Determining Soil Water Evaporation and Subsurface Evaporation Zones

确定土壤水蒸发和地下蒸发区

• 批准号: 0809656
• 负责人: Robert Horton
• 金额: $ 38.24万
• 依托单位: Iowa State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-10-01 至 2012-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809656&HistoricalAwards=false
• 关键词: Determining; Soil; Water; Evaporation; Subsurface

Evaporation from the soil largely determines both water availability in terrestrial ecosystems, and the partitioning of solar radiation between sensible and latent heat. It is key to both hydrology and climate. The evaporation process is complex, involving movement and phase change of water, varying with depth and time. Following water inputs, evaporation occurs at the soil surface, controlled by atmospheric demand. As surface soil water is depleted, evaporation becomes soil-limited and shifts below the surface; nonetheless it is generally viewed as a strictly surface process. As a result, measurement methods and understanding of these near-surface phenomena have lagged behind demand for accurate data. Much current research emphasizes large-scale areal estimates of soil moisture and temperature, but poor understanding of the soil water evaporation process causes low accuracy in water and energy balances. This poor understanding is largely due to our current inability to make the needed measurements.The purpose of the proposed research is to develop and test a new approach to measure evaporation within the soil. Recently developed sensors and concepts enable us to quantify sensible heat transferred into and out of mm-scale near-surface soil layers, as well as the change in sensible heat stored within each layer. Combined with conservation of energy, these measurements can locally quantify subsurface evaporation, showing the temporal patterns of in situ evaporation. Research will test four hypotheses: (1) that a sensible heat balance method can accurately estimate the mass of water evaporated from subsurface soil layers, (2) that the heat balance method can be extended to determine the latent heat flux from the soil surface layer (0-3 mm), (3) that through combined heat and mass balance, estimates of other hydrological components (transpiration and soil water flow) will be quantified or constrained, and (4) that the sensible heat balance method can quantitatively partition ET into evaporation and transpiration. Hypotheses 1-3 will be tested with both laboratory and field experiments, and Hypothesis 4 only by field experiments. Laboratory experiments will measure soil thermal properties, water content, and water flux under a combination of 2 energy regimes, 3 surface conditions, and 3 soils. Calculated evaporative loss via heat balance will be compared to evaporation measured by mass balance. In the field experiments, independent measurements of evaporation and transpiration will allow rigorous testing of heat balance estimates of transpiration and soil water evaporation.The intellectual merit of the proposed work is a new measurement-based methodology for quantifying soil water evaporation. The proposed research addresses current knowledge gaps by developing and testing in situ soil water evaporation measurement with novel sensors and analysis. Information obtained in the study will elucidate important evaporative processes. The research will quantify observation of soil water evaporation at and below the soil surface. This represents a notable advancement over descriptions of evaporation as a surface-only process.The proposed work carries broader impact by providing educational, scientific, and societal opportunities. Fundamental experience is provided for an early-career scientist, graduate students (including a minority student who is a NSF AGEP Fellow), and undergraduates. Results will be widely disseminated to the scientific community via website and published articles, and measurement techniques will have immediate repercussions for weather, climate, and environmental monitoring. Achieving the project goals will significantly improve our understanding of fundamental critical-zone properties and processes, enable better environmental monitoring and management, and enhance our predictions of large-scale hydrological and climate dynamics.

土壤蒸发在很大程度上决定了陆地生态系统中水的可用性以及太阳辐射在显热和潜热之间的分配。它对于水文和气候都很关键。蒸发过程很复杂，涉及水的运动和相变，随深度和时间的变化而变化。输入水后，蒸发发生在土壤表面，受大气需求控制。随着表层土壤水的耗尽，蒸发会受到土壤的限制并转移到地表以下；尽管如此，它通常被视为严格的表面过程。因此，测量方法和对这些近地表现象的理解已经落后于对准确数据的需求。目前许多研究强调对土壤湿度和温度进行大范围的区域估计，但对土壤水分蒸发过程的了解不足导致水和能量平衡的准确性较低。这种认识不足很大程度上是由于我们目前无法进行所需的测量。拟议研究的目的是开发和测试一种测量土壤内蒸发的新方法。最近开发的传感器和概念使我们能够量化传入和传出毫米级近地表土壤层的显热，以及每层中存储的显热的变化。结合能量守恒，这些测量可以局部量化地下蒸发，显示原位蒸发的时间模式。研究将检验四个假设：(1) 感热平衡方法可以准确估计从地下土壤层蒸发的水量，(2) 热平衡方法可以扩展以确定土壤表层 (0-3 mm) 的潜热通量，(3) 通过热质平衡，可以量化或约束其他水文成分（蒸腾作用和土壤水流量）的估计，以及 (4) 显热平衡法可以定量地将ET分为蒸发量和蒸腾量。假设1-3将通过实验室和现场实验进行检验，假设4仅通过现场实验进行检验。实验室实验将在 2 种能量状态、3 种表面条件和 3 种土壤的组合下测量土壤热特性、含水量和水通量。通过热平衡计算的蒸发损失将与通过质量平衡测量的蒸发进行比较。在现场实验中，蒸发和蒸腾的独立测量将允许对蒸腾和土壤水蒸发的热平衡估计进行严格的测试。所提出的工作的智力优点是一种新的基于测量的量化土壤水蒸发的方法。拟议的研究通过使用新型传感器和分析开发和测试原位土壤水蒸发测量来解决当前的知识差距。研究中获得的信息将阐明重要的蒸发过程。该研究将量化对土壤表面及以下土壤水分蒸发的观测。与将蒸发描述为仅表面过程相比，这代表了显着的进步。拟议的工作通过提供教育、科学和社会机会而产生更广泛的影响。为早期职业科学家、研究生（包括 NSF AGEP 研究员的少数族裔学生）和本科生提供基础经验。结果将通过网站和发表的文章广泛传播给科学界，测量技术将对天气、气候和环境监测产生直接影响。实现项目目标将显着提高我们对关键区域基本特性和过程的理解，实现更好的环境监测和管理，并增强我们对大规模水文和气候动态的预测。