Process-based models of fluid flow and heat transport in fluvial systems can be used to quantify 17 unknown spatial and temporal patterns of hydrologic fluxes and to predict system response to 18 change. In this study, a deterministic stream heat budget model, the HFLUX Stream 19 Temperature Solver (HFLUX), is developed and evaluated using field studies. Field studies are 20 conducted across two sites with different streamflow rates (0.07 vs 1.4 m 3 /s), and point sources 21 versus diffuse sources of groundwater discharge, to demonstrate model transferability. A winter 22 versus summer comparison at one site suggests latent heat flux should be derived using energy-23 based methods in summer and mass balance approaches during winter. For each field study, 24 HFLUX successfully modeled stream temperatures through space and time with normalized root 25 mean square errors of 3.0 to 6.2%. Model calibration to observed temperature data in order to 26 quantify groundwater contributions and a sensitivity analysis are demonstrated using HFLUX. 27

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