Big Data for Small Streams: Leveraging a Coarse National Model to Enhance Fine‐Resolution Stream Temperature Predictions

Abstract Predictive stream temperature models are commonly used to address spatial and temporal gaps in observational data because stream temperature information is essential for environmental management decisions. National models capture broad patterns driven by climate, topography, and seasonality, but often lack the spatial resolution to simulate fine‐scale variation in groundwater discharge, riparian shading, or tributary inflows. In contrast, watershed models may simulate finer‐resolution patterns but miss regional drivers or be limited by scarcity of observations. To generate watershed‐extent, fine‐resolution stream temperature predictions, we developed 18 multiscale machine‐learning models that leverage (a) a broad‐extent, coarse‐resolution stream temperature model and (b) finer‐resolution observations and predictor data. The coarse‐resolution model simulates daily stream temperature across the conterminous United States on the National Hydrologic Geospatial Fabric (13.7 km median reach length; 865,364 km total). The fine‐resolution models simulate daily stream temperature on the National Hydrography Dataset Plus version 2 (1.5 km median; 5,192,808 km total). We compared multiscale models to seven purely broad‐ (national) or watershed‐extent models, testing them in eight watersheds (436–985 km 2 ) with differing hydrologic settings and under realistic data availability scenarios. Especially when observations within a watershed were scarce, multiscale models generated more accurate predictions than national or watershed‐specific models. A recurrent graph convolution neural network pretrained on coarse predictions and fine‐tuned on watershed‐specific observations was most accurate across watersheds and data scenarios. These results highlight that leveraging both coarse, national results and watershed‐specific data can increase model accuracies, particularly when observations within a watershed are spatially or temporally limited.