Heat transfer, evaporation and carbon dioxide transfer in soil
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Latent heat flux associated with soil water evaporation connects the surface water balance with the surface energy balance. Soil water evaporation and soil carbon dioxide (CO2) fluxes both involve soil gas transport processes and properties, and both impact the soil environment and physical, chemical, and biological processes occurring in the soil. Accurate and dynamic measurements of soil water evaporation and soil CO2 fluxes enhance the understanding of water, energy, and carbon partitioning at the soil-atmosphere interface and the mechanisms of mass and energy movement in the soil. Most previous work focused on measurements made above the soil surface, and quantitative determinations of in situ water evaporation and carbon dioxide fluxes within soil profile were absent. The objectives of this dissertation were to accurately determine transient soil water evaporation and soil CO2 fluxes with depth in bare soil and in different management zones of a corn field and to evaluate in situ measurement techniques.
Three-needle heat pulse sensors were used to measure subsurface soil water evaporation at depths of 3 mm and below in a bare field. The daily evaporation estimated from the heat pulse method agreed well with the daily evaporation estimated from Bowen ratio and micro-lysimeter methods. The results showed that heat pulse sensors alone could accurately determine subsurface soil water evaporation with time and depth, and surface and subsurface evaporation could be accurately determined with heat pulse measurements combined with Bowen ratio measurements in a bare field. Newly designed 11-needle heat pulse sensors were used at the following locations within a corn field: within-row (ROW), between-rows with roots (BR), and between-rows without roots (BRNR). The findings showed that heat pulse sensors measured the dynamic soil water evaporation at the three locations. The daily heat pulse evaporation estimates agreed well with micro-lysimeter measurements of daily soil water evaporation at ROW and BR in the corn field. In addition to heat pulse measurements for soil water evaporation, plant transpiration and evapotranspiration (ET) were measured using stem flow gauges and an eddy covariance system in the corn field. The evapotranspiration estimated from the sum of heat pulse evaporation and stem flow transpiration (E+T), eddy covariance ET, and potential evapotranspiration, ET0, estimated from the Penman-Monteith equation had similar trends. ET0 was larger than the individually measured E+T and eddy covariance ET. The individually measured E+T and ET0 had similar values but eddy covariance measurements underestimated ET.
Bare soil CO2 fluxes were determined using a concentration gradient method with in situ measured soil CO2 concentrations and model estimated gas coefficients during natural wetting and drying periods. Results showed that CO2 fluxes decreased with depth and most of the CO2 was produced at shallow soil depths. CO2 fluxes decreased with depth from 0 to 90 mm, and kept stable at depths of 90 to 200 mm. The gradient method determined CO2 fluxes agreed well with surface closed-chamber measured CO2 fluxes. For 10 out of 12 days the daily mean gradient CO2 flux values were within the ranges of the closed-chamber CO2 fluxes values during a soil drying period.
The conclusions of the dissertation were that the heat pulse sensors were able to accurately determine soil water evaporation with time and depth in a bare field and in different soil management zones in a corn field. Soil CO2 fluxes and soil CO2 production rates with depth in a bare field were accurately determined using a concentration gradient method with in situ CO2 concentration profiles. These simultaneous soil water evaporation and soil CO2 flux measurements could serve as a foundation for testing the numerical models of coupled heat, water, and gas transfer in soil, and could enhance further understanding of the complex soil system, and could guide the management of soil properties and processes.