Determining Soil Water Evaporation and Subsurface Evaporation Zones

土壤蒸发在很大程度上决定了陆地生态系统中的水分供应，以及太阳辐射在显热和潜热之间的分配。它是水文和气候的关键。蒸发过程是复杂的，涉及水的运动和相变，随深度和时间而变化。水输入后，蒸发发生在土壤表面，由大气需求控制。随着表层土壤水分的耗尽，蒸发变得仅限于土壤，并转移到地表以下;尽管如此，它通常被视为严格的地表过程。因此，测量方法和对这些近地表现象的了解落后于对准确数据的需求。目前的许多研究强调大规模的面积估计土壤水分和温度，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低。这种认识不足主要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新的方法来测量土壤中的蒸发。最近开发的传感器和概念使我们能够量化毫米级近地表土壤层的显热传递，以及每层内存储的显热的变化。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）感热平衡法可以准确地估算出土壤表层蒸发的水分质量;（2）热平衡法可以推广到确定土壤表层（0-3 mm）的潜热通量;（3）通过热质平衡，（4）感热平衡法可以将蒸散量定量地划分为蒸发量和蒸散量。假设1-3将通过实验室和田间试验进行检验，假设4仅通过田间试验进行检验。实验室实验将测量土壤的热性能，水含量，和水通量的组合下2能源制度，3表面条件，和3土壤。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。在田间试验中，蒸发和蒸腾的独立测量将允许严格测试蒸腾和土壤水分蒸发的热平衡估计，所提出的工作的智力价值是一个新的基于测量的方法来量化土壤水分蒸发。拟议的研究通过开发和测试新型传感器和分析的原位土壤水分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。这项研究将量化土壤表面和土壤表面以下的土壤水分蒸发观测。这代表了一个显着的进步，描述蒸发作为一个表面只有过程。拟议的工作进行更广泛的影响，提供教育，科学和社会的机会。基本经验是为早期职业科学家，研究生（包括少数民族学生谁是NSF AGEP研究员）和本科生提供。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。实现项目目标将大大提高我们对关键区基本特性和过程的理解，实现更好的环境监测和管理，并加强我们对大规模水文和气候动态的预测。

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.