Understanding Soil Water
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Transcript of Understanding Soil Water
Understanding Soil Water
What is the term for the moisture status of a soil
when its pores are 100% full of water?
Saturation
Why do large soil pores (aka macropores)
normally drain within a few days?
GRAVITY
Large soil pores are drained by
Why doesn’t gravity drain all the water out of soil pores ?
Capillarity and surface attraction combine to pull more
strongly than gravity on: 1) water in “micropores” and
2) water close to the “soil skin”.
Field capacity Wilting point
When plants have extracted
as much water as they can When water is no longer
drained by gravity
Why doesn’t gravity drain all the water out of soil pores ?
Field capacity Wilting point
Some water is held
too tightly to be
pulled away by roots
When plants have extracted
as much water as they can When water is no longer
drained by gravity
Pull of the soil matrix on H2O
+
surface attraction + capillarity
So
il Sk
in H
H
O
Hydrogen bonding
H
H
O
H
H
O
?
cohesion
+
adhesion
pull H2O
into small
pores
Pull of the soil matrix on H2O
+
surface attraction + capillarity
So
il Sk
in H
H
O
Hydrogen bonding
H
H
O
H
H
O
?
cohesion
+
adhesion
pull H2O
into small
pores Water is pulled into
the micropores and
toward the soil skin
by matric forces
What do I mean by “soil skin”?
humus clay minerals
http://www.ccma.csic.es/dpts/suelos/ Brady and Weil (2002)
Plant available water
10-30 μm
Gravitational water in
drainage pores
Unavailable water
Adapted from Buol (2000)
Most available
Soil circulatory system
~0.2
μm
les
s
availab
le
Field Capacity
Wilting point
Saturation model soil pore
Plant available water
Unavailable water
Adapted from Buol (2000)
Most available
Soil circulatory system
~0.2
μm
les
s
availab
le
Field Capacity
Wilting point
model soil pore
Unavailable water
Adapted from Buol (2000)
Soil circulatory system
~0.2
μm
Wilting point
model soil pore
So
il skin
Thickness of water film
Unavailable
water
high energy H2O Low energy H2O
high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
So
il skin
Thickness of water film
Unavailable
water
high energy H2O Low energy H2O
high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
So
il skin
Thickness of water film
Unavailable
water
high energy H2O Low energy H2O
high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
So
il skin
Thickness of water film
high energy H2O Low energy H2O
high energy H2O = molecules bouncing around
low energy H2O = molecules moving slowly
There is still some water in air dry soils!
Mars Lander probe finds no water in Martian soils
A conductivity probe on the Mars Lander sensed rising and falling humidity levels in the
Martian atmosphere, but when stuck into the ground, the probe found “Martian soil” to
be completely and perplexingly dry.
On Earth, “if you have water vapor in the air, every surface exposed to that air will have
water molecules adhering to it that are somewhat mobile, even at temperatures well
below freezing," said Aaron Zent , lead scientist for the Lander’s conductivity probe.
~ 1
m
15
0 m
10
,00
0 m
Wilting point Air-dry
There are many
other methods
of expressing
soil water
tension
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 created by a hanging column of water
-1500 kPa
-15 bars
-100,000 kPa
-1000 bars
Field capacity
~ 1
m
15
0 m
10
,00
0 m
Wilting point Air-dry
There are many
other methods
of expressing
soil water
tension
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 created by a hanging column of water
You should be
familiar with
these units
-1500 kPa
-15 bars
-100,000 kPa
-1000 bars
Field capacity
Saturation
Are all of the water
molecules in this pore
under the same tension ?
Field
Capacity
Wilting
point
-1500 kPa
Air-dry
-100,000 kPa
Ψtotal + Ψmatric + Ψosmotic = Ψgravitational
Understanding soil water tension
Pull
of
gravity
Pull by
micropores
and soil skin ?
Understanding osmotic tension
Salt added
?
Understanding osmotic tension
Salt added
What causes fertilizer burn?
Osmotic tension
The same phenomena that causes “dishwashing hands”
Osmotic tension
How do
water
molecules
get from the
soil to the
top of a
plant?
Continuous
chains of water
molecules
move upward
through the
xylem
The chain
moves upward
if there is a
negative energy
gradient
H20
H20 H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20 H20
H20
H20
Continuous
chains of water
molecules
move upward
through the
xylem
Solar energy
drives
transpiration
Plants provide
the conduit
H20
H20 H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20
H20 H20
H20
H20
Transpiration = air conditioning for plants
~ 4000 gallons H2O
per acre on a hot
sunny day
~ 30 gallons H2O
per corn plant per
season
The tallest living tree is a coast redwood that stands 112 meters
(367 feet, 6 in.), or ~ five stories higher than the Statue of Liberty.
Cohesion
theory
Why don’t trees grow
any taller ?
Hydrogen bonding is only
strong enough to hold
together ~ 400’
of water
molecules
Soil water is a switch that activates and
deactivates soil biology
Water is biologically available, when soil
organisms are able to win the
“tug of war” with the soil
What is meant by the term water content?
Up to this point, we have been discussing
water tension
Determining gravimetric soil moisture content
Collect sample. Weigh moist. Weigh after oven drying.
g.m.c. = (moist – dry soil mass) / dry soil mass
Water content can also be
expressed volumetrically
v.m.c. = volume of water in soil / total soil volume
volume
of H2O
volume
of dry
soil
mass of H2O
mass of dry soil
mass of dry soil
volume of dry
soil
Converting from gravimetric to volumetric MC
volume of H2O
mass of H2O * * =
inappropriate
for expansive soils
Bulk density
Gravimetric
moisture
content Density
of H2O
Volumetric
moisture
content
Why would you want to do this conversion?
Gravimetric MC is easier to measure
but volumetric MC is more useful
for managing irrigation
0
Translating between
water tension (aka potential)
and water content using
a “characteristic curve”
A characteristic
curve (aka
water release
curve) describes
the relationship
between water
tension and
water content
for a specific
soil.
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 ceramic plate.
Why are
all those
bolts
needed?
Different soils have different characteristic curves
Field capacity
Wilting point
Brady and Weil, 2002
Different soils have different characteristic curves
Field capacity
Wilting point
0.09 – 0.02 = 7%
Brady and Weil, 2002
Different soils have different characteristic curves
Field capacity
Wilting point
34
% - 8
% =
26
%
Brady and Weil, 2002
Different soils have different characteristic curves
Field capacity
Wilting point 54
% - 2
4%
= 3
0%
Brady and Weil, 2002
Use the diagram to interpret how much water
is held in the clay @ saturation, FC and WP.
Calculate how many inches of water are
needed to bring a 3’ rooting zone of this soil
from 50% of FC to FC.
0.54 * 36” = 19.4” of water @ FC
The volumetric water content @ FC = 0.54
50% of 19.4” = 9.7”
Real soils rarely hold more than 2.5” of
plant available water per foot… based on
this fact, do you think the characteristic
curve for the clay soil is realistic?
The volumetric water content @ FC = 0.54
The volumetric water content @ WP = 0.24
PAW = 0.30
0.3* 12” = 3.6” >> 2.5”
So how does compaction impact soil water relationships ?
So how does compaction impact soil water relationships ?
Loss of drainage
pores
Gain in
small
pores
Field capacity line
Which soil texture can hold the
most plant available water?
Brady and Weil, 2002
Plant
Available
water
~ 2.5” of plant
available water
(PAW) per foot
Wilting point line
How does SOM affect PAW?
Adapted from Brady and Weil
How does SOM affect PAW?
Adapted from Brady and Weil
Prairie soil Farm field
Impressive example of the impact of
soil organic matter on
water holding capacity
So when should you irrigate a clay soil?
Wimpy crops
Tough crops
So when should you irrigate a clay soil?
Wimpy crops
Tough crops
So when should you irrigate a loam soil?
So how does one measure soil water tension in the field?
Brady and Weil, 2002
A tensiometer is a
water filled tube
with a porous
ceramic tip on one
end and a vacuum
gauge on the other.
Water tension in the
tube equilibrates
with the water
tension outside the
porous tip.
Tensiometers are
useful for montioring
tensions between
0 and -85 kPa (-0.85 bars)
a range that includes
about half the water in
most soils.
When soils are too dry
(> -85 KPa), air is drawn
in through the porous tip
and the vacuum fails.
Measuring soil
moisture as a
function of
electrical
resistance
Gypsum block
Brady and Weil, 2002
Measuring soil
moisture as a
function of
electrical
resistance
Calibration is
critical !!
Gypsum block
Brady and Weil, 2002
Resistance
drops as
gypsum
starts to
dissolve
What is this gizmo?
What is this gizmo?
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
Measuring infiltration rate
http://soilquality.org/images/infiltration_photo1.jpg
Why do the wetting fronts have different shapes?
http://www.ext.colostate.edu/mg/gardennotes/images/213-7.jpg
Capillarity pulls the water farther in finer textured soils
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
Capillary rise in a sandy soil
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
Capillary rise in a silt loam
What happens when capillary rise lifts water to the soil surface?
http://www.ag.ndsu.edu/pubs/plantsci/soilfert/sf1087.pdf
How fast does water move through soil ?
Flow rate = Area*Ksat *pressure/length Brady and Weil, 2002
Darcy’s
Law
Hyd
raulic
co
nd
uctivity
Hydraulic conductivity = permeability
Flow rate ~ pore radius4
Coarse textured layer
Fine textured layer
How does the presence of a coarse textured
layer under a fine textured layer affect
percolation ?
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.
Does a layer
of sandy soil
improve
drainage ?
NO !
Layer with sandy texture
Layer with sandy texture
Soil suitability for septic drainfields
http://organicearthsolutions.wordpress.com/2012/02/16/
What happens if a septic drainfield
does not drain adequately?
Can a drainfield drain too well?
Interstream
divide
SOIL
DRAINAGE
CLASSES
Poorly
drained
Somewhat
poorly
drained
Moderately
well drained
Poorly
drained
Well
drained
Valley floor
Backslope
Shoulder
LANDSCAPE
POSITIONS
N.C. Agric. Res. Bull. 467
Common in IL
Impact of topography on drainage
Illinois’
natural drainage
classes
http://www.il.nrcs.usda.gov/technical/soils/Suite_Maps.html
What is a
hydric soil?
A hydric soil is a soil that
formed under conditions of
saturation, flooding or
ponding long enough during
the warm season to develop
anaerobic conditions in the
upper horizons.
Soils in which the hydrology
has been artificially modified
are still considered hydric if
the soil, in an unaltered
state, was hydric.
htt
p:/
/ww
w.il.n
rcs.u
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/Suite_M
aps.h
tml
Hydric soils are dominated by low chroma colors
http://www.wtamu.edu/~crobinson/soils/clayskn05s.jpg
Mottles are
indicative of a
fluctuating
water table.
Some hydric soils in McDonough Cty
http://www.ars.usda.gov/SP2UserFiles/Place/36251500/TheExtentofFarmDrainageintheUnitedStates.pdf
Artificial drainage in the United States
% of land drained
IL has experienced some very wet
springs in recent years
?
Yield maps
have made
drainage
problems
more
obvious
Increasing yield by installing drainage
By Mindy Ward, Missouri Farmer Today
BOONVILLE --- For more than 100 years, the
Hoff family has fought to farm wet areas of their
fields.
For Eddie Hoff, the fourth generation to farm the
creek bottom ground in Cooper County, the loss
of yield and added expense of working the
ground was ultimately affecting his bottom line.
“We were losing 60 to 70 bushels per acre in
some spots,” he says.
We were working the ground over and over. I
just wanted to no-till and save some cost.”
So, he decided to drain the soils with pattern tile.
Could this story be about your farm?
http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm
Pattern Tiling in Ontario
http://www.omafra.gov.on.ca/english/engineer/facts/10-091.htm
http://www.fastline.com/flimages/internet/032/169/3959312_4.jpg
Installing corrugated plastic tile with a tile plow
Ontario Ministry of Ag and Food
Why do crops on tiled-drained land tend to
be more drought resistant ?
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.
?
http://wrc.umn.edu/prod/groups/cfans/@pub/@cfans/@wrc/documents/asset/cfans_asset_212844.jpg
Bioreactor
filled with
woodchips
Controlled
drainage system
Conservation Drainage
Maximum conveyance Crop productivity Environmental quality
Artificial drainage has greatly increased the number of days when
soils in the Upper Midwest are suitable for field operations and
deep root growth
but has also
contributed
to some
environmental
problems
Pollution of
water resources Loss of SOM
Which is worse??
Saturated soil is
less compressible
than wet soil
Compaction
probably extends
several feet deep
What is the
optimum soil
moisture for
compacting
soil?
Soils are most compactible
near field capacity because
the particles are well
lubricated and the large
pores are empty and most
collapsible
Soil resistance to penetration is very
related to soil moisture content.
Healthy crops tend to use more
water which can result in higher
penetrometer readings.
heat capacity of water =
1 calorie / gram / degree C
Understanding Heat Capacity
How much will the temperature of the water increase in cup A if
300 calories of thermal energy are added? How about cup B?
A
B
Why does soil heat up faster than water ?
The heat capacity of
water is ~ 5 times
higher than the heat
capacity of dry soil.
As a result, moist soils heat up and
cool down more slowly than dry soils.
Water has a
high thermal
conductivity
Air has a
low thermal
conductivity
What can be done to
maximize geothermal
heat transfer ?