1. W1188 Progress Report Tyson E. Ochsner USDA-ARS St. Paul, MN Jan. 3, 2005

2. The surface energy balance G r = heat flux at the reference depth z r  S = rate of change of heat storage in the soil above the reference depth G 0 = heat flux at the soil surface

3. Methods for measuring soil heat flux 1.Plate method  Most common by far  Thermopile measures vertical temperature difference across a plate embedded in the soil  Flux through the plate may not equal flux through the soil

5. Methods for measuring soil heat flux 1.Plate method  Most common by far  Thermopile measures vertical temperature difference across a plate embedded in the soil  Flux through the plate may not equal flux through the soil 2.Gradient methods  Rarely used  Soil temperature gradient and thermal conductivity measured  In situ thermal conductivity measurements are challenging

6. Leads Epoxy Thermocouple Resistance heater SS tubing 6 mm 1.3 40 mm

7. Methods for measuring soil heat flux 1.Plate method  Most common by far  Thermopile measures vertical temperature difference across a plate embedded in the soil  Flux through the plate may not equal flux through the soil 2.Gradient methods  Rarely used  Soil temperature gradient and thermal conductivity measured  In situ thermal conductivity measurements are challenging 3.Self-calibrating plate method  Relatively new  Heater on top of plate permits in situ correction for heat flux divergence  Large plate and unpublished theory

10. Flux estimates from HFT1.1 plates ~25% smaller than from three-needle gradient method. Bare soil site – 2001 Two independent gradient methods gave similar flux estimates.

11. Corn site – 2002 Flux estimates from HFT1.1 plates ~25% smaller than from three-needle gradient method. Two independent gradient methods gave similar flux estimates.

13. Soybean site – 2004 Flux estimates from HFT1.1 plates ~25% smaller than from three-needle gradient method. Self-calibrating plates and three- needle gradient method gave similar flux estimates.

14. Bare soil site – 2001 Flux estimates by the HFT1.1 plates were better than those from the other three types we tested. Regression statistics for plate versus three needle gradient heat flux estimates. PlateSlopeInterceptr2r2 Mean absolute difference W m -2 HFT1.10.7512.280.90814.2 CN30.6401.820.91317.4 GHT-1C0.5223.490.85222.8 6100.3281.190.91229.4

15. Conclusions Heat flux plates typically underestimate the magnitude of soil heat flux. The best of the four types of plates we tested underestimated the flux by about 25%. The three-needle gradient method and the self- calibrating plate method are promising alternatives. Further development of these methods are needed for frozen soil conditions.

16. Collaborators Thomas J. Sauer, USDA-ARS, National Soil Tilth Laboratory, Ames, IA Robert Horton, Dep. of Agronomy, Iowa State University, Ames, IA