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

干旱地区的供水有限，给水资源管理带来了独特的挑战。 这些挑战随着人口增长和气候变化而增加。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温度的持续上升而变化非常重要。在美国西部，气候通常是干燥的。水从地表流失到大气层是各地水循环的一部分，但水的流失在干旱的生态系统中尤为重要。陆地向大气中流失水分有两种主要方式：（1）从土壤中直接蒸发;（2）通过蒸腾作用，这是一种生物过程，植物通过其组织将水分转移到空气中。这会影响土壤中残留的水量，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研讨会，向科学家和自然资源管理人员介绍了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有关的课程计划。他们将为北方亚利桑那大学和新墨西哥州大学的研究生和本科生创造实践研究经验。这项研究将集中在美国西南部半干旱生态系统中的水分流失。这项工作的中心是将蒸散量（ET）分为其组成部分（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）蒸发和蒸腾在生长季节的不同时间主导蒸散（ET），并提供了在不同时间哪些过程是重要的见解，（2）蒸发和蒸腾对环境驱动因素的不同组合敏感，生态系统内和生态系统间的ET各组分与其环境驱动因子之间的时间关系各不相同;（3）过去的环境条件对ET的控制很重要，尤其是蒸腾组分，在更干旱的地方，过去的条件的重要性被放大了。将蒸散量分为蒸发量和蒸腾量可以采用多种方法，但不同的方法可能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，然后使用贝叶斯统计方法调和矛盾的结果。他们将依赖于现有的ET和液流（蒸腾作用的代理）测量，土壤，茎和大气中的水的同位素的新测量，以及建模技术的组合来划分ET，以确定重要的驱动程序，并评估这些驱动程序控制ET的时间尺度。这些结果将被纳入现有的过程为基础的模型（SOILWAT），以评估其对整个网站的生态系统水平的水预算的影响，并模拟不同的环境条件对当地水预算的影响。研究生和本科生将积极参与该项目的现场，实验室和分析工作。此外，该小组将把通过该项目开发的工具纳入科学家和自然资源管理人员的讲习班。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。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.