Soil Physics 2010

1. Outline • Announcements • Soil Thermal Regime • Evaporation Soil Physics 2010

2. Announcements • Review sessions this week: • Today, 11-1 in G217 • Wednesday, 11-1, 1581 • No quiz today! Soil Physics 2010

3. Homework 6, question 3a You are monitoring soil temperature, wetness, and CO2 concentrations, and want to calculate the CO2 efflux. A spreadsheet of your data (SP_HW6_gasdiff.xls) is available on the course website. Plot the calculated (a) CO2 diffusivity as a function of depth and time. (see pdf online for more details) Soil Physics 2010

4. Homework 6, question 3b Plot the calculated (b) CO2 efflux (diffusion across the soil surface) as a function of time for the period given. State your assumptions. What is D(q,T) for ? How do you average diffusivity (or conductivity)? Conductors in series use a harmonic mean (in parallel, they use the arithmetic mean) Soil Physics 2010

5. Homework 6, question 3b Plot the calculated (b) CO2 efflux (diffusion across the soil surface) as a function of time for the period given. State your assumptions. parallel: arithmetic mean =average(1,2,3…) in excel y-axis units are ppm / cm2 s series: harmonic mean =harmean(1,2,3…) in excel Soil Physics 2010

6. Back to the soil thermal regime Ta= Average Temperature A0 = amplitude of temperature at the surface w = 2p / period (say, 24 hours): normalizes the “clock time” t to the 2p sine wave period. d = “damping depth”: depth z at which thermal amplitude is A0/e:normalizes “physical depth” z to exponential function depth. Specifically, Soil Physics 2010

7. The sine part This is about the soil surface warming during the day, and cooling at night. Soil Physics 2010

8. More sine stuff Clock time at the surface, normalized to 2p Phase shift with depth Phase constant: adjust so peak is at the right time of day 3:00 pm 6:00 am midnight noon midnight For a period of 24 hours, and a peak at the surface at 3:00 pm (the 13th hour), Soil Physics 2010

9. The e-z part exponential decay, half-lives, etc. Soil Physics 2010

10. Summary • Thermal properties (specifically DT) appear only in the definition of damping depth: • Phase shifts (delays) as sine wave propagates downward • Amplitude decreases as the wave propagates downward • Temperature is constant at infinite depth Soil Physics 2010

11. Applications • The questions we ask this equation are usually about either • timing and phase shift, or • amplitude • but not both. • When it’s a timing question, focus on the sin() part • When it’s about amplitude, concentrate on the e-z/d part Soil Physics 2010

12. Example application On the coldest day of the year, at what depth is the warmest soil found? Translation: what depth z is ½ cycle (i.e., p) later than the surface? ½ cycle delay requires that , where and , so Soil Physics 2010

13. Example application Around June 20, the soil surface temperature may have an amplitude of 15 °C in one day. At what depth is the amplitude only 2.5 °C in one day? This is an amplitude problem, so we are only concerned with the e-z/d part. Soil Physics 2010

14. Evaporation Soil wets: Infiltration Soil dries: Drainage Transpiration Evaporation – John? Soil Physics 2010