Final - Soil Physics, Iowa State University

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Outline
• Announcements
• Final exam
• Final lecture
• Final comments
Soil Physics 2010
Announcements
• Last review session:
• 11:00 – 1:00 today in G217
• Homework 7 answers & spreadsheet
are posted
• List of review topics is posted
• Practice questions won’t happen
• Textbook typos taken till terminal
testing (accepted until the final exam)
Soil Physics 2010
Final exam I
• 9:45 – 11:45, Monday, May 3, 2010, in
room 2026 Agronomy Hall
• Bring a calculator, pens and/or pencils,
and 1 sheet of letter paper
• The (cheat-) sheet rules are the same as
earlier: 1 side only, anything you want,
cannot be scanned or photocopied,
your name on the back
Soil Physics 2010
Final exam II
• The final exam is comprehensive
• My target is 50% covering material since
the last exam, and 50% from the first 2/3
of the semester
• Same general format and kinds of
questions as previous exams
• Some questions will be recycled from
earlier tests and quizzes
Soil Physics 2010
Final exam III
• Feel free to send me candidate questions.
• Raffiniert ist der Toby, aber boshaft ist er
nicht
Soil Physics 2010
Final lecture
Soil Physics 2010
Solute transport
Summary:
• CDE says dispersion is like diffusion
• Streamtubes say dispersion is about
different flow velocities
• Neither upscales reliably
Soil Physics 2010
But don’t despair!
• Allen Hunt, critical path analysis (CPA)
• How important is diffusion
> a few pores?
Log [at
x scales
(m) ]
• Allen Shapiro,
1000 km BTC with
2 tracers
-2
2
4
6
5
1
Soil Physics 2010
Field, porous (65)
Field, fractured (26)
Laboratory (85)
Haggerty (159)
Vanderborght (635)
2-D ordered
2-D disordered
3-D ordered
3-D disordered
2-D inv/dis
-1
-3
-5
Log [ α (m) ]
3
Thoughts
I may not have gone where I intended to go, but
I think I have ended up where I intended to be.
~ Douglas Adams
Physics is mathematical not because we know so
much about the physical world, but because we
know so little; it is only its mathematical properties
that we can discover.
~ Bertrand Russell
Soil Physics 2010
Objectives
1. To learn and understand basic concepts
of soil physics
2. To apply soil physics concepts as
appropriate
3. To recognize soil physics when you see
it outside the classroom
4. To enhance your overall scientific,
quantitative, and computer skills
Soil Physics 2010
What we’ve seen
Follow the units
No gradient, no flow (sort of like Newton’s 1st law)
Newton’s first law is under-rated
Lots of history. All science is a continuing story. Remember
that 2 months in the lab can spare you 2 hours in the library.
A transient equation is a steady-state equation combined with
a conservation equation
A physical explanation should make physical sense
Soil Physics 2010
What we’ve seen
Cultivate your ability to see what effects are important, and
what are secondary. You can solve a complex problem much
more easily by ignoring most of the complexities.
But also… include all the information, and you’ll get a better
picture (like with homework 7)
Measurement error: when to measure directly with big error,
when to estimate from better or easier measurements?
Example: porosity.
The same equations have the same solutions
But also… thinking of a process as analogous to diffusion can
be really useful (e.g., heat transfer), or really misleading (e.g.,
hydraulic diffusivity and the CDE).
Soil Physics 2010
Successes of soil physics
Processes in the soil are subject to the same physics as rivers,
engines, and planetary orbits: Newton’s laws, Darcy’s law, Fick’s
law, Fourier’s law, and so on.
Fundamental basis for managing drainage, irrigation, soil
engineering
Soil physics processes are inextricably linked with soil chemitry,
soil microbiology, pedology, plant growth, weather and climate,
soil management,…
Soil Physics 2010
Successes of soil physics
Historically robust connection between field, laboratory, and
theory.
Recognition of soil in context: hillslope hydrology, preferential
flow
Methods of measurement and analysis continue to improve
The best soil physicists frequently borrow from other disciplines:
remote sensing, microCT,
lattice-Boltzmann modeling, molecular dynamics, surface chemistry
percolation theory, graph theory, constructal theory
geostatistics, fractals, simulation models
In fact, the best soil physicists often come from different
disciplines: math, physics, engineering, agriculture, and so on.
Soil Physics 2010
Failures of soil physics
Constitutive relationships are disjointed, missing important
linkages. For example, the air entry pressure ha is presumably
related to Ks, to typical field saturation, to soil structure, and
perhaps to qr. These connections hold promise, but get little
attention.
Little exploration at the extremes. For example, predicting and
measuring unsaturated hydraulic conductivity at low water
contents. Likewise, dispersion is almost never studied in
unsaturated systems
Structure, structure, structure, at many scales. Way beyond
what soil morphology or geostatistics can deal with.
Soil Physics 2010
Failures of soil physics
Coupled processes are poorly understood rigorously,
theoretically, quantitatively. How does water move heat, or a
thermal gradient move salt?
Soil is never isothermal, but most soil physics equations are. As
a soil heats up, the particles expand, water viscosity and surface
energy decrease, and air expansion makes the soil breathe out.
Pay attention!
Too many things are assumed uniform in both space and time.
Typically we assume that particle size distribution, porosity,
wetting angle, organic matter content, structure, and even
temperature, don’t change.
Soil Physics 2010
Issues and opportunities in soil physics
In soils, randomness and structure occur at all scales,
and can have a huge impact on transport properties.
We don’t know how to measure structure, nor to use
what structural information that we do have.
Scales of heterogeneity: we need to use a combination of
traditional physics, and stochastic thinking, to get the
picture right. Expect more statistical physics in the
future.
Scale issues in general: we measure things at the cm
scale, and predict at the km scale. Surprise! It doesn’t
work. Something fundamental and perplexing is going
on here.
Soil Physics 2010
Issues and opportunities in soil physics
Soil physics knowledge is not well integrated into
meteorology, hydrology, and other neighbors. Talk with
the neighbors! We need each other.
We’ve made a lot of assumptions, both in this class and
beyond. That has allowed progress, but it also leaves
behind lots of opportunities. Assumptions are questions
that we have deferred, maybe because we didn’t dare go
there. Someday, someone will dare.
We used to think that if we knew one, we knew two,
because one and one are two. We are finding that we
must learn a great deal more about ‘and’.
In N. Rose, Mathematical Maxims and Minims
Soil Physics 2010
Final thoughts
All science is either physics or stamp collecting. ~ Ernest Rutherford
Only mathematics and mathematical logic can say as
little as the physicist means to say. ~ BertrandqRussell
 
t

 



K
q

1



z 
 z

Science is built up of facts, as a house is built of stones; but an
accumulation of facts is no more a science than
a heap
of stones
I♥
Darcy
is a house. ~ Henri Poincaré
There is something fascinating about science. One gets such
wholesale returns of conjecture out of such a trifling investment
of fact. ~Mark Twain
It is nothing short of a miracle that modern methods of
instruction have not yet entirely strangled the holy curiosity of
inquiry. ~ Albert Einstein
Soil Physics 2010
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