Publication Date

8-1-2025

Date of Final Oral Examination (Defense)

4-9-2025

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Supervisory Committee Chair

Anna Bergstrom, Ph.D.

Supervisory Committee Co-Chair

Lejo Flores, Ph.D.

Supervisory Committee Member

Jim McNamara, Ph.D.

Supervisory Committee Member

Shad O'Neel, Ph.D.

Abstract

Water supply and demand are encountering unique challenges in the western United States as climate change impacts the availability of water resources. Much of the west relies on high mountain snowpack to maintain consistent streamflow throughout the dry, warm season months. Temperatures often rise above freezing in the cold season, diminishing the snowpack size, and altering the timing of snowmelt. As droughts become more frequent, water managers are increasingly concerned about the risk of reduced summer streamflow. To understand changing hydrologic patterns, this study utilizes phase changes in precipitation, specifically the distinction between cold and warm season precipitation, and their influence on streamflow dynamics. It examines how defining seasonal precipitation end members affects streamflow partitioning. To do this, stable water isotopes are used, specifically δ^18O, a naturally distinct tracer, to define warm and cold season precipitation end members and determine their contributions to streamflow.

While end-member mixing equations are commonly used in hydrologic research, there is no standardized method for identifying warm vs. cold end members for streamflow partitioning of precipitation due to the diverse range of watershed characteristics and the influence of climate change on precipitation patterns. This research identifies alternative end member definitions that consider the complexity of watershed dynamics, with the aim of addressing the influence of precipitation phase contributions on streamflow during warmer winters. Climate change predictions for the western United States will impact ecosystem services that rely on mountain snowpack for water resources, due to shifts in the predominant precipitation types. I applied the alternative definitions to the Mores Creek Watershed, a large, semi-arid to subalpine, snow-dominated watershed in Idaho to understand system variability in identifying end members. This approach will be helpful for systems that are undergoing climate forcings such as changes in peak snow water equivalent or snowmelt timing. This watershed is at risk of transitioning from a snow-dominated system to a mixed precipitation regime.

The results of this study are compared with findings from other research that employs different methods to classify warm and cold season end members. Key trends in selecting end member methods, based on climate and watershed characteristics, are identified by comparing the results. Findings indicate that assessing cold season temperature variability across the entire watershed is crucial to determine whether a single or multiple precipitation collection sites are necessary for identifying seasonal end members. Understanding regional climate projections, including snow-to-rain transition zones, can aid in selecting specific sites for accurately capturing cold and warm season precipitation in a study. Weighted isotope samples can help correct isotope biases when the catchment area or elevation relief significantly affects precipitation phase and amount. Refining definitions for effectively characterizing seasonal water sources can reduce errors in researchers' modeling efforts to understand the timing and magnitude of water availability. As climate-induced change alters hydrologic processes, drought will impact snowpack storage, melt timing, and summer water availability. By enhancing the accuracy in identifying stream source contributions, this study will assist water managers in refining a framework for quantifying streamflow predictions and preparing for future water availability in the western United States.

DOI

https://doi.org/10.18122/td.2416.boisestate

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