Collaborative Research: From Roots to Rock - Linking Evapotranspiration and Groundwater Fluxes in the Critical Zone

水循环描述了水在地表上、地表上和地表下的运动，并确定了水存在的位置。通量量化了地下水、地表水和大气水蒸气等水库之间的水运动速率。蒸散发是支持地表到大气水通量的主要机制；它是地表水体蒸发和植物从土壤中吸收水分并从叶片表面蒸发的蒸腾作用的综合作用。美国国家研究委员会（National Research Council）已经确定，理解蒸发蒸腾和地下水通量之间的相互联系，是当今水文学家面临的最重要挑战之一。该项目解决了一个关键的知识缺口，即地下水储存如何调节蒸散发和地下水动力学（如地下水位高度和流速）之间的联系。阐明蒸发蒸腾和地下水补给之间的联系可能会限制灌溉农业和含水层抽水，这与社会对食物和水的需求直接相关。这项研究集中在俄勒冈州的一个长期研究地点。根据太平洋西北地区的全球气候模型，长期蒸散作用对地下水造成的压力可能对下游社区的水供应越来越重要。这项工作的结果将被纳入本科课程。未被充分代表的本科生将在整个项目中参与和指导。已经开发了两个概念模型来解释蒸散-基流相互作用-河岸截流和水力泵-但它们对临界区模型的影响尚未探索。该项目将通过同位素测量和地下成像来测试这些概念模型。该项目将(1)利用土壤和树木木质部水分同位素的时空变化来研究蒸腾、地下水和溪流之间的地下联系；(2)经风化腐岩进入未风化临界带的地下含水率图像变化；(3)评估地下性质和前向湿度对蒸散发信号向河流传递的重要性。该研究将提供新的贡献：(1)确定地下水文响应与山坡尺度树木生理过程耦合的机制；(2)整合实时观测、同位素分析和地球物理方法，以确定蒸发蒸腾-地下水相互作用如何随空间、时间和之前的湿度变化。这些结果可以改变对地表水、地下水和土壤水分之间相互作用的理解，以及植被动态控制陆地生态系统多尺度水文功能的作用。

The water cycle describes the movement of water on, above, and below Earth's surface and establishes where water exists. Fluxes quantify the rate of water movement among reservoirs such as groundwater, surface water, and atmospheric water vapor. Evapotranspiration is the primary mechanism supporting the surface-to-atmosphere water flux; it is the combined effect of evaporation from surface-water bodies and transpiration by plants drawing water from the soil and evaporating it from leaf surfaces. The National Research Council has identified understanding the interconnections between evapotranspiration and groundwater fluxes to be one of the most important challenges facing hydrologists today. This project addresses a critical knowledge gap in how subsurface water storage mediates the connection between evapotranspiration and groundwater dynamics such as water-table elevation and flow rates. Elucidating the connections between evapotranspiration and groundwater recharge that may limit irrigation agriculture and aquifer pumping is directly relevant to societal needs for food and water. The study focuses on a long-term research site in Oregon. Based on global climate models of the Pacific Northwest, stresses placed on groundwater by prolonged evapotranspiration are likely to become increasingly important to water availability for downstream communities. Results from this work will be incorporated into undergraduate curriculums. Underrepresented undergraduates will be engaged and mentored throughout the project.Two conceptual models have been developed to explain evapotranspiration-baseflow interactions - riparian interception and hydraulic pumping - but their implications for critical zone models have yet to be explored. This project will test these conceptual models through isotopic measurements and subsurface imaging. The project will (1) use the temporal and spatial change in soil and tree xylem water isotopes to examine subsurface connections between transpiration, groundwater and streamflow; (2) image changes in moisture content in the subsurface through the weathered saprolite and into the unweathered critical zone; and (3) assess the importance of subsurface properties and antecedent moisture on the transfer of the evapotranspiration signal to the stream. The research will provide novel contributions by (1) identifying the mechanisms by which subsurface hydrological responses are coupled to tree physiological processes at the hillslope scale and (2) integrating real-time observations, isotopic analysis, and geophysical approaches to identify how evapotranspiration-groundwater interactions vary with space, time, and antecedent moisture. These results can transform the understanding of the interactions among surface water, groundwater, and soil moisture and the role of vegetation dynamics controlling the multi-scale hydrological functioning of terrestrial ecosystems.