Date of Award
Doctor of Philosophy
Coupled soil heat and water processes are critical for terrestrial life at all scales. Yet detailed understanding of these processes is limited. Inability to measure fine-scale, transient, one-dimensional (1-D) heat and water redistribution encumbers laboratory and field experiments and restricts testing of theory. The impetus for this work is to strengthen understanding of soil heat and water processes through improved measurement. Objectives were to (1) Develop closed soil cells with 1-D, non-isothermal conditions; (2) Measure soil temperature, water content, and thermal conductivity distributions under transient, 1-D conditions; (3) Test diffusion-based coupled heat and water transfer theory; and 4) Measure in situ soil water evaporation under dynamic field conditions. Soil-insulated, closed soil cells were developed to achieve 1-D conditions. These cells provided a 1:0.02 ratio between intended axial and unintended radial temperature gradients. The cells were instrumented with thermo-TDR sensors to measure transient temperature, water content, and thermal conductivity for two soils (sand and silt loam), two initial moistures, and ten boundary temperature gradients. Thermo-TDR water content measurements provided root mean square error (RMSE) <0.02 m3 m-3 versus gravimetric measurements. Co-located inflection points in temperature, water content, and thermal conductivity distributions indicated heat and water redistribution consistent with coupled transfer. These data were used to calibrate and test transfer theory. Adjustment of calculated vapor and liquid fluxes via the vapor enhancement factor and saturated hydraulic conductivity, respectively, reduced RMSE by an average of 36% for water content and temperature. Predictions from calibrated theory agreed with measurement when boundary and initial conditions changed gradually, but showed more disparity for drastic changes in boundary temperature conditions. In the field, a measurement-based soil heat balance was used to track the transient evaporation zone within the soil. Heat-pulse sensors measured soil temperature and thermal properties under bare surface conditions during multiple natural wetting/drying cycles. The heat-balance approach revealed the diurnal and inter-diurnal pattern of in situ soil water evaporation. Comparison of the heat balance approach to independent evaporation estimates gave RMSE of 0.11 mm d-1. Overall, the experiments demonstrated the utility of improved measurement for describing coupled soil heat and water processes.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Joshua L. Heitman
Heitman, Joshua L., "Measurement of coupled soil heat and water processes" (2007). Retrospective Theses and Dissertations. 15947.