Monthly River Temperature Trends Across the US Confound Annual Changes
Citation:
Kelleher, C., H. Golden, AND S. Archfield. Monthly River Temperature Trends Across the US Confound Annual Changes. Environmental Research Letters. IOP Publishing LIMITED, Bristol, Uk, 16(10):104006, (2021). https://doi.org/10.1088/1748-9326/ac2289
Impact/Purpose:
Though river temperature trend analyses exist in the literature, few studies have examined river temperature trends at sub-annual timescales. As we show, through both annual and monthly trend analyses for 20-year (n = 142 sites) and 40-year (n = 40 sites) periods, annual temperature trends mask extensive monthly variability. While most sites exhibited annual warming trends, these annual trends obscured sub-annual cooling at many sites. Surprisingly, monthly trends were spatially organized, with persistent regional patterns at both reference and human-impacted sites. Monthly trends frequently exhibited larger magnitudes than annual trends, emphasizing the need to consider sub-annual behavior when managing the ecological impacts of water temperature throughout river networks.
Description:
Climate variations and human modifications of the water cycle continue to alter the Earth's surface water and energy exchanges. It is therefore critical to ascertain how these changes impact water quality and aquatic ecosystem habitat metrics such as river temperatures. Though river temperature trend analyses exist in the literature, studies on seasonal trends in river temperatures across large spatial extents, e.g. the contiguous United States (US), are limited. As we show through both annual and monthly trend analyses for 20 year (n = 138 sites) and 40 year (n = 40 sites) periods, annual temperature trends across the US mask extensive monthly variability. While most sites exhibited annual warming trends, these annual trends obscured sub-annual cooling trends at many sites. Monthly trend anomalies were spatially organized, with persistent regional patterns at both reference and human-impacted sites. The largest warming and cooling anomalies happened at human impacted sites and during summer months. Though our analysis points to coherence in trends as well as the overall impact of human activity in driving these patterns, we did not investigate the impact of river temperature observation accuracy on reported trends, an area needed for future work. Overall, these patterns emphasize the need to consider sub-annual behavior when managing the ecological impacts of river temperature throughout lotic networks.
