Soil Water TensionSoil Water Tension

Department of Agricultural and Biological Engineering University of Illinois at Urbana-Champaign

+h

-h

Soil Water Characteristic Soil Water Characteristic (Release Curve)(Release Curve)

Matric PotentialMatric Potential

Tensiometer for Measuring Soil Water Tensiometer for Measuring Soil Water PotentialPotential

Porous Ceramic Tip

Vacuum GaugeVacuum Gauge

Water ReservoirWater Reservoir

Variable Tube Length (12 in- 48 in) Based on Root Zone Depth

Soil Water Soil Water PotentialPotential

Measure of the energy status of the soil waterMeasure of the energy status of the soil water Important because it reflects how hard plants Important because it reflects how hard plants

must work to extract watermust work to extract water Units of measure are normally bars or Units of measure are normally bars or

atmospheresatmospheres Soil water potentials are negative pressures Soil water potentials are negative pressures

(tension or suction)(tension or suction) Water flows from a higher (less negative) Water flows from a higher (less negative)

potential to a lower (more negative) potentialpotential to a lower (more negative) potential

ComponentsComponents

tt = total soil water potential = total soil water potential gg = gravitational potential (force of gravity = gravitational potential (force of gravity

pulling on the water)pulling on the water) mm = matric potential (force placed on the = matric potential (force placed on the

water by the soil matrix – soil water “tension”)water by the soil matrix – soil water “tension”) oo = osmotic potential (due to the difference in = osmotic potential (due to the difference in

salt concentration across a semi-permeable salt concentration across a semi-permeable membrane, such as a plant root)membrane, such as a plant root)

Matric potential, Matric potential, mm, normally has the greatest , normally has the greatest effect on release of water from soil to plantseffect on release of water from soil to plants

t g m o Soil Water PotentialSoil Water Potential

Water held in large pores

Available for crop use

CapillaryWater

GravitationalWater

FieldCapacity1/3 bar

Water adheres to soil particles

Water drains through soil profile

HydroscopicWater

WiltingPoint

15 bars

Water Movement in Plants

Illustration of the energy differentials which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere. The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere.

w~ -1.3 MPa

w~ -1.0 MPa

w~ -0.8 MPaw~ -0.75 MPa

w~ -0.15 MPa

s~ -0.025 MPa

Leaf air spaces

Xylem

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CoarseCoarseTexturedTexturedSoilSoil

Matric PotentialMatric Potential

Fine Fine TexturedTexturedSoilSoil

Effect of Soil TextureEffect of Soil Texture

Height of capillary Height of capillary rise inversely related rise inversely related to tube diameterto tube diameter

Matric Potential and Soil TextureMatric Potential and Soil TextureThe tension or suction created by small The tension or suction created by small capillary tubes (small soil pores) is greater capillary tubes (small soil pores) is greater that that created by large tubes (large soil that that created by large tubes (large soil pores). At any given matric potential coarse pores). At any given matric potential coarse soils hold less water than fine-textured soils.soils hold less water than fine-textured soils.

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CompactedCompactedSoilSoil

Matric PotentialMatric Potential

AggregatedAggregatedSoilSoil

Effect of Soil StructureEffect of Soil Structure

[1 + (h)n]m

= r + (s - r)

= empirical parameter h = soil water pressure headn = empirical parameter that affects the shape of the curvem = empirical parameter that affects the shape of the curve (m = 1 - 1/n)r = residual water content (an extrapolated parameter)s = saturated water content = volumetric water content at head, h.

Van Genuchten RepresentationVan Genuchten Representation

(hd/h) = r + (s - r) = empirical parameter hd = soil water pressure headr = residual water content (an extrapolated parameter)s = saturated water content = volumetric water content at head, h.

Brooks and Corey RepresentationBrooks and Corey Representation

A moist sand sample has a volume of 456 cm3 in the natural state and a weight of 843 g. The dry weight is 763 g and the specific gravity of the soil particles is 2.65. Determine the porosity, volumetric moisture content, and degree of saturation (water filled porosity/total porosity)

A soil sample with a volume V=100 cm3 was placed in a pressure plate apparatus and brought to apparent saturation. The following outflow volumes were measured at equilibrium for the given pressure settings.

After equilibrium was obtained at the 500 cm pressure, the sample was found to have a dry bulk density of 1.64 g/cm3, and a gravimetric water content (mass of water/mass of solids) of 0.031. Evaluate and plot the soil water retention curve from these data

Pressure (cm) 0 10 20 30 40 50 60 70 80 100 150 200 500

Outflow Volume (cm3) 0 0.3 1.4 3.1 3.6 3.4 3.0 2.2 1.5 1.4 1.6 1.5 2.1

Determine the volume (depth) of water that is released from Tama Silt Loam when the water table falls from the soil surface to 1 m below the soil surface.

Depth (cm)

Van Genuchten Parameters for Tama

r s

(1/cm) n

0 - 18 0.052 0.39 0.0063 1.3418 - 48 0.057 0.44 0.0100 1.3048 - 80 0.091 0.47 0.0055 1.46

80 - 100 0.088 0.59 0.0057 1.53100 - 150 0.081 0.59 0.0052 1.56

This poorly drained "prairie soil" formed in 40 to 60 inches of loess over Wisconsinan drift. It occupies nearly 1.6 million acres. It has a high management level productivity index (PI) of 150.

Drummer Silty Clay

Loam