Thesis
What controls forest evapotranspiration at different concentrations of atmospheric carbon dioxide: vapor pressure deficit or soil moisture?
Washington State University
Master of Science (MS), Washington State University
2017
Handle:
https://hdl.handle.net/2376/101380
Abstract
Evapotranspiration (ET), the combined effect of soil-surface evaporation and plant transpiration, is a vital element of the global water balance. It is also important to plants, providing them with water, nutrients, and cooling, and helping them regulate carbon dioxide entry through the opening and closing of their stomata. The quantification of ET in forested environments is an area of ongoing research. Complex physiological responses, climatic variation, and the difficulty of taking wide-spread measurements make ET one of the least understood components of forest water balance. The world is being exposed to increased carbon dioxide (CO2) concentrations, one of the main causes of global warming, and one study indicates that this is limiting plant transpiration by reducing stomatal opening. To understand the impact of climate change and water limitation on plant functionality and productivity, this study primarily focuses on understanding the water use by plant at different soil moisture conditions with increasing CO2 and vapor pressure deficit. We use the WEPP model as a platform by modifying its ET by incorporating the Penman-Monteith approach with an empirical stomatal conductance equation. The model was calibrated and evaluated for five forest sites with different climate gradients. We found reasonable R2 and NSE values, varying from 0.5 to 0.85 and 0.36 to 0.83 respectively, among the sites. A sensitivity test suggests that the direct reduction of ET by rising CO2 concentrations can be expected to be moderate for wet climates, with a reduction of around 10.4 under a future climate, and a slight (1.6%) reduction for dry climates. We also found that ET is proportional to vapor pressure deficit (VPD) up to a certain limit, after which it reacts inversely to increasing VPD. For dry climates, increasing VPD causes a small negative decrease in ET due to soil-moisture limitation. Finally, we found that elevated VPD acts heterogeneously: forest sites differ in VPD and soil moisture conditions at different levels of CO2. This study will be helpful for quantifying ET in forest ecosystems to improve our understanding and to predict drought-induced tree mortality and forest fire under future climatic events.
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Details
- Title
- What controls forest evapotranspiration at different concentrations of atmospheric carbon dioxide
- Creators
- Golam Arif Rabbani
- Contributors
- Jennifer C. Adam (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Civil and Environmental Engineering, Department of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; [Pullman, Washington] :
- Identifiers
- 99900525099301842
- Language
- English
- Resource Type
- Thesis