Soil Physics 2010
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Transcript of Soil Physics 2010
Soil Physics 2010
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
Competition for pores
m
draining
wetting
When both water and air are present (middle range):
• Water is in the smaller pores
• Air is in the larger pores
Where were we?
Matric potential
Overview
Soil Physics 2010
Where’s the wilting point?
1.5 MPa(15 bars)
Soil Physics 2010
Measuring water potential
TensiometerPsychrometerEmbedded medium
Soil Physics 2010
Tensiometer
porous ceramic cup
water-filled shaft
cap, tightly sealed
sealed refiller (optional)
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
Wanted:m here
Soil Physics 2010
TensiometerWater in shaft equilibrates with soil water through the ceramic cup.
Water in the tensiometer is at negative pressure
Water cavitates
around -0.085 MPa
• 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
Pressure transducer
Mercury& water
manometer
Vacuum gage
Soil Physics 2010
Reading the tensiometer
Like a certain exam question… equal force pulling down on
each side
Mercury& water
manometerSoil Physics 2010
Reading the mercury tensiometer
m
w
HgHgwm hh
Equal downward force on each side
Tube has water and mercury – no air
hw
hHg
w
HgHgh
m
w
wwh
Soil Physics 2010
New: polymer tensiometerMartine 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 psy:
0
lnp
p
M
RTompsy
Soil Physics 2010
Embedded medium
Noborio
• Insert into the soil a material with a known (m) relationship
• The known material equilibrates with the soil
• Measure in the known material
• Read the soil’s m from the known (m) curve
Decagon
Soil Physics 2010
Unsaturated flow
You’re on, Joyce(and then Richards’ equation was derived)
Soil Physics 2010
m
draining
wetting
• 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
Why does K decrease as decreases?
Rawls. Brakensiek, and Saxton, 1982
Ks versus K(S), 1982 predictions
Soil Physics 2010