| Summary: | Includes bibliographical references. 2017 Summer. The watershed response to climate warming in cold regions is a critical area of water resources research because slight changes in temperature can trigger abrupt changes in hydrologic regimes. A regime shift occurs when the cumulative effect of small perturbations causes an abrupt change in system behavior. As mountainous cold regions warm, potential regime shifts include the loss of snow and glaciers, permafrost thaw, and transitioning from snow-dominated to rainfall-dominated streamflow patterns. Cold regions occupy over half of the land area in the northern hemisphere, and encompass a broad range of physiographic features and climates; this research into regime change focuses on a subset of cold regions situated in mountainous terrain at mid- and high-latitudes where streamflow is dominated by seasonal snow and glacier meltwater. The science questions guiding this dissertation research explore the hydrologic regime changes that may occur as cold region landscapes continue to warm. To facilitate this research, and to address the absence of a parsimonious water balance model with integrated glacier dynamics and permafrost features, an established USGS water balance model was substantially enhanced with representations of multiple cold region processes. Modeling of hydroclimatic interactions spanning the time period 1950 to 2100 was performed in two different cold-region landscapes: the snow-dominated Rio Grande Headwaters (RGH) in the Colorado Rocky Mountains and the heavily glacierized Copper River watershed in Southcentral Alaska. The mid-latitude RGH reveals little streamflow sensitivity to historical changes in climate, but it does exhibit vulnerability to projected future climate change; by the end of this century, the RGH may no longer possess the characteristics of a snow-dominated watershed that define it today. In the Copper River, discharge is projected to surge by almost 50% by the end of this century as precipitation increases and water is released from perennial storage in snow and ice; however, as glacier mass is exhausted, the inevitable decline of glacier contributions to streamflow will be evident in some tributaries as early as the 2050s.
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