrostatic pressure if water is ponded


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http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm


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http://www.montcalm.org/montcalmold/media/planningeduc/tn_gwa5.jpg


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Saturation

unsaturated

wet

dry


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Unsaturated flow

Soil moisture content changing with depth


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Unsaturated flow – most common in soils

  • Occurs along soil surfaces, not through large pores.

  • Driven by matric forces that are much stronger than gravity.

    Gravity is not sufficiently strong to exert a significant influence on unsaturated flow because much of the soil water adheres to solid surfaces.

  • Unsaturated flow is slow.

  • Even though the driving force is usually greater than for saturated flow, the resistance to flow is enormous.

  • Water will flow toward a lower (more negative) potential regardless of direction (up, down, laterally). In other words it will flow towards:            drier medium            salty solution            finer texture (small pores)


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http://www.maf.govt.nz/mafnet/schools/activities/swi/swi-04.htm


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http://wwwlb.aub.edu.lb/~webeco/SIM215soilwater_files/image004.gif


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