Date of Award

Spring 2023

Project Type

Thesis

Program or Major

Hydrology

Degree Name

Master of Science

First Advisor

Alix Contosta

Second Advisor

Elizabeth Burakowski

Third Advisor

Anne Lightbody

Abstract

The timing of snowmelt and the onset of spring is changing in the Mountain West of the United States. The period between snow disappearance and vegetation green-up, known as the vernal window, is also changing, with implications for ecohydrology, the regional water supply, and the economy. Here, I present the first detailed analysis of the vernal window in the Mountain West at the ecosystem scale, focusing specifically on northwestern Wyoming in the Greater Yellowstone Area (GYA). I used in-situ and satellite data to address four objectives: 1) to understand how winter intensity and duration are changing, 2) to understand when and in what order springtime transitions occur, 3) to understand how these transitions and the lags between them (including the vernal window) might be changing, and 4) to understand the relationship between winter intensity and springtime transitions and lags, including the vernal window. I found that winters have become less intense, with significantly warmer temperatures during late winter and significantly earlier snow disappearance over the past six decades, a finding that may contribute to changes in water availability if the trend continues. Further, I found that some springtime transitions occur in different orders at different elevations, and that vegetation often greens up before snow disappears at high elevations in the study area, differing from findings in the eastern U.S. There were no significant trends in springtime transition timing or lag lengths over time, potentially due to the short period of record from which I drew observations. Finally, despite trends in late winter warming and earlier snow disappearance, there were no relationships between winter intensity indicators and snow disappearance date, vegetation green up, spring discharge, or vernal window length. Soil thaw timing did show a strong positive relationship with winter intensity.

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