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Soil Physics 2010

Outline. More Wikipedia stuff Where were we? Measuring soil wetness. Soil Physics 2010. Wikipedia stuff. 8 (out of 16) students have now claimed topics. On Monday, Jan 25, I will start assigning topics to students who have not yet chosen their own. Soil Physics 2010. Wikipedia topics:.

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Soil Physics 2010

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  1. Outline • More Wikipedia stuff • Where were we? • Measuring soil wetness Soil Physics 2010

  2. Wikipedia stuff • 8 (out of 16) students have now claimed topics. • On Monday, Jan 25, I will start assigning topics to students who have not yet chosen their own. Soil Physics 2010

  3. Wikipedia topics: • Topic must fall within the realm of soil physics • Topic must currently have poor coverage in Wikipedia • Ideally, topic relates to your research or specific interests • Once topic is chosen and approved, we figure out what class topic it goes with, and which day you present it. Soil Physics 2010

  4. Where were we? Volume wetness: Air Ma Volumetric water content Water volume fraction Units? Range? Water Mw t Mt Solid Ms Soil Physics 2010

  5. Engineers prefer to norm to Vs or Ms In agricultural & environmental soil physics, we tend to use f and q, not w and e Mass wetness: Unitless Soil Physics 2010 Soil Physics 2010

  6. Fig. 6.1?! 1.6 1.3 1.0 0.7 sand loam rb, kg/ m3 clay 0.0 0.1 0.2 0.3 0.4 0.5 0.6 w, kg water / kg soil Anyone notice that Fig. 6.1 wasn’t what it was supposed to be? What does this mean? Some clays swell on wetting, decreasing the bulk density Soil Physics 2010 Soil Physics 2010

  7. Typo policy: Anyone pointing out a new typographical error in the text (i.e., one that I haven’t yet found) gets a bonus 5 points on the next quiz. Soil Physics 2010 Soil Physics 2010

  8. Measuring soil wetness Why? When (how often)? Where? How? Agriculture, hydrology Daily (at least) Everywhere Lots of methods Soil Physics 2010 Soil Physics 2010

  9. Measuring soil wetness: scale issues Bottom line: Chose a method that fits your requirements in spatial and temporal resolution Soil Physics 2010 Soil Physics 2010

  10. Direct method Collect sample Weight it Dry it Weight it Where? Seal it! How? Soil Physics 2010 Soil Physics 2010

  11. Direct method Advantages: Collect sample Weight it Dry it Weight it Direct The standard against which other methods are calibrated Low-tech Soil Physics 2010 Soil Physics 2010

  12. Direct method Disadvantages: Labor-intensive Destructive Standard drying t & T? Gives w, not q from Robinson et al., VZJ 7, 2008 Soil Physics 2010 Soil Physics 2010

  13. q and w quick review: Need rb to get q from w Which number is greater, w or q? Suppose a soil has q = 0.2. What is w? What do you need to know to get w? Soil Physics 2010 Soil Physics 2010

  14. Electrical resistance (gypsum or nylon block) Bury blocks in soil Wait for equilibration Measure electrical resistance Convert resistance to wetness Soil Physics 2010 Soil Physics 2010

  15. Electrical resistance (gypsum or nylon block) Advantages: Good in dry soil Low tech Easy to set up with dataloggers & multiplexers Calibration is for the block – not specific to each soil Bury block(s) in soil Wait for equilibration Measure electrical resistance Convert resistance to wetness Soil Physics 2010 Soil Physics 2010

  16. Electrical resistance (gypsum or nylon block) Disadvantages: Indirect: needs calibration Blocks change over time (especially in acidic soils) Slow equilibration, hysteresis Unreliable in high-salinity soils Temperature-sensitive Not useful in wet soil Bury block(s) in soil Wait for equilibration Measure electrical resistance Convert resistance to wetness Soil Physics 2010 Soil Physics 2010

  17. Neutron Scattering (thermalization, moderation) Probe emits fast neutrons and counts slow neutrons. Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010

  18. Neutron Scattering (thermalization, moderation) Advantages: Measurements repeated at exact same location No temperature issues – even works in frozen soil! Pretty reliable Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010

  19. Neutron Scattering (thermalization, moderation) Disadvantages: Radioactive material: need special training & licensing Indirect: need soil-specific calibration Slow & labor-intensive Doesn’t work near surface Issues with non-water H, O, C, Al, Fe, etc. Test volume varies with wetness Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to q Soil Physics 2010 Soil Physics 2010

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