Estimating the Impact of Impounding an 85‐km ² Hydropower Reservoir in a Subarctic Environment on the Local Radiation Balance

Abstract This study focuses on the net radiative forcing caused by the impoundment of an 85‐km 2 hydroelectric reservoir in the subarctic Côte‐Nord, Quebec, Canada (50.69°N, 63.24°W, mean depth of 44 m). Using spectral bands from the Landsat 7 ETM+ and Landsat 8 OLI/TIRS satellites, we observed spatial and temporal variations in albedo and surface temperature on 104 dates between 2000 and 2023. By integrating these data with ERA5‐Land meteorological reanalyses and in situ measurements, we investigated seasonal and interannual variations in shortwave and longwave radiative fluxes and net radiation before (2000–2013) and after (2015–2023) reservoir impoundment. The flooded area initially consisted of 79% coniferous forest (mainly black spruce), 15% water, 4% dry soil, and 2% wet soil. Before the impoundment, the average winter albedo was 0.25 ± 0.02 and the summer albedo was 0.08 ± 0.002. After the impoundment, the winter albedo increased significantly, reaching between 0.70 and 0.90 in the presence of fresh snow on the ice (January to May), while in summer it decreased to about 0.05 ± 0.003. Surface temperatures exhibited pronounced pre‐ to postimpoundment contrasts, with the reservoir consistently 2–7°C warmer in autumn and winter but 1–5°C cooler in spring and summer. The reservoir also reaches its maximum temperature 3–5 weeks later. The impoundment induced a net radiative cooling of −21 W m −2 compared to forest, featuring a pronounced winter deficit (−60 to −140 W m −2 ) due to snow cover albedo effects, partially compensated by summer surpluses (+5 to +50 W m −2 ).