MEASURING SOIL WATER FLUXES DUE TO EVAPORATION AND FREEZING

Robert Horton, Iowa State University

Joshua Heitman, North Carolina State University

Heat transfer and associated temperature variations are fundamental drivers of water phase changes within the hydrologic cycle. Yet, our understanding of soil water phase changes in the presence of temperature gradients remains limited. Newly-developed instrumentation provides detailed, fine-scale measurements of soil thermal properties, temperature and water content. Combined with conservation of energy and mass, these measurements allow calculation of in situ latent heat sinks and soil water fluxes, thus revealing both time and depth dynamics of soil water phase change. Research will test a central hypothesis about the evaporation process, that the depth of the evaporation front is controlled by the magnitude of the liquid water flux to the front, within a context considering both heat and water transfer. This hypothesis will be evaluated through a series of non-isothermal laboratory experiments using thermo-TDR equipped soil columns for a series of surface boundary conditions and two soil types. Measurements of liquid water and water vapor flux profiles, soil surface temperature conditions, and mass balance will offer unprecedented information about both heat and water transfer occurring with soil water evaporation. Concurrently, research will address a second hypothesis about quantifying soil freezing, that a combined measurement-based energy and water balance can accurately characterize the rate of soil freezing, ice contents, liquid water contents and liquid water fluxes at the freezing front in partially frozen soil. This hypothesis will first be addressed through numerical experiments aimed at testing system-imposed limitations, and microcosm experiments and inverse numerical analysis aimed at adaptation of thermal property measurements for temperatures near the freezing point. Incorporating findings from these studies, the hypothesis will be tested in a series of freezing soil column systems, instrumented with thermo-TDR sensors to measure soil water and ice contents, and associated heat and water fluxes.

Soil water phase changes -- evaporation/condensation and freezing/thawing -- drive the hydrological cycle and determine water availability for biological, chemical, and physical processes occurring throughout the terrestrial environment. These phase changes also tightly couple water and energy budgets. They involve both sensible and latent heat transfer, and both liquid water and water vapor fluxes. To date, understanding of the interplay of water and energy fluxes with soil water phase changes remains extremely limited. This research will carefully examine water and energy budgets together using newly developed, fine-scale instrumentation in order to improve understanding of the hydrologic cycle, soil water evaporation, and soil freezing. Implications for this research include improved capabilities for land-surface modeling and remote sensing of surface processes, as well as direct application to understanding carbon and trace gas transmissions and myriad other biogeochemical processes.