Graduate Thesis Or Dissertation
 

Calculated and field-measured soil heat flux : a comparison under two soil water regimes

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  • Heat and water vapor flow in summer-fallowed fields in the Pacific Northwest may significantly affect the position and supply of water for germination of fall sown crops. New tillage tools and drill openers used in reduced tillage systems leave a trashy, residue covered surface in contrast to the traditional bare fallow systems. It is likely that these new management techniques cause changes in heat and water flow that produce unique profiles of soil temperature and water content. There is a need to elucidate heat and mass flow theory under field conditions appropriate to these new tillage systems. However, heat and mass flow theory has been studied almost exclusively in the laboratory using uniformly packed homogeneous soil. Heat flow theory in these experiments has generally consisted of two components: de Vries theoretical estimation of soil thermal conductivity and the Philip formulation of total heat flux consisting of heat flow responses to thermal and moisture gradients. The Philip-de Vries theory was field tested to determine if such theory could be used to project the water conservation aspects of reduced tillage systems. Soil heat flux was calculated using the theory of Philip-de Vries and then compared with soil heat fluxes experimentally determined in a Walla Walla silt loam using a null-alignment of soil temperatures and calorimetric heat flow. Comparisons at 20 depths in the top 60 cm of soil were made during mid-summer on no-till field plots with two soil moisture regimes. All components of heat flow including vapor heat flux terms were used in the theoretical calculations, but isothermal vapor flux accounted for less than 1% of the total vapor flux. Measured net daily heat flux at 0.25 cm was 22.3 and 21.7 cal cm⁻² day⁻¹ for the moist and dry treatments, respectively. Theoretical heat flux calculations at this same depth generally gave a tenfold overestimation of measured heat flux during those times of the day when large temperature gradients existed. At 9 cm, approximately the seeding depth of winter wheat, measured net daily heat flux was 14.5 and 20.6 cal cm⁻² day⁻¹ for the moist and dry treatments, respectively; respective theoretical fluxes were 15.7 and 32.8 cal cm⁻² day⁻¹. Calculated total fluxes in the top 35 cm generally overestimated by 20 and 40 percent, as compared to measured fluxes, for the moist and dry treatments, respectively. Theoretical heat flux vapor components were calculated as the difference between total and conduction heat fluxes. Soil water changes predicted by vapor flux theory agreed with measured seedzone diurnal water content changes on the dry plot. At 35 cm vapor fluxes were negligible. Heat flux by conduction at this same depth over estimated measured flux by 10% on both plots. At all depths theoretical heat flux by conduction was more seriously overestimated than the vapor components of heat flux. The Philip-de Vries heat flux model satisfactorily predicted water changes in dry soil seedbeds by vapor flux theory. Overestimation of the conduction component of the de Vries thermal conductivity produced an overestimation of calculated heat flux on both plots at all depths. Further calibration of the conduction components of the model will be needed to continue assessment of heat and vapor flux in dry seedbeds.
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