Collaborative Research: Ecohydrological controls on evapotranspiration across a semiarid elevation gradient

干旱地区获得水资源的机会有限，对水资源管理提出了独特的挑战。随着人口增长和气候变化，这些挑战也在增加。了解这些地区的水循环对于管理水资源和预测它们将如何随着温度继续上升而变化非常重要。在美国西部，气候通常是干燥的。从陆地表面到大气的水分流失是各地水循环的一部分，但水分流失在干旱生态系统中尤为重要。土地向大气中流失水分主要有两种方式：(1)土壤的直接蒸发；(2)通过蒸腾作用，即活的植物通过其组织将水分转移到空气中的生物过程。这会影响土壤中剩余的水量，从而影响干旱的严重程度。这也会影响该地区的降雨量和/或降雪量。这个团队的目标是更好地了解蒸发和蒸腾的环境驱动因素，以提高我们预测干旱地区水循环变化的能力。这项研究的更广泛影响包括两个讲习班，向科学家和自然资源管理人员介绍了分析本项目收集的数据类型的现代方法。研究人员还将与当地一名中学教师合作，制定与干旱地区水循环有关的课程计划。他们将为北亚利桑那大学和新墨西哥大学的研究生和本科生创造实践研究经验。本研究将重点研究美国西南部半干旱生态系统的水分损失，研究蒸散发（ET）分解为其组成部分（蒸发和蒸腾）。它将在该地区常见的六个半干旱生态系统中测试三个主要假设：(1)蒸发量和蒸腾量在生长季节的不同时期主导蒸散发（ET），并提供了在不同时期哪些过程是重要的信息；(2)蒸发量和蒸腾量对不同环境驱动因素的组合很敏感，蒸散发各组成部分与其环境驱动因素之间的时间关系在生态系统内部和不同生态系统之间存在差异；(3)过去的环境条件对控制蒸散发具有重要意义。特别是蒸腾成分，过去条件的重要性在更干旱的地点被放大。将蒸散发分解为蒸发量和蒸腾量的方法多种多样，但不同的方法可能导致结果相互矛盾。因此，在每个生态系统中，研究人员将采用创新的方法，使用两到五种方法来划分蒸发和蒸腾，然后使用贝叶斯统计方法来调和矛盾的结果。他们将依靠现有的蒸散发和液流（蒸腾的代表）测量的组合，土壤、茎干和大气中水分同位素的新测量，以及划分蒸散发的建模技术，以确定重要的驱动因素，并评估这些驱动因素控制ET的时间尺度。这些结果将被纳入现有的基于过程的模型（SOILWAT），以评估它们对整个站点的生态系统水平水收支的影响，并模拟不同环境条件对当地水收支的影响。研究生和本科生将积极参与本项目的实地、实验室和分析工作。此外，该团队将把通过该项目开发的工具整合到为科学家和自然资源管理者举办的讲习班中。最后，他们将把我们对蒸散发驱动因素的改进理解纳入亚利桑那州Flagstaff当地中学生的课程计划中。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Limited access to water in dry regions poses unique challenges to managing water resources. These challenges increase as human populations grow and climate changes. Understanding the water cycle in these regions is important for managing water resources and predicting how they will change as temperatures continue to rise. In the western U.S., the climate is generally dry. The loss of water from the land surface to the atmosphere is part of the water cycle everywhere, but water losses are particularly important in dry ecosystems. There are two major ways that the land loses water to the atmosphere: (1) direct evaporation from soil and (2) through transpiration, the biological process by which living plants move water through their tissues into the air. This affects the amount of water left in the soil, which impacts drought severity. This can also affect the amount of rain and/or snow that falls in the region. This team aims to better understand the environmental drivers of evaporation and transpiration to improve our ability to predict changes to the water cycle in dry regions. Broader impacts of this research include two workshops that introduce scientists and natural resource managers to modern methods for analyzing the types of data collected in this project. Researchers will also work with a local middle-school teacher to develop lesson plans related to the water cycle in dry regions. They will create hands-on research experiences for graduate and undergraduate students at both Northern Arizona University and University of New Mexico.This study will focus on water losses in semiarid ecosystems on the Southwestern U.S. The work centers on the partitioning of evapotranspiration (ET) into its components (evaporation and transpiration). It will test three main hypotheses across six semiarid ecosystems common to the region: (1) evaporation and transpiration dominate evapotranspiration (ET) at different times of the growing season, and provide insights into which processes are important at different times, (2) evaporation and transpiration are sensitive to different combinations of environmental drivers, and the temporal relationships between these components of ET and their environmental drivers vary within and between ecosystems, and (3) past environmental conditions are important for governing ET, especially the transpiration component, and the importance of past conditions is amplified at more arid sites. Partitioning ET into its evaporation and transpiration components can be accomplished using various methods, but the different methods can lead to conflicting results. Therefore, in each ecosystem, researchers will take the innovative approach of using two to five methods to partition evaporation and transpiration and then reconcile contradictory results using a Bayesian statistical approach. They will rely on a combination of existing ET and sap flow (a proxy for transpiration) measurements, new measurements of isotopes in water in soil, stems, and the atmosphere, and modeling techniques to partition ET, to identify important drivers, and to evaluate the timescales over which these drivers control ET. These results will be incorporated into an existing process-based model (SOILWAT) to evaluate their implications for ecosystem level water budgets across the sites and to simulate the impact of varying the environmental conditions on local water budgets. Graduate and undergraduate students will take an active role in conducting field, lab, and analytical work for this project. Further, the team will integrate the tools developed through this project into workshops for scientists and natural-resource managers. Finally, they will incorporate our improved understanding of drivers of evapotranspiration into lesson plans for local middle school students in Flagstaff, AZ.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.