Soil Physics 2010

1. Outline • Announcements • Final exam • Final lecture • Final comments Soil Physics 2010

2. 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

3. 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

4. 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

5. Final exam III • Feel free to send me candidate questions. • Raffiniert ist der Toby, aber boshaft ist er nicht Soil Physics 2010

6. Final lecture Soil Physics 2010

7. Solute transport • Summary: • CDE says dispersion is like diffusion • Streamtubes say dispersion is about different flow velocities • Neither upscales reliably Soil Physics 2010

8. But don’t despair! • Allen Hunt, critical path analysis (CPA) • How important is diffusion at scales > a few pores? • Allen Shapiro, 100 km BTC with 2 tracers Soil Physics 2010

9. 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

10. Objectives • To learn and understand basic concepts of soil physics • To apply soil physics concepts as appropriate • To recognize soil physics when you see it outside the classroom • To enhance your overall scientific, quantitative, and computer skills Soil Physics 2010

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Final thoughts I ♥ Darcy 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 Russell 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 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