Long-term monotonic trends in annual and monthly stream temperature metrics at multi-source monitoring locations in the United States

The U.S. Geological Survey (USGS) Water Resources Mission Area (WMA) is working to address a need to understand where the Nation is experiencing water shortages or surpluses relative to the demand for water need by delivering routine assessments of water supply and demand and an understanding of the natural and human factors affecting the balance between supply and demand. A key part of the Integrated Water Availability Assessments (IWAAs) Trends and Drivers project is identifying long-term national trends in water availability, including groundwater and surface water quantity, quality, and use. This data release contains Mann-Kendall monotonic trend analyses for 55 observed annual (calendar, water, and climate years) and monthly stream temperature metrics. Data were collated (Oliver et al., 2024) from the USEPA/USGS Water Quality Portal (WQP), the USGS National Water Information System (NWIS) and EcoSHEDS, and the USDA NorWeST databases. Metrics were calculated at a total of 2,080 stream temperature monitoring locations within the conterminous United States, Alaska, Hawaii, and Puerto Rico that passed initial screening criteria, and are also included as part of this data release. Stream temperature metrics include monthly and annual summaries, extreme (i.e., min/max) and central (i.e., mean) tendencies, variability, and timing characteristics. Monthly ("mean_[month]") and annual ("mean") mean, annual maximum of seven-day averages ("high7d"), and annual sinusoidal regression metrics ("ampl_median" and "phase_median") were calculated using daily mean values. Monthly ("high7dmax_[month]") and annual ("high7dmax") maximum of seven-day averages and monthly ("cvmax_[month]") and annual ("cvmax") coefficient of variation were calculated using daily maximum values. The monthly ("low7dmin_[month]") and annual ("low7dmin") minimum of seven-day averages were calculated using daily minimum values. Trend magnitudes were computed for 1,967 qualifying monitoring locations as a modified form of the Theil-Sen slope that accounts for missing values. Trend analyses were computed between years 1948-2022 and trend periods are between 10-72 years long. Metric time series analyzed for trends satisfied two requirements to be considered complete records: (i) have values in at least eight out of every 10 years (i.e., 80 percent) within the entire trend period and (ii) have values in at least eight out of the first and last 10 years of the trend period. Trends at each site are available for four main periods: (i) the longest possible ≥10-year period that meets completeness criteria at each site, (ii) 1980-2020, (iii) 1990-2020, and (iv) 2000-2020. Additionally, trends for various ≥10-year sub-periods, between 1949-2022, are included. Caution must be exercised when utilizing monotonic trend analyses conducted over periods of up to several decades (and in some places longer ones) due to the potential for confounding deterministic gradual trends with multi-decadal climatic fluctuations. In addition, trend results for USGS locations (site_id prefix "USGS-") are only available for post-reservoir construction years to avoid including abrupt changes arising from the construction of larger reservoirs in periods for which gradual monotonic trends are computed. Reservoir impacts on non-USGS sites were not evaluated. Other abrupt changes, such as changes to water withdrawals and wastewater return flows, or episodic disturbances with multi-year recovery periods, such as wildfires, are also not evaluated for any site. Sites with pronounced abrupt changes or other non-monotonic trajectories of change may require more sophisticated trend analyses than those presented in this data release.