MEASURING SOIL WATER FLUXES DUE TO EVAPORATION AND FREEZING

罗伯特霍顿，爱荷华州州立大学乔舒亚海特曼，北卡罗来纳州州立大学热传递和相关的温度变化是水文循环中水相变化的基本驱动力。然而，我们对温度梯度存在下土壤水相变化的理解仍然有限。新开发的仪器提供了对土壤热特性、温度和含水量的详细、精细的测量。结合能量守恒和质量守恒，这些测量允许计算原位潜热汇和土壤水通量，从而揭示土壤水相变的时间和深度动态。研究将测试关于蒸发过程的中心假设，即在考虑热量和水分转移的情况下，蒸发前沿的深度由流向前沿的液态水通量的大小控制。这一假设将通过一系列的非等温实验室实验进行评估，使用热TDR配备一系列的表面边界条件和两种土壤类型的土柱。测量液态水和水蒸气通量剖面、土壤表面温度条件和质量平衡将提供有关土壤水分蒸发发生的热量和水分转移的前所未有的信息。与此同时，研究将解决第二个假设量化土壤冻结，结合测量为基础的能量和水平衡可以准确地表征土壤冻结率，冰含量，液态水含量和液态水通量在冻结锋在部分冻结的土壤。这一假设将首先通过数值实验，旨在测试系统施加的限制，和微观实验和逆数值分析，旨在适应的温度接近冰点的热性能测量。为了证实这些研究的结果，将在一系列冻结土壤柱系统中测试这一假设，该系统装有热TDR传感器，以测量土壤水分和冰含量，以及相关的热量和水通量。土壤水分的相变-蒸发/冷凝和冻结/解冻-驱动水文循环，并确定生物，化学，以及整个陆地环境中发生的物理过程。这些相变也将水和能源预算紧密结合在一起。它们涉及感热和潜热的传递，以及液态水和水汽通量。到目前为止，对水和能量通量与土壤水相变化的相互作用的理解仍然非常有限。这项研究将仔细检查水和能源预算一起使用新开发的，精细规模的仪器，以提高对水文循环，土壤水分蒸发和土壤冻结的理解。这项研究的意义包括提高陆地表面建模和遥感表面过程的能力，以及直接应用于了解碳和痕量气体传输和无数其他地球化学过程。

Robert Horton, Iowa State UniversityJoshua Heitman, North Carolina State UniversityHeat 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.