1 / 15

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 )

darius-pitts
Download Presentation

Soil Physics 2010

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. 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

  3. Where were we? Matric potential Competition for pores • When both water and air are present (middle range): • Water is in the smaller pores • Air is in the larger pores ym wetting draining q Soil Physics 2010

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

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

  6. Tensiometer Water in shaft equilibrates with soil water through the ceramic cup. cap, tightly sealed sealed refiller (optional) Water in the tensiometer is at negative pressure water-filled shaft Water cavitates around -0.085 MPa Wanted: ym here porous ceramic cup Soil Physics 2010

  7. Tensiometer Water in shaft equilibrates with soil water through the ceramic cup. • 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 Water in the tensiometer is at negative pressure Water cavitates around -0.085 MPa Soil Physics 2010

  8. Reading the tensiometer Vacuum gage Pressure transducer Mercury & water manometer Like a certain exam question… equal force pulling down on each side Soil Physics 2010

  9. Reading the mercury tensiometer hHg hw Equal downward force on each side Tube has water and mercury – no air Mercury & water manometer ym Soil Physics 2010

  10. 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

  11. 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: Soil Physics 2010

  12. Embedded medium Noborio Decagon • Insert into the soil a material with a knownq(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

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

  14. 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 ym wetting draining q Soil Physics 2010

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

More Related