| Across the western USA, many watersheds are experiencing warmer temperatures, drier
summers, and declining snowpacks, all of which affect how snowmelt is partitioned into
streamflow and evapotranspiration. These changes are particularly important in mountainous
headwater streams that link high-elevation snowpacks and forests to downstream water users.
Our ability to accurately predict water resources in seasonally snow-covered areas hinges on our
understanding of how climate and vegetation interactions affect streamflow generation. To
further our understanding of how warmer temperatures and lower snowpacks affect headwater
streamflow, I present three chapters that show how vegetation’s response to climatic conditions
mediate groundwater storage and streamflow across Idaho’s upper Snake River. In Chapter 1, I
use a causal inference model to demonstrate that local climate and plant water use are consistent
drivers of headwater stream hydrology and stream drying. This study demonstrates that the
interactions between riparian vegetation, local climatic conditions, and stream channel water are
complex, change over time, and affect the local groundwater system. Chapter 2 extends this
framework to compare two years with low snowpack, or a snow drought, and a wet year. During
both the snow drought years, data show that stream channel water is made up of a lower portion
of snowmelt than in the wet year, which reflects smaller groundwater contributions. Applying
the framework from Chapter 1 reveals that this lower contribution is driven by complex climate,
vegetation, and water interactions that deplete near-stream groundwater during snow drought
years. Given the important role of snowmelt-driven mountain groundwater recharge in dictating
downstream water availability, lower recharge and more complex dynamics in snow drought
years have important implications for future water supply reliability across the region. As a next
step in improving runoff predictions, Chapter 3 presents a new metric that quantifies how closely
aligned water inputs and the growing season are for a given year, which can be used to infer
groundwater storage deficits or surpluses that dictate the following year’s runoff efficiency.
Chapter 3 demonstrates the utility of this metric in improving summer streamflow predictions in
the Henrys Fork of the Snake River.
Keywords: mountain hydrology, ecohydrology, drought, headwaters, critical zone, groundwater
storage |