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Soil Physics 2010 Outline Announcements Soil Thermal Regime Evaporation

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

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Soil Physics 2010 You are monitoring soil temperature, wetness, and CO 2 concentrations, and want to calculate the CO 2 efflux. A spreadsheet of your data (SP_HW6_gasdiff.xls) is available on the course website. Plot the calculated (a) CO 2 diffusivity as a function of depth and time. (see pdf online for more details) Homework 6, question 3a

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Soil Physics 2010 Plot the calculated (b) CO 2 efflux (diffusion across the soil surface) as a function of time for the period given. State your assumptions. Homework 6, question 3b Temperature CO 2 10 0 30 – 63 96 – 12 1512 – 1815 2118 – 24 2724 – 3027 3630 – 36 4236 – 4245 60 10075 What is D( ,T) for How do you average diffusivity (or conductivity)? Conductors in series use a harmonic mean (in parallel, they use the arithmetic mean) ?

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

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Soil Physics 2010 Back to the soil thermal regime = 2 / period ( say, 24 hours ): normalizes the “clock time” t to the 2 sine wave period. d = “damping depth”: depth z at which thermal amplitude is A 0 /e: normalizes “physical depth” z to exponential function depth. Specifically, T a = Average Temperature A 0 = amplitude of temperature at the surface

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

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

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

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Soil Physics 2010 Summary Thermal properties (specifically D T ) 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

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Soil Physics 2010 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

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Soil Physics 2010 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., ) later than the surface? ½ cycle delay requires that, where and, so

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Soil Physics 2010 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.

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

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