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

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Presentation on theme: "Soil Physics 2010 Outline More Wikipedia stuff Where were we? Measuring soil wetness."— Presentation transcript:

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

2 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.

3 Soil Physics 2010 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.

4 Soil Physics 2010 Where were we? Volume wetness: Air Water Solid t MtMt MsMs MwMw MaMa Volumetric water content Water volume fraction Units? Range?

5 Soil Physics 2010 Engineers prefer to norm to V s or M s Mass wetness: Unitless In agricultural & environmental soil physics, we tend to use  and , not w and e

6 Soil Physics 2010 Fig. 6.1?! Anyone notice that Fig. 6.1 wasn’t what it was supposed to be? w, kg water / kg soil  b, kg/ m sand loam clay What does this mean? Some clays swell on wetting, decreasing the bulk density

7 Soil Physics 2010 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.

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

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

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

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

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

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

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

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

16 Soil Physics 2010 Bury block(s) in soil Wait for equilibration Measure electrical resistance Convert resistance to wetness Electrical resistance (gypsum or nylon block) 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 Disadvantages:

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

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

19 Soil Physics 2010 Insert access tubes in soil Lower neutron probe down the tube Record the count ratio Convert count ratio to  Neutron Scattering (thermalization, moderation) 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 Disadvantages: Soil Physics 2010


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