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Due to temperature differences of groundwater and streamwater, localized groundwater inflows into small streams can directly be detected with ground‐based thermographic systems in summer or winter. Infrared radiation temperatures of surface water were used to determine mixing length and to calculate the relative fraction of groundwater inflow to downstream discharge. These fractions were comparable to groundwater inflow fractions derived from electrical conductivity, kinetic water temperatures and discharge measurements. This approach advances the immediate detection and quantification of localized groundwater inflow for hydrology, geology and ecology.