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

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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.