Outline
• Announcements
• Where were we?
• Measuring water potential
• Unsaturated flow
Soil Physics 2010
Announcements
• Giant clam-eating shark found in
Kansas (no kidding! See
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8530000/8530995.stm)
• Homework 4 due March 3. File
posted online:
http://www.agron.iastate.edu/soilphysics/SP_HW4.pdf
Soil Physics 2010
Where were we?
Competition for pores
When both water
and air are
present (middle
range):
Matric
potential
ym
• Water is in the
smaller pores
• Air is in the
larger pores
Soil Physics 2010
q
Overview
1.5 MPa
(15 bars)
Soil Physics 2010
Where’s the
wilting point?
Measuring water potential
Tensiometer
Psychrometer
Embedded medium
Soil Physics 2010
Tensiometer
sealed refiller
(optional)
Water in shaft
equilibrates
with soil water
through the
ceramic cup.
water-filled shaft
Water in the
tensiometer is
at negative
pressure
cap, tightly sealed
porous ceramic cup
Soil Physics 2010
Water
Wanted:
cavitates
ymaround
here -0.085
MPa
Tensiometer
• Use degassed water to
delay cavitation
• Many caps have a
manual vacuum pump
• Check and refill
frequently when soil is
dry
• Volume of water added
to surrounding soil is
small
Soil Physics 2010
Water in shaft
equilibrates
with soil water
through the
ceramic cup.
Water in the
tensiometer is
at negative
pressure
Water
cavitates
around -0.085
MPa
Reading the tensiometer
Like a certain exam question…
equal force pulling down on
each side
Pressure
Vacuum gage
transducer
Mercury
& water
manometer
Soil Physics 2010
Reading the mercury tensiometer
 Hg
w
y m  hw
 hHg
w
w
hHg
Equal downward
force on each side
 Hg
y m  hw  hHg
w
Soil Physics 2010
Tube has
water and
mercury –
no air
hw
ym
Mercury
& water
manometer
New: polymer tensiometer
Martine van der Ploeg
• Polymer swells on
absorbing water
• Pressure transducer
measures pressure
due to swelling
• Good to -1.6 MPa
Soil Physics 2010
Psychrometer
• Small soil sample placed in holder
• Close door, start
• Sample equilibrates with chamber air (2-5 minutes)
• Measure water vapor pressure p of chamber air
• Calculate ypsy:
y psy
RT  p 
 y m y o 
ln 
M  p0 
Soil Physics 2010
Embedded medium
• Insert into the soil a material with
a known q(ym) relationship
• The known material equilibrates
with the soil
• Measure q in the known material
• Read the soil’s ym from the known
q(ym) curve
Soil Physics 2010
Noborio
Decagon
Unsaturated flow
You’re on, Joyce
(and then Richards’ equation was derived)
Soil Physics 2010
Why does K decrease as q decreases?
• Water is only in the
smaller pores, which
have lower flow
(Poiseuille’s law)
• Air fills the larger
pores, so:
• Less cross-sectional
area for flow
• More tortuous
flowpath, because
water has to flow
around big pores
Soil Physics 2010
ym
q
Ks versus K(S), 1982 predictions
Soil Physics 2010
Rawls. Brakensiek, and Saxton, 1982