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

水循环描述了水在地球表面上、上面和下面的运动，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移动速率。蒸发蒸腾是支持地表到大气水分通量的主要机制，它是地表水体蒸发和植物从土壤中汲取水分并从叶表面蒸发的综合作用。国家研究理事会已经确定，了解蒸散和地下水通量之间的相互关系是当今水文学家面临的最重要挑战之一。该项目解决了地下水储存如何协调蒸散作用和地下水动态，如地下水位高度和流量之间的联系的关键知识空白。阐明可能限制灌溉、农业和含水层抽水的蒸散和地下水补给之间的关系，直接关系到社会对粮食和水的需求。这项研究的重点是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对下游社区的水资源供应可能变得越来越重要。这项工作的结果将被纳入本科课程。代表不足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-河岸截留和水力抽水-但它们对临界区模型的影响还有待探索。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同位素的时间和空间变化来研究蒸腾作用、地下水和径流之间的地下联系；(2)通过风化腐泥岩并进入未风化的临界区的地下含水率的图像变化；(3)评估地下特性和以前的湿度对向河流传递蒸散信号的重要性。这项研究将通过以下方式提供新的贡献：(1)确定地下水文响应与山坡尺度上树木生理过程相耦合的机制；(2)结合实时观测、同位素分析和地球物理方法，确定蒸散-地下水相互作用如何随空间、时间和先前湿度而变化。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.