soil physics 2010 outline more wikipedia stuff where were we? measuring soil wetness

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:

• 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

Where were we?

t

w

V

VVolume wetness:

Air

Water

Solid

t Mt

Ms

Mw

Ma

Volumetric water content

Water volume fraction Units? Range?

Soil Physics 2010Soil Physics 2010

Engineers prefer to norm to Vs or Ms

Mass wetness:s

w

M

Mw Unitless

b

ww

w

bw

In agricultural & environmental soil physics, we tend to use and , not w and e


Fig. 6.1?!Anyone notice that Fig. 6.1 wasn’t what it was supposed to be?

w, kg water / kg soil

b, k

g/ m

3

1.6

1.3

1.0

0.70.0 0.1 0.2 0.3 0.4 0.5 0.6

sand

loam

clay

What does this mean?

Some clays swell on wetting, decreasing the bulk density


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.


Measuring soil wetness

Why?

When (how often)?

Where?

How?

Agriculture, hydrology

Daily (at least)

Everywhere

Lots of methods


Measuring soil wetness: scale issues

Bottom line:Chose a method that fits your requirements in spatial and temporal resolution


Direct method

Collect sample

Weight it

Dry it

Weight it

weightdry

weightdryweightwetw

Where?Seal it!

How?


Collect sample

Weight it

Dry it

Weight it

Direct

The standard against which other methods are calibrated

Low-tech

Advantages:

Direct method


Labor-intensive

Destructive

Standard drying t & T?

Gives w, not

Disadvantages:

Direct method

from Robinson et al., VZJ 7, 2008


and w quick review:

b

ww

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


Bury blocks in soil

Wait for equilibration

Measure electrical resistance

Convert resistance to wetness

Electrical resistance(gypsum or nylon block)


Bury block(s) in soil





Good in dry soil

Low tech

Easy to set up with dataloggers & multiplexers

Calibration is for the block – not specific to each soil

Advantages:


Bury block(s) in soil





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:

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

Neutron Scattering(thermalization, moderation)







Measurements repeated at exact same location

No temperature issues – even works in frozen soil!

Pretty reliable

Advantages:

Soil Physics 2010






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

soil physics 2010 outline more wikipedia stuff where were we? measuring soil wetness

Documents

soil wetness slide

soil advantages

soil b

realm of soil physics

measuring soil wetness

w slide

wet soil disadvantages

e slide