Experimental and numerical injections of warm water into a free surface flow. Different flow regimes were tested and detection thresholds were determined. Experiments were done in an outdoor flume, in June 2016. They were monitored by Fiber-Optic Distributed Temperature Sensing with a double-ended configuration and 20s time resolution. Two flume flow rates Q were tested. For each, a few injection flow rates were done, leading to a total of 10 experiments characterized by an injection ratio R. Only calibrated data are uploaded. Simulations were carried out on HEC-RAS 5.0, a 1D software developed by the US Army Corps of Engineers, and based on the experimental flume geometry. A total of 90 simulations were tested with different flow rates, injection percentage and thermal contrast ∆T between flowing water and injected water. Supplementary data can be found to understand the flume geometry, hydraulic characteristics (Reynolds, Peclet and Froude numbers) and exchanges with the atmosphere for both simulations and experiments.