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Articles by J. L. Heitman
Total Records ( 2 ) for J. L. Heitman
  T. M. DeSutter , T. J. Sauer , T. B. Parkin and J. L. Heitman
  Carbon dioxide concentrations in the soil can vary both temporally and spatially. Methodology was developed to semicontinuously measure subsurface concentrations of CO2 using expanded, porous Teflon (ePTFE) tubing. Lengths of ePTFE tubing (7.6 m) were buried at 0.02, 0.1, and 0.18 m below the soil surface in a Harps loam soil (fine-loamy, mixed, superactive, mesic Typic Calciaquoll) in central Iowa, and also positioned directly on the soil surface (0 m). Soil atmospheric gases that diffused through the walls of the tubing were circulated in a closed-loop design through solid-state CO2 sensors to determine the concentration of CO2 at each depth. Independent measures of CO2 concentrations were also determined by sampling the in-line gas stream of the ePTFE system and from samples extracted from gas wells positioned near the buried tubing. Good agreement (r2 > 0.95) was observed between the ePTFE system and the independent measures, with the ePTFE having biases of 1.2 and 1.37 times greater than the in-line and gas well samples, respectively. The soil-gas diffusion coefficient of CO2 (Ds) was determined using intact soil cores and values were about 2.5 times less than two popular models used to predict Ds in soil. Estimates of CO2 flux using Fick's Law, six approaches to determine the vertical CO2 concentration gradient, and three methods to determine Ds ranged from >800 to <1 µmol m–2 s–1 on Day of the Year 239.5. Although Fick's Law is commonly used to estimate CO2 flux from soil, the approach used to determine the vertical CO2 concentration gradient and method used to determine Ds can both include sources of uncertainty.
  J. L. Heitman , R. Horton , T. Ren , I. N. Nassar and D. D. Davis
  Diffusion-based coupled soil heat and water transfer theory includes capability to describe transient behavior. Unfortunately, laboratory tests of theory typically include a single initial water content distribution with a single set of boundary conditions, rather than providing a set of experimental conditions with a range of measurements for comparison with predictions. Agreement between theory and measurements can result from calibration, but this provides an incomplete test of theory. The objective of this work was to test diffusion-based coupled heat and water transfer theory by comparing theory-based predictions with measured transient temperature and water content distributions. Data from a single boundary condition were used for calibration of each of two soils, silt loam and sand. Subsequent testing was performed at additional boundary and initial conditions using measurements from the same soil. Results indicate that the theory can be calibrated for a single boundary condition with adjustment of soil saturated hydraulic conductivity and/or the vapor enhancement factor, which adjust the liquid and vapor fluxes, respectively. For silt loam, calibration reduced Root Mean Square Error (RMSE) by 67 and 18% for water content and temperature distributions, respectively. For sand, RMSE was reduced by 14 and 46% for water content and temperature, respectively. Using this calibration, there was agreement between calculated and measured distributions for additional boundary and initial conditions with RMSE ≤ 0.03 m3m–3 and 1.28°C for water content and temperature distributions, respectively. However, when the boundary temperature gradient was instantly reversed, noticeable differences occurred between measured and calculated patterns of heat and moisture redistribution. The theory described observations well when boundary temperature conditions were changed gradually, but results suggested a need for further development of coupled heat and water transfer theory combined with testing under transient conditions to make improvements in the description of transfer mechanisms.
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