soil physics 2010 outline announcements where were we? measuring water potential unsaturated flow
TRANSCRIPT
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
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
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?