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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Jun 2018

Research article | 25 Jun 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

Principle components of thermal regimes in mountain river networks

Daniel J. Isaak, Charles H. Luce, Gwynne L. Chandler, Dona L. Horan, and Sherry P. Wollrab Daniel J. Isaak et al.
  • U.S. Forest Service, Rocky Mountain Research Station, Aquatic Sciences Lab, Boise, ID 83702

Abstract. Description of thermal regimes in flowing waters is key to understanding physical processes, enhancing predictive abilities, and improving bioassessments. Spatially and temporally sparse datasets, especially in logistically challenging mountain environments, have limited studies on thermal regimes but inexpensive sensors coupled with crowd-sourced data collection efforts provide efficient means of developing large datasets for robust analyses. Here, thermal regimes are assessed using annual monitoring records spanning a five-year period (2011–2015) at 226 sites across several contiguous montane river networks in the northwestern U.S. Regimes were summarized with 28 metrics and principle components analysis (PCA) was used to determine those metrics which best explained thermal variation on a reduced set of orthogonal axes. Four principle components (PC) accounted for 93.4% of the variation in the temperature metrics, with the first PC (49% of variance) associated with metrics that represented magnitude and variability and the second PC (29% of variance) associated with metrics representing the length and intensity of the winter season. Another variant of PCA, T-mode analysis, was applied to daily temperature values and revealed two distinct phases of spatial variance – a homogeneous phase during winter when daily temperatures at all sites were <3°C and a heterogeneous phase throughout the year's remainder when variation among sites was more pronounced. Phase transitions occurred in March and November, and coincided with the abatement and onset of subzero air temperatures across the study area. S-mode PCA was conducted on the same matrix of daily temperature values after transposition and indicated that two PCs accounted for 98% of the temporal variation among sites. The first S-mode PC was responsible for 96.7% of that variance and correlated with air temperature variation (r=0.92) whereas the second PC accounted for 1.3% of residual variance and was correlated with discharge (r=0.84). Thermal regimes in these mountain river networks were relatively simple and responded coherently to external forcing factors, so sparse monitoring arrays and small sets of summary metrics may be adequate for their description. PCA provides a computationally efficient means of extracting key information elements from large temperature datasets and could be applied broadly to facilitate comparisons among more diverse stream types and develop classification schemes for thermal regimes.

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Daniel J. Isaak et al.
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Daniel J. Isaak et al.
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Short summary
Description of thermal regimes in flowing waters is key to understanding physical processes and improving bioassessments but has been limited by sparse datasets. Using a large annual temperature dataset from a mountainous area of the western U.S., we explored thermal regimes using principle components analysis. A small number of summary metrics adequately represented most of the variation in this dataset given strong temporal coherence among sites.
Description of thermal regimes in flowing waters is key to understanding physical processes and...