Degree

Doctor of Philosophy (PhD)

Department

School of Renewable Natural Resources

Document Type

Dissertation

Abstract

Floodplains are hydrologically dynamic, receiving water from overbank events, hyporheic flows, local precipitation, and regional groundwater sources. These sources are variously important contributors to the heterogeneous floodplain water pool that includes matrix water in soil micropores, mobile water in soil macropores, groundwater below the rooting zone, ephemeral to seasonal surface storage, and permanent surface water features such as oxbow lakes, sloughs, and other secondary channels. All sources may be ecologically relevant for floodplain vegetation, but the exact roles of each source in both controlling soil water and shallow groundwater recharge and in controlling floodplain water drainage are not well understood, particularly in large, fine-grained floodplains. The objective of this research is to better characterize the relationship between source waters and floodplain water pools by using stable isotope and remote sensing tools in combination with classical hydrometric techniques at four floodplains in the southeastern United States: Old Sabine Bottoms WMA, Boeuf WMA, Big Lake WMA, and White River NWR. Specifically the objectives were to 1. use remote sensing tools, specifically time series of land surface temperature to identify spatial and temporal patterns in tree water stress to determine processes controlling water limitation in forested floodplains (Chapter 2), 2. determine the factors controlling fluctuation of shallow groundwater of large, fine-grained floodplains and the main sources of recharge across floodplains with different degrees of modification and geomorphologies (Chapter 3), and 3. determine the sources of water for floodplain trees in large, fine grained floodplains by investigating the important sources of and controls on floodplain soil moisture (Chapter 4). Spatial and temporal analysis of water stress indicated the importance of growing season precipitation in controlling water limitation of the Old Sabine floodplain. Similarly, precipitation was found to be an important source of shallow groundwater recharge during the growing season and the most important source of water for seasonal wet up periods across all floodplains. River water likely plays a role in prolonging the duration of seasonal wet periods by contributing to surface subsidies and limiting drainage during high stages. Shallow groundwater is not a homogenous pool and instead is made up of variably perched macropore water (VPMW) that is dynamic in time and space. This structure mediates the relationship between shallow groundwater and river water fluctuations. In addition, the soil structure likely has important implications for isotopic composition of soil water and tree rooting behavior including the possible role of air filled macropores acting as vapor pathways that lead to the dominance of a partial distillation signal in the soil-water isotopic compensation. Overall this project has helped shed light on the hydrological behavior of large, fine-grained floodplains and will help to guide managers in the face of future climatic uncertainty.

Date

7-7-2020

Committee Chair

Keim, Richard F.

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