Short-term air sensitivity predicted 2000-2012 to 2013-2025 summer warming at 356 US stream gauges, but Nov-Mar water warmed about 0.1 C more than predicted, mainly in the East

Question: does the month-to-month sensitivity of stream temperature to air temperature and discharge predict the decadal change? Approach: at USGS NWIS sites with long daily water temperature records, we fitted the sensitivity on monthly anomalies in 2000-2012 only and predicted the change in mean water temperature to 2013-2025. Air temperature came from GHCN_CAMS (primary), nClimGrid and Berkeley Earth. Intervals come from a 5-degree spatial block bootstrap. Result: for July-August at 356 sites, the ratio of mean observed to mean predicted change was 0.86 (0.40 to 1.20) with GHCN_CAMS, 1.05 with nClimGrid and 1.11 with Berkeley Earth, and the predictions reduced site-level squared error by 30% relative to no change. Annual means warmed more than predicted (ratio 1.26 to 1.52 over the three products). At 281 year-round sites, the predicted monthly pattern of change agreed with the observed pattern (correlation 0.92 to 0.96), but November-March water warmed 0.11 C (0.04 to 0.17) more than predicted. This excess was mainly east of 100 W (cold-minus-warm residual +0.14 to +0.23 C; West near 0). A placebo (2000-2006 vs 2007-2012, sensitivity from 2013-2025) gave no all-site or eastern contrast, but it gave a western contrast of +0.14 C, so regional residuals near 0.1 C can occur in this design. The excess was not larger at ice-affected or high-baseflow sites and did not increase with local air warming, so a simple groundwater-lag explanation does not fit. Limitations: non-random, partly regulated sites; 13-year periods; gridded air temperature; linear monthly model; the cause of the cold-season excess is not identified.

stream temperature · air-water temperature sensitivity · climate change · USGS NWIS · out-of-sample test · groundwater · winter warming · nClimGrid · GHCN_CAMS · placebo test