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Measurement of Near-Surface Soil Heat Storage in Energy Balance Studies

Measurement of Near-Surface Soil Heat Storage in Energy Balance Studies. T.E. Ochsner 1 T.J. Sauer 2 R. Horton 3 1 USDA-ARS St. Paul, MN 2 USDA-ARS Ames, IA 3 Iowa State University. Soil heat storage in the energy balance. G 0 = heat flux at the soil surface.

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Measurement of Near-Surface Soil Heat Storage in Energy Balance Studies

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  1. Measurement of Near-Surface Soil Heat Storage in Energy Balance Studies T.E. Ochsner1 T.J. Sauer2 R. Horton3 1 USDA-ARS St. Paul, MN 2 USDA-ARS Ames, IA 3 Iowa State University

  2. Soil heat storage in the energy balance G0 = heat flux at the soil surface Gr = heat flux at the reference depth zr DS = rate of change of heat storage in the soil above the reference depth

  3. Is DS negligible? Neglecting DS causes underestimates of the magnitude of G. Neglecting DS delays peaks in G.

  4. Why not make zr shallow? • Potential LE error up to 100 W m-2 (Buchan) • Obstruction of water and heat transfer • Plastic disks buried at 2 cm increased water content and temperature gradients 200 to 300%. • Greater risk of poor thermal contact due to soil drying or freezing

  5. DS depends on C and T • C = soil volumetric heat capacity • T = soil temperature • A diversity of ways to determine C in current practice • Objective to compare methods • Estimation by soil sampling (CSS) • Estimation by Theta Probe (CTP) • Direct measurement by heat pulse sensors (CHP)

  6. Simultaneous C measurements When C was determined using all three techniques simultaneously, the results agreed to within 6% on average.

  7. Temporal variability in C • Temporal variability of C was best recorded with the heat pulse sensors • Sampling three times per week was not frequent enough to consistently record the temporal variations in C of the near-surface soil.

  8. Spatial variability in C Both soil sampling and heat pulse sensors are suitable for describing variations of C with depth in the top few centimeters of the soil.

  9. Definition: Simplification: Discretization: Including dC/dt: Calculating DS X

  10. Hourly DS values The neglect or inclusion of dC/dt had larger effects on DS than did the measurement technique used to determine C.

  11. Effects of neglecting dC/dt • Large underestimates of DS during infiltration • 246 W m-2 at the bare soil site • 282 W m-2 at the soybean site • 90 W m-2 at the corn site • Small persistent overestimates of DS during drying

  12. Recommendations • In long-term surface energy balance studies C determinations should be automated and frequent. • Heat pulse sensors or dielectric sensors should be used to perform these frequent, automated C determinations. • In short-term studies where frequent site visits are acceptable, determination of C by soil sampling or by use of the Theta Probe should be considered. • DS should be measured in all surface energy balance studies.

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