Why doesn’t gravity pull all the water out of soil ? When water is no longer drained by gravity...
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![Page 1: Why doesn’t gravity pull all the water out of soil ? When water is no longer drained by gravity When plants have extracted as much water as they can Capillarity.](https://reader036.fdocuments.in/reader036/viewer/2022062422/56649ef65503460f94c09d71/html5/thumbnails/1.jpg)
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Why doesn’t gravity pull all the water out of soil ?
When water is no longer drained by gravity
When plants have extracted as much water as they can
Capillarity and surface interactions combine to pull more strongly than gravity on the water in micropores and the water close to
the surfaces of soil particles.
Field capacity Wilting point
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Matric forces
+
Hydrogen bonding
cohesion +
adhesion pull H2O
into small pores
surface interactions + capillarity
H
H
O= H 2
O
Water is pulled into the micropores and toward the soil skin
by matric forces
So
il Skin
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Plant available water
10-30 μm
Drainage pores
Unavailable water
Adapted from Buol (2000)
Most available
Soil circulatory system
~0.2
μm
less
av
aila
ble
Field Capacity
Wilting point
Saturation
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Soi
l ski
n
Distance from soil skin
Unavailable water
high energy H2O Low energy H2O
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http://piru.alexandria.ucsb.edu/~geog3/concept_illus/279_sc.jpg
Adhesivewater
Plant available water
Air dry
Gravitationalwater
Low energy H2O high energy H2O
Soil feels wet
00
Relationship between water film thickness and moisture tension
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1-3.3 m 150 m 10,000 m
Field capacity Wilting point Air-dry
There are many other methods of expressing
soil water potential
All of the following are equivalent:
-1 m of H2O-100 cm of water
-75 mm of mercury-10 kPa
-0.01 MPa-0.1 bars
-0.0987 atmospheres-1.45 PSI
Soil water tension (aka potential) can be visualized as the suction from a hanging column of water
You should be familiar with these units
-1500 kPa-15 bars
-10 to -33 kPa-0.1 to -0.33 bars
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Saturation
Do all of the water molecules in this pore have the same energy
status ?
Field Capacity
-10 K
Pa
-20 K
Pa
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Wiltingpoint
-1500 kPa
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Air-dry
-100,000 kPa
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Ψtotal + Ψmatric + Ψosmotic= Ψgravitational
Understanding soil water potential
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Osmotic potential
Salt added
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H20
H20
H20H20
A continuous chain of water molecules is
pulled up through the
plant
Solar energy drives the process
Plants provide the conduit
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Soil water is a switch that activates or deactivates soil biology
Water is considered biologically available, when soil organisms are able to win the
“tug of war” with the soil
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The soil matrix presents its inhabitants with many challenges
Structural rigidity
Tortuous, loosely connected and highly constricted porosity
Low quality nutritional resources
Moisture fluctuations
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0
Translating betweenwater potential (aka tension) and water content using a “characteristic curve”
A characteristic curve describes the relationship between water
tension and water content for a specific
soil.
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A pressure plate system can be used to bring soil to specific
water tensions
A known positive pressure is applied inside the chamber. Soil water is pushed
out through a porous plate.
After intact cores of soil are
brought to specific
tensions, the moisture
content at those tensions can be
determined.
Why are all those
bolts needed?
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Different soils have different characteristic curves
Field capacity
Wilting point
0.09 – 0.02 = 0.07
34% - 8%
= 26%
54% - 24%
= 30%
Brady and Weil, 2002
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So how does compaction impact soil water relationships ?
Loss of drainage pores
Gain in small pores
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Impact of texture on soil water
Available water
Brady and Weil, 2002
35 - 14
21%
21% of 12”
~ 2.5”
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SOM increases plant available H20
Adapted from Brady and Weil (2002)
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Determining gravimetric soil moisture content
Collect sample. Weigh moist. Weigh after oven drying.
g.m.c. = (moist – dry soil mass) / dry soil mass
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volumeof H2O volume of dry soil
mass of H2Omass of dry soil
mass of dry soilvolume of dry
soil
Converting from gravimetric to volumetric
volume of H2Omass of H2O* * =
inappropriate for expansive soils
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So when should you irrigate ?
Wimpy crops
Tough crops
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TensiometerBrady and Weil, 2002
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Measuring soil moisture as a function of electrical resistance
Calibration is critical !!
Gypsum block
Brady and Weil, 2002
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“An affordable and practical substitute for tensiometric measurement in most agricultural and landscape irrigation
environments.”
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The technique involves determination of the propagation velocity of an electromagnetic pulse sent down a fork-like probe installed in the soil. The velocity is determined by measuring the time taken for the pulse to travel down the probe and be reflected back from its end. The propagation velocity depends on the dielectric constant of the material in contact with the probe (i.e. the soil). Water has a much higher dielectric constant than soil.
Time Domain Reflectometry
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A neutron probe contains a source of fast neutrons and slow neutron detector. Neutrons are emitted from a radioactive source (e.g. Radium or Americium-beryllium) at a very high speed. When these fast neutrons collide with a small atom such as the hydrogen contained in soil water, their direction of movement is changed and they lose part of their energy. These “slowed” neutrons are measured by a detector tube and are calibrated to indicate the amount of water present in the soil.
Neutron probe
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Measuring infiltration rate
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12”
6”
50% porosity
satu
rati
onMacropores
Plantavailable
H2O50% plant
available H2O
2.5”1.25”3.5”
Total water at field capacity
50% solids
6”
Visualizing water in a 1 foot layer of soil
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50% plantavailable H2O
1.25”
How much water is need to bring the soil to field capacity ?
What will happen if more than 1.25” of water infiltrates into this soil ?
Water will percolate deeper than 1’
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How fast does water move through soil ?
Flow rate = Area*Ksat *pressure head/lengthBrady and Weil, 2002
Darcy’s Law
Hyd
raul
ic c
ondu
ctiv
ity
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Permeability = Hydraulic conductivity
Flow rate ~ pore radius4
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Coarse textured layer
Fine textured layer
How does the presence of a coarse textured layer under a fine textured layer affect
percolation ?
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http://www.personal.psu.edu/asm4/water/drain.html
Coarse textured layer
Water will not enter the coarse
textured layer until the upper layer is near saturation
After water enters the coarse textured layer, it
will percolate more quickly.
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Does a thin layer of coarse
material improve
drainage ?
NO !
Thin layer with coarser texture
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Slit filled with coarse material
Soil capped slit
Systems for rapidly draining surface water should be open to the surface
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Slit trenching equipment
Outlets are needed !!
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The current guide reflects recent developments in drainage science and technology. Most of
these are related to new equipment and materials, widespread use of computers, and
water quality considerations. It includes information not in the previous edition on
pipeline crossings, water and sediment control basins, drain fields for septic systems, design of drainage water management systems, and
design charts for smooth-walled pipes.
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In Illinois soil drainage groups are assigned a number (1 to 4) and a capital letter (A or B). The number
indicates the degree of soil permeability. The letter indicates the natural drainage.
1
Rapidly permeableMore than 6 inches per hour
Moderately rapidly permeable
2 to 6 inches per hour
2Moderately permeable
0.6 to 2 inches per hour
3Moderately slowly permeable
0.2 to 0.6 inch per hour
4
Slowly permeable0.06 to 0.2 inch per hour
Very slowly permeable
less than 0.06 inch per hour
IL Permeability classes
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Fields at the Allison Farm
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Open field ditch
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Bioreactor vs. standard tile outlet
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One calorie is the amount of thermal energy required to raise the temperature of one gram of water by one Celsius degree.
3000 calories of thermalenergy enters each cup.The temperature of thewater on the left rises by30 Celsius degrees.
By how much does thetemperature of thewater in the cup on theright rise ??
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Why does soil heat up faster than water ?
The heat capacity of water is ~ 5 times higher than the heat capacity dry soil.
As a result, moist soils heat up and cool down more slowly than dry soils.
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Water has a high thermal conductivity
Air has a low thermal conductivity
What can be done to maximize geothermal
heat transfer ?
compacted vs. loose ?
moist vs. dry ?