A Comparison of Methods for Simulating Watershed Evapotranspiration and Runoff under Changing Climatic Conditions

Project ID: 2024
Principal Investigator: Michael Tansey
Research Topic: Water Operation Models and Decision Support Systems
Priority Area Assignments: 2013 (Climate Change and Variability Research)
Funded Fiscal Years: 2013
Keywords: evapotranspiration, watershed runoff, native vegetation, carbon dioxide, climate change

Research Question

The R&D office in collaboration with other Federal agencies identified knowledge gaps and research priorities for climate change assessment and adaptation planning. This proposal addresses the high priority gap -4.02 WH: Understanding how climate change should impact potential evapotranspiration, and how it is represented in watershed hydrologic models.

Along with rising global temperatures, increasing levels of atmospheric CO2 will affect both plant growth and ET and consequently watershed runoff. Extensive research has demonstrated that plant water use under elevated CO2 concentrations is a function of several counter-balancing processes including increased leaf area and biomass production contributing to higher ET and decreased stomatal conductance contributing to lower ET. Future warming will also cause growing season shifts and changes in length that will impact ET in several ways. For perennials, warmer spring and fall temperatures will extend the growth period and contribute to higher ET. For annuals, warmer temperatures may shorten the growth period resulting in reduced ET. For some plant species warmer temperatures may exceed their optimal growth range and contribute to ET reductions. Shifts in growing season may also result in changes in other meteorological conditions affecting plant growth and ET. Shifts into spring and fall seasons when solar radiation is lower may contribute to reduced ET and plant growth. Growing season shifts may correspond with periods of greater cloudiness further reducing solar radiation. While warmer temperatures may increase the saturation vapor pressure potentially causing higher ET, increased atmospheric humidity may offset this potential increase in the vapor pressure deficit. Shifts in growing seasons may also be accompanied by changes in wind speed, another factor directly affecting ET. Understanding and modeling these processes is necessary for assessing how climate change will impact both watershed ET and runoff.

Need and Benefit

Climate change will be one of the major challenges affecting the sustainability of Reclamation projects in the 21st century. Understanding how climate change will impact evapotranspiration (ET) and how it may represented in watershed hydrologic models was identified as a high priority knowledge gap (4.02 WH) in Reclamation's "Addressing Climate Change in Long-Term Water Resources Planning and Management, User Needs for Improving Tools and Information" report(January, 2011).

In Reclamation's recently released "Secure Water Act Section 9503(C) - Reclamation Climate Change and Water 2011" report, a consistent, detailed analysis of the effects of potential climate changes on water supplies for major Reclamation basins was performed. However, the effects of increasing concentrations of atmospheric carbon dioxide on watershed ET and runoff were not included. Furthermore, improved methods developed under a WaterSMART grant to the Desert Research Institute (DRI) for estimation of the solar radiation, atmospheric humidity, and wind speed terms in the Penman-Monteith equation were not available for the analysis.

This research project will address these gaps in several ways. A comprehensive review of previous studies and an assessment of existing methods and models for quantifying the effects of climate change on watershed ET and runoff will be performed. The review component will include a thorough review of existing scientific literature on climate-vegetation interactions, evaluation methods, and existing models used for simulating evapotranspiration and growth of native vegetation present in upland watersheds throughout the western United States. Special emphasis will be given to the effects of CO2 as uncertainties associated with its importance have been expressed in some studies. The results of the literature review and database development performed in Task 1 will directly benefit Reclamation programs by providing a comprehensive reference to studies, data, methods and models that can be used in future climate change studies.

This research will also benefit Reclamation's climate change modeling efforts in several other ways. By selecting several watersheds where comparisons between the Variable Infiltration Capacity (VIC) model used for Reclamation's West-wide Climate Risk Assessment and the CV_WEAP, ParFlow and WEAP_DSSAT can be made, inter-model comparison of ET and runoff can be made to develop insights into how different modeling approaches, levels of calibration and watershed representations (gridded versus catchment objects) affect results. The WEAP intra-model comparisons using both the Penman-Monteith and PG_PM approaches will also provide a way to assess the importance of simulating the effects of shifts in the growing season, changes in the growth period length and the effects of elevated CO2 on vegetation growth and ET-runoff relationships.

Another benefit of the research will be the development of augmented climate projections in which improved estimates of solar radiation, humidity and wind speed are obtained to better characterize the selected watershed conditions. This will allow the benefits of augmenting climate projection datasets to be evaluated. Furthermore, the development of additional climate projections based on 800m spatial resolution NASA Earth Exchange (NEX) and the DRI statistical and dynamically downscaled climate projections will allow the benefits of improved spatial resolution and different downscaling methods to be examined.

The overall results of the study will also be useful to Reclamation and others by helping to identify and prioritize needs for future research on the gaps addressed by this study.

Contributing Partners

Contact the Principal Investigator for information about partners.

Research Products

Research products will focus on connecting with Reclamation mission related activities by providing briefings and workshops describing study methods, assumptions and simulation results through webinars and coordination with WWCRA, Basin Study and LCC activities. A technical report will prepared and distributed to partners and other interest groups. One or more manuscripts for publication in scientific journal will also be prepared.

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Last Updated: 6/22/20