MEASURING SOIL WATER FLUXES DUE TO EVAPORATION AND FREEZING

Robert Horton，爱荷华州立大学joshua Heitman，北卡罗莱纳州立大学传热和相关的温度变化是水文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详细的、精细的土壤热特性、温度和含水量测量。结合能量和质量守恒，这些测量可以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。研究将检验一个关于蒸发过程的中心假设，即蒸发锋的深度是由到达蒸发锋的液态水通量的大小控制的，同时考虑到热量和水的传递。这一假设将通过在一系列表面边界条件和两种土壤类型下使用热tdr装备的土壤柱进行一系列非等温实验室实验来评估。液态水和水蒸气通量剖面、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热量和水分传递的前所未有的信息。同时，研究将解决第二个关于土壤冻结量化的假设，即基于测量的能量和水相结合的平衡可以准确表征部分冻土冻结锋的土壤冻结率、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施加限制的数值实验，以及旨在适应冰点附近温度的热性能测量的微观实验和反数值分析来解决。结合这些研究的结果，将在一系列冻土柱系统中对这一假设进行测试，这些系统配备了热- tdr传感器，以测量土壤水和冰的含量，以及相关的热和水通量。土壤水的阶段变化——蒸发/凝结和冻结/融化——驱动水循环，并决定整个陆地环境中发生的生物、化学和物理过程的水可用性。这些阶段的变化也将水和能源预算紧密结合在一起。它们既包括感热和潜热传递，也包括液态水和水蒸气通量。迄今为止，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。本研究将使用新开发的精细仪器仔细检查水和能量预算，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地表过程遥感的能力，以及直接应用于理解碳和微量气体传输和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.