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

干旱地区有限的用水机会给水资源管理带来了独特的挑战。这些挑战随着人口的增长和气候的变化而增加。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气温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所有地方水循环的一部分，但在干燥的生态系统中，水分损失尤为重要。陆地向大气流失水分的主要方式有两种：(1)直接从土壤蒸发；(2)通过蒸腾作用，这是活着的植物将水分通过组织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。该团队旨在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分析该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相关的教案。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。这项研究将专注于美国西南部半干旱生态系统的水分损失。工作中心是将蒸散(ET)划分为其组成部分(蒸发和蒸腾)。它将在该区域常见的六个半干旱生态系统中检验三个主要假说：(1)蒸发和蒸腾在生长季节的不同时间主导蒸散(ET)，并提供关于哪些过程在不同时间是重要的洞察；(2)蒸发和蒸腾对不同环境驱动因素的组合很敏感，ET的这些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和生态系统之间有所不同；(3)过去的环境条件对ETs的控制很重要，特别是蒸腾作用，过去条件的重要性在更干旱的地点被放大。将ET分解为蒸发组分和蒸腾组分可以用不同的方法来完成，但不同的方法会导致相互矛盾的结果。因此，在每个生态系统中，研究人员将采取创新的方法，使用两到五种方法来划分蒸发和蒸腾作用，然后使用贝叶斯统计方法协调相互矛盾的结果。他们将依靠现有的ET和树液流量(蒸腾作用的替代)测量的组合，土壤、茎和大气中水中同位素的新测量，以及划分ET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)，以评估它们对各个地点的生态系统水平水收支的影响，并模拟环境条件变化对当地水收支的影响。研究生和本科生将在该项目的现场、实验室和分析工作中发挥积极作用。此外，该团队将把通过该项目开发的工具整合到科学家和自然资源管理人员的研讨会中。最后，他们将把我们对蒸散驱动因素的更好理解纳入亚利桑那州弗拉格斯塔夫当地中学生的课程计划中。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.