Sponsor
Portland State University. Department of Environmental Science and Management
First Advisor
Kelly Gleason
Term of Graduation
Summer 2026
Date of Publication
9-10-2026
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Environmental Science and Management
Department
Environmental Science and Management
Language
English
Subjects
hydrological modeling, post-fire snow hydrology
Physical Description
1 online resource (xi, 88 pages)
Abstract
The invaluable water resources stored in snowpack face mounting threats from rising temperatures, drought, shifts in precipitation regimes, and increased fire frequency, which pose challenges to operational hydrologic modeling in Oregon's sensitive maritime snowpacks. The gridded temperature index (GTI) approach in operational models like the Hydrological Engineering Center's Hydrologic Modeling System (HEC-HMS) from the U.S. Army Corps of Engineers remains in common use for its minimal data requirements, but how GTI parameterizations should adapt to post-fire conditions is largely unknown.
This research assessed the impact of the 2020 Lionshead Fire on HEC-HMS performance in the North Santiam River basin across ten pre-fire and five post-fire water years (WY2011–WY2025). Monte Carlo sensitivity analysis identified ground melt rate, rain-snow threshold (Px), and base temperature as the dominant drivers of peak SWE, while streamflow and runoff generation were governed by a mix of snow, soil, and canopy parameters — establishing snow parameters as central to both snow and flow outputs. A streamflow-calibrated model degraded across four streamflow and ten snow metrics post-fire, most severely in the most-burned Breitenbush subbasin, with snow NSE dropping 0.44–0.48 at all three basins. A snow-calibrated model showed no significant post-fire snow degradation and only one significant streamflow effect, with no statistically significant streamflow tradeoff between calibration strategies.
Empirically derived parameters at in-situ burned stations had lower Px temperatures, lower dry ATI-melt rates, and elevated wet melt rates than unburned reference sites. Base temperatures representing melt onset did not differ between burned (38.4°F) and unburned stations (38.5°F). Integrating these empirical parameters showed that post-fire ATI-melt rates transferred usefully to the model, but Px and base temperature did not — suggesting structural disruption of the surface energy balance relationships GTI models rely on.
Combined, these results show that fire widens the gap between empirical post-fire snowpack and its representation within temperature index models, and that calibration of snow parameters towards assimilated SWE datasets, compared to calibration on streamflow records alone, can limit how much of that gap surfaces as error in post-fire model performance. These findings have direct applications to downstream water managers, communities, and ecosystems in the Oregon Cascades and to similar snow-dominated watersheds of the Pacific Northwest facing similar hydrologic shifts following fire.
Rights
© 2026 Logan Hastings
In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).
Persistent Identifier
https://archives.pdx.edu/ds/psu/45150
Recommended Citation
Hastings, Logan, "Representing Post-Fire Reality: An Empirically Informed Temperature Index Approach to Post-Fire Hydrologic Modeling in Oregon's Western Cascades" (2026). Dissertations and Theses. Paper 7216.