Soil Water
Chapter 5
Water Potential
Components
Matric potential (tension, suction, -pressure)
Capillarity
Soil Moisture Characteristic Curve
Water Flow
Saturated
Unsaturated
Vapor
Plant-Available Water
Terms
Factors affecting
Energy of water affected by
Gravity
Pressure
Attraction by soil solids
Solutes
Reference
Pure water at atmospheric pressure at
specified elevation
Energy per unit quantity of water
Water potential
The 2 kinds of quantities commonly used as a basis for water potential are
volume and weight (not mass). Energy per unit volume (E / V = [F x L]/ [A x L] =
F / A = P) has the dimensions of pressure, whereas energy per unit weight, a force
(E / W = F x L / F = L) has the dimension, length. Water potential in pressure is
commonly used with biological systems, and water potential as length is used
in engineering.
Gravitational Potential
Higher potential, higher in the gravitational
field, no, so water moves downhill.
Pressure Potential
Higher
Here
or
Here
Obviously, down there.
P = 2/ R
The pressure differential, air > water,
is given by the above expression, where
γ is surface tension and R is radius of the
sphere. Notice the bubble is concave into
the water.
Matric Potential
Attraction by
soil solids
reduces
water potential
- relative to
atmospheric
pressure
So, concave into water film means the
pressure in the water is < pressure in air.
Unsaturated soil
Osmotic Potential
Solutes reduce
water potential
relative to
pure water
Water enters the osmometer through a
semipermeable membrane and rises in
the tube. The top of the water in the beaker
is at atmospheric pressure (call it zero net
pressure) but at the same level inside the
osmometer, there is a standing column of
water. The negative osmotic potential just
balances the positive pressure potential.
Total Potential
TP = GP + (PP or MP) + OP
There can be only positive pressure potential (below a free water surface, or water
table, in saturated soil) or negative matric potential (unsaturated soil), not both.
Matric potential is negative pressure, or tension.
In the absence of a semipermeable membrane, only gravitational potential and
either pressure or matric potential affect water flow.
Attraction to surfaces
due to adhesion and
cohesion
..
..
H-bonding
..
..
Dipole interaction
These properties of water
are responsible for its
cohesion unto itself and its
adhesion to hydrophilic
surfaces. So, when some
water molecules stick to a
surface, they bring their
buddies along.
Responsible for capillarity
Yes, like with capillarity. This
is a derivation of the capillary
rise equation. Note height of
rise, h, is
inversely
related to
radius of tube,
R. ρ is density
of water and
g is acceleration
due to gravity.
bot
Ptop = -2/ R
Pbot = gh
Ptop + P
=0
h = 2/ (gR)
Pressure outside the capillary tube at the
water surface is zero and also inside the tube
because the negative pressure in the water
at the top of the tube is just balanced by the
positive pressure of the water column.
Capillary bundle
model for
soil water
Soil physicists have used
capillary tubes of different
radii as models for soil pores
to explain water movement in
soil. Note that soil pores of a
particular radius are filled with
water only to the height that
corresponding capillary tubes
are filled.
Of course soil pores are very
short, not long capillaries.
Here are two
capillary pores,
one with a small
radius and one
with a larger radius.
When tension is
applied to the
bottom of each, the
larger one empties
first because it can’t
withstand as great
of tension as the
smaller capillary.
The same thing
holds for water in
soil pores. Consider
the next slide.
Relationship between matric potential and
water content --soil moisture characteristic
The decrease in
water content of a
soil as tension on it
increases is due to
pores draining, first
the largest and lastly
the smallest. There
is a continuum of pore
sizes so this decrease
is smooth.
Matric Potential
These dots are
supposed to be
pore size distributions. So, which
would be the sand
and which would
be the loam and
clay? And what
would be the effect
on the soil moisture
characteristic curve?
Shape depends on texture and structure
Clay or
Sand
Besides the rapid decrease in water content with increasing tension (more
and more negative matric potential), you might suspect the pink soil to be
the sand also based on the lower water content at saturation (recall, generally
higher bulk density, thus, lower total porosity, in sand than clay).
Good structure or compacted
Same idea holds here.
It’s a matter of pore
size distribution –
compact a soil and you
decrease total porosity
and reduce the
number of
large
pores.
How to Measure Soil Water
Content
Tension
Content
Gravimetric
Neutron Probe
TDR
You’ve done gravimetric. It’s very
accurate but destructive. Attenuation of
fast neutrons by interaction with H nuclei
(calibrate the instrument at different know soil water contents) can be related
to water content. Time domain reflectometry is newer technology that relates
changes in dielectric constant to water content. Non-destructive methods.
Tension (Matric Potential)
Tensiometer
Simple device consisting of rigid tube
with a porous ceramic cup on the end.
Fill with water, cap and stick in the soil.
The greater the tension in the soil water,
the greater the tension in the tube.
The latter is read by vacuum gage or
pressure transducer. Works fine at
lower tensions, i.e., not dry soil.
Water Movement
Saturated
Unsaturated
Vapor Phase
Always moves from higher to lower potential
Darcy’s Law
These considerations allow you to sort of derive
this. Obviously, volumetric flow (e.g., cm3 h-1) is
directly related to cross
sectional area.
Everything else the
same (soil, length of
it and area), there is
greater flow when
there is more standing
water but there will always be some
minimum flow provided you add water.
Thus, flow is proportional to water depth.
However, flow is inversely related to
length of flow through soil –resistance.
At this point, we’ve Q is proportional to A x (D + C) / L. At zero depth of water
on the surface of saturated soil of different lengths of flow, there is the same flow,
and the only way this can be the case is if the unknown constant, C, is actually L.
Thus, Q is proportional to A x (D + L) / L but it is going to vary with the soil, i.e.,
with the pore size distribution, bigger pores, faster flow and conversely.
Sand
Are the conductivities, Ksat,
the same?
Clay
If K = 1 cm h-1,
what’s the flow?
Q = K A ([D + L] / L)
Darcy’s Law
Unsaturated Flow
Wet
This is the typical case, unsaturated.
Here, despite no difference in gravitational
Potential, water moves from left to right.
This, of course,
is due to the
difference in
matric potential.
Dry
More complex than saturated flow
Application of Darcy’s
Law is not straightforward
for unsaturated soil. A
big issue is that conductivity decreases as
water content decreases.
This is because the area
for flow decreases as the
soil dries and the path
that water moves becomes
longer. More importantly,
flow is restricted to smaller
and smaller pores, through
which water moves slower.
Decreasing Water Content
Why water
goes to roots
Like is always the case
everywhere and every time,
it goes down a potential
gradient, from higher to lower.
Vapor Phase Movement
From higher to lower partial pressure
So, does water vapor spontaneously move
Hot
Cold
Non-saline
Salty
Wet
Dry
Well, isn’t the vapor pressure of water
higher above relatively hot water than
cold water? Don’t solutes reduce water
vapor pressure? In fact, the soil may
become so dry that adhesion of film water
to soil solids actually reduces water
vapor pressure.
If you let a saturated soil drain, it drains fast at first but slows. This is
the behavior of a clay and a sand. The early thinkers on the matter
concluded that water was draining under the influence of gravity and since
drainage was so fast, that portion of the maximum water content was not
really available to plants. So, gravitational water was plant-unavailable.
Field Capacities of a Clay and Sand
Time
Regardless of soil type, the tension of soil water when this gravitational
water has drained is about - 0.2 or - 0.3 or - 0.33 bar (depending on authority).
Plant-Available Water and Related
Gravitational
Water
This is a soil
moisture
characteristic
curve, no?
-0.2 bar, Field Capacity
Matric Potential
Gravitational
Plant-Available
Unavailable
Plants are goners
when they
can’t uptake
water against
the tension at
which it is held
by soil solids.
They permanently
wilt and the
associated
tension is
about -15 bar.
So, plantavailable water
is in between, no?
-0.2 Bar
-15 Bar, Wilting Point
Hygroscopic water
Texture Affects Plant-Available Water
Plant available
water depends
on texture.
Field capacity
and wilting
points are
determined from
moisture
characteristic
curves.
Max at about
silt loam.
Organic Matter Affects Plant-Available Water
Organic
matter is good
in this way, too,
no?