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

2. 8 (out of 16) students have now claimed topics.
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. Topic must fall within the realm of soil physics
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: Ma Mw Mt Ms Volumetric water content
Water volume fraction
Units?
Range?
Air
Water
Solid t Mt Ms Mw Ma
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. Anyone notice that Fig. 6.1 wasn’t what it was supposed to be?
w, kg water / kg soil
rb, kg/ m3
1.6
1.3
1.0
0.7
What does this mean?
Some clays swell on wetting, decreasing the bulk density
sand
loam
clay
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. Agriculture, hydrology Daily (at least) Everywhere Lots of methods
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 Where? Seal it! Weight it Dry it How?
Soil Physics 2010
Soil Physics 2010

11. Direct method Advantages: Collect sample Direct Weight it
Dry 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
from Robinson et al., VZJ 7, 2008
Labor-intensive
Destructive
Standard drying t & T?
Gives w, not q
Soil Physics 2010
Soil Physics 2010

13. Need rb to get q from w q and w quick review:
Which number is greater, w or q?
Suppose a soil has q = What is w?
What do you need to know to get w?
Need rb to get q from 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 Advantages:
(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 Disadvantages:
(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 Insert access tubes in soil
(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 Advantages:
(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 Disadvantages:
(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