MEASURING SOIL WATER FLUXES DUE TO EVAPORATION AND FREEZING

罗伯特·霍顿，爱荷华州立大学北卡罗来纳州立大学的约书亚·海特曼热传递和相关的温度变化是水循环中水相变化的基本驱动因素。然而，我们对存在温度梯度的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性质、温度和水分的详细、精细的测量。结合能量守恒和质量守恒，计算了土壤潜热汇和土壤水分通量，从而揭示了土壤水分相变的时间和深度动态。研究将检验关于蒸发过程的一个中心假设，即在同时考虑热量和水转移的情况下，蒸发锋面的深度由流向锋面的液态水通量的大小控制。这一假设将通过一系列非等温实验室实验来验证，该实验使用了配备热TDR的土柱，用于一系列地表边界条件和两种土壤类型。液态水和水蒸气通量分布、土壤表面温度条件和质量平衡的测量将提供有关土壤水分蒸发过程中发生的热量和水分转移的前所未有的信息。同时，研究将解决关于量化土壤冻结的第二个假设，即基于联合测量的能量和水平衡可以准确地表征部分冻结土壤中冻结前沿的土壤冻结速率、冰含量、液态水含量和液态水通量。这一假设将首先通过旨在测试系统强加的限制的数值实验，以及旨在适应冰点附近温度的热物性测量的微观实验和反向数值分析来解决。结合这些研究的结果，这一假设将在一系列冻结的土柱系统中得到验证，这些系统配备了温度-TDR传感器来测量土壤水分和冰的含量，以及相关的热量和水通量。土壤水的相变化--蒸发/凝结和冻结/融化--驱动着水文循环，并确定整个陆地环境中发生的生物、化学和物理过程的水的可用性。这些阶段的变化也将水和能源的预算紧密地联系在一起。它们既涉及感热输送，也涉及潜热输送，还涉及液态水和水汽通量。到目前为止，对水分和能量通量与土壤水相变化的相互作用的了解仍然非常有限。这项研究将使用新开发的精细仪器仔细检查水和能量的收支，以便更好地了解水文循环、土壤水分蒸发和土壤冻结。这项研究的意义包括改进陆面建模和地表过程遥感的能力，以及直接应用于了解碳和痕量气体传输以及无数其他生物地球化学过程。

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.