1. Soil Physics 2010 Outline Announcements Where were we? Measuring water potential Unsaturated flow

2. 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

3. Soil Physics 2010 Competition for pores m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

4. Overview Soil Physics 2010 Where’s the wilting point? 1.5 MPa (15 bars)

5. Soil Physics 2010 Measuring water potential Tensiometer Psychrometer Embedded medium

6. 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

7. Soil Physics 2010 Tensiometer 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 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

8. 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

9. Mercury & water manometer Soil Physics 2010 Reading the mercury tensiometer Equal downward force on each side Tube has water and mercury – no air hwhw h Hg mm

10. Soil Physics 2010 New: polymer tensiometer Martine van der Ploeg Polymer swells on absorbing water Pressure transducer measures pressure due to swelling Good to -1.6 MPa

11. 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 :

12. 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

13. Soil Physics 2010 Unsaturated flow You’re on, Joyce (and then Richards’ equation was derived)

14. Soil Physics 2010 m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?

15. Rawls. Brakensiek, and Saxton, 1982 K s versus K(S), 1982 predictions Soil Physics 2010