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Drought and forest disturbance impacts on hydrology under changing climate conditions
Slinski, Kimberly M.
Slinski, Kimberly M.
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2018
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2019-06-13
Abstract
Freshwater is one of the world’s most important natural resources. It is essential to human health, ecosystem function, and livelihoods, but is a finite resource and needs to be appropriately managed. As global demand grows, there is increasing competition for resources between the water, agriculture, energy, livestock, fisheries, forestry, mining, transport, and other sectors. Conceptual frameworks for water policy such as integrated water resources management and the water-energy-food nexus have emerged to balance these competing interests. However, the policy challenges to meeting societal demands for water while maintaining ecosystem function are exasperated by the modification of water availability due to changing land use and climate conditions. Reliable methods to monitor and predict water availability are essential for understanding the vulnerability of water resources to future changes in land use, climate, and policy. The overarching objective of this dissertation is to better understand how changing patterns of land use and climate conditions impact the distribution and availability of freshwater resources. This dissertation first examines the impact of the current mountain pine beetle (MPB, Dendroctonus ponderosae) outbreak in the western United States (US) on surface water resources. In the Western US, the current MPB epidemic has affected more than five million hectares since its start in 1996, including headwater catchments that supply water to much of the Western US. There is widespread concern that the hydrologic consequences of the extensive pine tree die-off will impact water supply across the Western US. While forest disturbance studies have shown that streamflow increases in response to tree harvest, the actual effect of bark beetle infestations on water supply remains widely debated among MPB researchers. This study evaluates watershed-level response following bark beetle outbreak for 33 watersheds in seven western states. Streamflow records were investigated to assess whether the timing and amount of stream discharge during bark beetle outbreak and early recovery periods were significantly different to pre-outbreak conditions. Results show no significant modification in peak flows or average daily streamflow following bark beetle infestation and that climate variability may be a stronger driver of streamflow patterns and snowmelt timing than chronic forest disturbance. The second part of this dissertation examines how surface water resources in the Awash River Basin, in Ethiopia, were impacted by the 2015 regional drought and their subsequent recovery. This study presents a new method to develop accurate, high-resolution maps of waterbodies. Cloud-based computing resources and machine learning techniques are used to merge Sentinel 1 synthetic aperture radar (SAR) and Landsat observations to generate monthly waterbody maps at a 10-meter resolution. The accuracy of this method is shown to be comparable to waterbody map products generated by high performance computing resources. The technique is demonstrated by mapping surface water change over the Awash River basin in Ethiopia during the 2015 regional drought. Results indicate that the downstream sub-catchments were most strongly impacted by drought and that surface water in all catchments recovered to pre-drought surface water area after the 2016 summer rains. The mapping illustrates the acute impact the drought had on surface water area, but this study could not determine surface water volume because measurements of the change in water body depth are not available. The upcoming Surface Water and Ocean Topography (SWOT) mission will provide global satellite altimetry measurements of water levels. The APWC technique combined with SWOT measurements of water elevation will be a powerful tool for monitoring changes in surface water volume in data-sparse regions. To our knowledge, this study is the first to merge passive and active sensors to generate waterbody maps and the first to create waterbody maps at a 10-meter resolution. This technique will help earth scientists better monitor and understand the impact of environmental changes on global freshwater ecosystems. The final part of this dissertation examines how small, seasonal deformations induced by the West African Monsoon maybe used to better understand the distribution of water resources and geology of the Ara Watershed, a small catchment located in the Sudanian ecoregion of northern Benin, in West Africa. West Africa is undergoing unprecedented growth and development. Its population has been increasing by 2.75% each year and is expected to double in 25 years. This growth is intensifying pressure on regional and local water resources while changes in climate and land use are modifying the regional hydrology. The consequential changes to water availability are not fully understood, due in part to the scarcity of in-situ data across the region. This study uses interferometry of synthetic aperture radar (InSAR) techniques to map the dynamics of seasonal deformation across the Watershed. Seasonal deformation was found to be closely linked to monsoon precipitation. Riparian areas and the seasonally water-logged areas in the headwaters of small streams called bas-fonds in French-speaking West Africa may experience larger and more rapid deformations than adjacent upland areas, regions of lateritic covers, and areas where quartzite/quartz dykes have been identified. Two mechanisms are proposed to explain these patterns of deformation. This result may be due to the larger seasonal changes in water mass in the bas-fond and riparian regions as the monsoon precipitation accumulates as perched water and streamflow in these areas and is subsequently removed by evapotranspiration during the dry season. The higher water mass upon onset of the monsoon would cause downward deformation, which would be reversed during the dry season. The anomalies could also be explained by shrinkage of the deeper clay layers as the permanent groundwater table continues to recede through the early part of the monsoon. The study results indicate that InSAR methods show promise for being able to identify relative changes in water mass in areas with sparse data coverage.
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