Date of Award

2023-05-01

Degree Name

Doctor of Philosophy

Department

Geological Sciences

Advisor(s)

Hugo A. Gutierrez

Abstract

Projected climate and land cover changes in the 21st century profoundly impact the functioning of the earth’s water cycle. Identifying the components that contribute to the persistence and resilience of watersheds in light of climate change constitutes a research priority of global relevance, and will enable detect the areas most sensitive to climate variability and landcover changes. Global vulnerability of ecosystem services, particularly water service provision, needs to be urgently and continually assessed, given the unceasing human and climate-induced changes in land cover conditions worldwide. Given the increasing rate of extreme weather events, we present three interrelated research studies evaluating the impact of climate variability and land cover changes on hydrologic responses while displaying the direction of hydrologic behavior and climate conditions. Previous studies have documented hydrologic responses to climate variability in a few catchments within geographically limited regions. Because of this limited geographic extent, other important landscape factors such as elevation, slope, and aspect, which influence climate variabilities, have not been assessed at a large scale. Thus, there is a lack of a global synthesis evaluating the hydrologic responses to climatic variability, while evaluating the role of the topography in altering the response. In terms of the effects of landcover changes and their impact on hydrologic responses, previous studies have mainly focused on the effect of forest cover loss on Temperate Coniferous forests and the tropical Amazonia region, emphasizing the need for understanding the impacts across other forest types or other basins of similar biome to reveal the directions that major water resources are heading in a global scale. Although there is evidence of changing water yield (increasing or decreasing) under forest disturbance, there is no consensus on the direction of hydrologic changes. Also, most of these studies have not been assessed quantitatively limiting our knowledge of the reliable extent of the land cover effects on hydrologic changes. Human-driven deforestation and wildfires are major causes of tree cover loss. Many of these events occur in highly threatened-biodiverse tropical rainforests and boreal ecosystems. However, their impacts on hydrologic processes have not been systematically studied and the directions in which hydrologic changes may be occurring are still unclear. For example, although the Amazon basin has been extensively studied (Chambers & Artaxo, 2017; Lawrence & Vandecar, 2015; Leite-Filho et al., 2021; Malhi et al., 2008; Spracklen Xu et al., 2022 & Garcia-Carreras, 2015), other important tropical basins that have a major influence on global climates, such as the Central African and Southeast Asian forests have received less attention (Lawrence & Vandecar, 2015). Furthermore, the frequency of fire and forest exploitation leading to forest clearing in boreal systems is proportionally second to that in tropical ecosystems (Hansen et al., 2013), but only a few studies evaluate these impacts on boreal hydrology (Pimentel & Arheimer, 2021). Compounded with the loss of tree cover, changes in land surface properties reinforce current climatic trends such as rising temperatures leading to increasing snowmelt in high-latitude regions. Studies evaluating the role of forest clearing in these dynamics are missing or fragmented (te Wierik et al., 2021; Valeo et al., 2003). Given the lack of clarity on the effects of forest disturbance on hydrologic responses and the thresholds driving changes in water yield and regional climate, reporting the tipping points leading to significant changes in the hydrologic sensitivity in the world’s major forests is critical to aid in forest management strategies to prevent irreversible or permanent changes in freshwater resources. First, we present a global assessment assessing the sensitivity of the world’s water landscapes to climate variability during 2001-2016, using a new metric called the Hydrologic Sensitivity Index (HSi) (Chapter 2). This equation is based on the well-known Budyko curve that uses annual values of Potential and Actual Evapotranspiration (PET and AET), and Precipitation (P), to assess the hydrologic behavior of a location under a given climatic condition by plotting the Evaporative Index (EI, AET/P) against the Dryness Index (DI, PET/P). For values 𝐻𝑆i ≥1: Sensitive and 𝐻𝑆i ≥1: Resilient. We also point out whether the sensitive areas resulted in a decrease or increase in water yield and warmer vs cooler climate conditions. Also, since elevation, slope, and aspect are the three defining factors in temperature and humidity regimes, we evaluate their influence on HSi. Moreover, the variables used for computing HSi are evaluated against in-site measurement to confirm the use of high-quality datasets throughout the entire study. Next, we document critical thresholds of deforestation in 45 regions that underwent extensive forest cover loss induced by either drought, fire, or clear-cutting during the 2001-2016 period and report if these trends lead to increasing or decreasing water yield and warmer or cooler climate conditions (Chapter 3). HSi is used in this study to evaluate departures in historic hydrologic behavior in the face of land cover disturbances such as tree cover loss using 3-year HSi averages of before and after-disturbance periods. The Hydrologic Sensitive Area (HSia) was obtained by computing the portion of the area with high HSi values (HSi>1) relative to the entire disturbed area. We identified the critical threshold in forest cover loss before hydrologic responses are detected and the speed at which the disturbed area attains complete sensitivity once this threshold is surpassed, while also detecting if the observed changes in hydrologic regime increase or decrease water yield and if they are accompanied by warmer or cooler climate conditions. Lastly, we document the most hydrologically sensitive ecoregions to tree cover loss within the Amazon basin by reporting the differences in response in HSia to the most recent extreme drought events (Amazon drought in 2005, 2010, and 2015) while exploring the underlying factors leading to such responses.

Language

en

Provenance

Recieved from ProQuest

File Size

p.

File Format

application/pdf

Rights Holder

Marisol Dominguez

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Included in

Hydrology Commons

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