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Advancements in Soil Heat Pulse Measurements and Water Dynamics

This study explores soil thermal properties, water content, liquid water flux, and heat transfer dynamics using heat pulse measurements. The impact of these factors on biological, chemical, and physical processes is investigated. Modeling coupled heat and water dynamics is challenging and requires precise parameters. Recent advancements in in-situ measurements have improved understanding. Techniques such as heat pulse probes are used to measure thermal properties, water content, and heat flux in soil. Data collected aids in determining soil thermal conductivity, diffusivity, and capacity. The study evaluates soil texture influences, factors affecting soil water content, and methods for estimating soil water evaporation. Measurements using heat pulse sensors provide insights into soil heat flux and energy balance, aiding in understanding evaporation dynamics.

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Advancements in Soil Heat Pulse Measurements and Water Dynamics

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  1. Heat Pulse Measurements to determine: soil thermal properties soil water content infiltrating liquid water flux sensible heat flux in soil latent heat flux (vaporization or fusion) upward liquid water flux Agron 405/505

  2. Soil heat and water dynamics • Impact biological, chemical, and physical, processes • Modeling coupled heat and water dynamics is difficult and requires many hard to measure parameters • Measuring in situ coupled heat and water dynamics has improved recently

  3. Sun rise Sun set Diurnal soil water content change 5, 6, and 7 days after irrigation Jackson. 1973. SSSA Spec. Publ., 5, 37–55

  4. Coupled Heat and Water Transfer Thermal gradients cause water to move in unsaturated soil. When water moves in soil, it carries heat. Because heat transfer and water movement affect one another they are coupled.

  5. Theory Water Flow Heat Transfer

  6. Some heat pulse probe possibilities • Measure soil thermal properties • Measure soil water content • Measure infiltrating liquid water flux • Measure sensible heat flux in soil • Measure latent heat flux (vaporization or fusion) • Measure upward liquid water flux

  7. Heat Pulse Probe

  8. Stainless steel tubing 40 mm Thermocouple Resistance heater 1.3 6 mm 6 mm Sketch of a heat pulse sensor Heat pulse probe

  9. Heat Pulse Method DC power 0.5 (tm, Tm) V A 0.4 0.3 DT (oC) 0.2 0.1 0.0 r 0 30 60 90 120 150 For a cylindrical coordinate, heat conduction Eq. and solution: Datalogger t(s)

  10. tm=30 s ΔTm=1.23 K Determining of soil thermal properties by heat pulse sensor Temperature increase (K) Time (s) Temperature response after applied t0=8 sheat pulse on the central heater needle

  11. Soil thermal diffusivity (J/m3C) Soil heat capacity C (J/m3C): é ù é ù 2 t r 1 1 = - m ln ê ú ê ú - - 4 ( t t ) t ( t t ) ë û ë û 0 0 m m m Soil thermal conductivity (W/mC): Determining of soil thermal properties by heat pulse sensor

  12. 0.6 0.5 0.4 Temperature increase (K) 0.3 0.2 0.1 10 20 30 40 50 60 70 80 Time (s) Example of heat pulse data By fitting a heat transfer model to the heat pulse data we determine the soil thermal properties.

  13. Thermal properties

  14. Soil thermal properties

  15. Influences of soil texture, rb and q on l rb

  16. Calculation of Volumetric Heat Capacity This equation can be used to estimate soil b or q with the heat-pulse technique.

  17. Factors Influencing Soil rc For mineral soils, rc increases linearly with q

  18. Thermo-TDR Water Content

  19. Upstream needle Heater Downstream needle 1 cm • Heat pulse measurements for estimating soil liquid Water Flux

  20. Heat transfer equations • The governing heat transfer equation is where J is the water flux [volume / (time x area)]

  21. A solution to heat transfer equation (Ren et al., 2000)

  22. The ratio of downstream and upstream T increase The relationship between water flux and the temperature ratio is very simple (Wang et al., 2002) When

  23. Temperature ratio is constant

  24. Measured heat pulse signals Sand

  25. Heat pulse signals converted to Td/Tu Sand

  26. Heat pulse flux estimates versus imposed unsaturated fluxes

  27. A Heat Pulse Technique for Estimating Soil Water Evaporation

  28. Sensible heat balance provides a means to determine latent heat (LE) used for evaporation. = 0 no evaporatio n LE =(H1 – H2) –DS > 0 evaporatio n < 0 condensation LE Sensible heat flux in, H1 Sensible heat storage change DS Sensible heat flux out, H2 Basic theory of HP method:

  29. 1 dT/dz1, H1 2 l1, C1, 3 DS dT/dz2, l2, C2, H2 T1 T3 T2 Soil layer Determining the dynamic soil water evaporation Heat-pulse sensor LE = (H1 – H2) –DS Sensible soil heat flux:H =-(dT/dz) Change in sensible heatstorage:ΔS = C (ΔZ) (dT/dt)

  30. 7cm Heat-pulse sensors arrangement. Six sensors were installed within the top 7 cm of the soil profile.

  31. T (C) C (MJ/m3 C) C Temperature (T ); Heat capacity (C) and thermal conductivity(λ) λ( W/ mC ) Day of year 2007

  32. Heat fluxes at 3 and 9 mm (H1,H2); heat storage change (∆S) at soil layer (3~9 mm) H and ∆S (W/m2) Day of year 2007

  33. 3~9 mm 1st depth 9~15 mm 2nd 15~21 mm 3rd 21~27 mm 4th Evaporation dynamics measured by heat pulse method Evaporation (mm/hr) 174 175 176 177 178 179 180 Day of year 2007

  34. Comparison of daily soil water evaporation (mm) from heat pulse with micro-lysimeters and Bowen ratio methods HP daily evaporation (mm) Micro-lysimeters daily evaporation (mm)

  35. Latent Heat in Soil Heat Flux Measurements

  36. Better Energy Balance Closure When the latent heat flux (LE) includes evaporation from soil, the depth at which we measure soil heat flux (G) is critical to accurately representing the surface energy balance. Objective: Characterize variations in G with depth near the soil surface.

  37. Materials and Methods Soil heat flux (G) measured via heat-pulse sensors installed at 3 depths: 1, 3, and 6 cm G = -l(DT/Dz)

  38. side view soil surface heat-pulse sensor 1 cm 3 cm 6 cm cutaway view

  39. Materials and Methods Evaporation (LE) determined via microlysimeters (per 24 h)

  40. Cumulative Soil Heat Flux at 1-cm Depth

  41. ‘G’ measured above the drying front isn’t really G – its G + LE.

  42. Accumulated Energy

  43. Conclusions • Shallow soil heat flux measurements may capture G + (soil-originating) LE • Leads to ‘double accounting’ for LE in energy balance closure based on above-ground measurements Recommendation:G must be measured at a depth below the expected penetration of the drying front (here, possibly as deep as 6 cm) in order to treat the surface energy balance as Rn – G = LE + H

  44. HP sensors installed in a corn field in 2009 In-row Bare soil Between-rows with roots Between-rows without roots

  45. Soil temperature at different locations Temperature (˚C) 240 242 244 246 248 250 252 254 256 258 240 242 244 246 248 250 252 254 256 258 Day of year 2009

  46. Soil water evaporation dynamics Evaporation (mm) 240 242 244 246 248 250 252 254 256 258 240 242 244 246 248 250 252 254 256 258 Day of year 2009

  47. Cumulative soil water evaporation at 3-mm soil depth Cumulative Evaporation (mm) Day of year 2009

  48. E Fu Water storage change DS Fd For a soil layer, ΔE is the evaporation rate (cm/h), Ft and Fb are the liquid water flux (cm/h) at top and bottom boundaries, and ΔS is the change in water storage (cm/h).

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