The thermal regime of hydroelectric reservoirs differs from that of lakes, as it is influenced not only by natural inflows and outflows of energy, but also by management rules through regulated downstream constraints and more importantly the electric demand through turbine flows. These advection terms are rarely assessed for hydroelectric reservoirs particularly in eastern North America, a region where they are abundant. This study contributes, using a series of unique observations, to the assessment of the water and energy balances of the 85-km 2 Romaine-2 northern reservoir (50.69°N; 63.24°W) with an average depth of 44 m. Two thermistor chains were deployed to monitor the dynamics of the vertical temperature profiles from 2018 to 2022. The surface energy balance components were measured using two eddy-covariance stations. Summer stratification occurs from June to November, and winter stratification from December to May. The maximum water temperature gradient of the metalimnion was 1°C m –1 in mid-September, and the maximum depth of the thermocline was 35 m in late October, before the autumn mixing period. We found that the water balance of the reservoir was mainly controlled by turbine operations, with a hydraulic residence time of 5.4 months. Net radiation was found to be the main source of energy to the reservoir (95.6% of the energy input), and the net advection of heat was weak (4.4%) in a steady state reservoir. Latent (58.5%) and sensible (16.5%) heat fluxes dominated the outflow energy balance. In short, this study highlights that the heat advection term represents a small fraction of the annual energy budget for the subarctic reservoir under study, despite being the dominant term in its water budget.

Water bodies such as lakes and reservoirs affect the regional climate by acting as heat sinks and sources through the evaporation of substantial quantities of water over several months of the year. Unfortunately, energy exchange observations between inland water bodies and the atmosphere remain rare in northeastern North America, which has one of the highest densities of lakes in the world. This study helps to fill this gap by analyzing field observations collected from a subarctic hydropower reservoir (50.69°N, 63.24°W) characterized by a mean depth of 44 m and a surface area of 85 km 2. Two eddy covariance (EC) systems, one on a raft and one onshore, were deployed from 27 June 2018 to 12 June 2022. The thermal regime of the reservoir was monitored using vertical chains of thermistors. Results indicate a mean annual evaporation rate of 590 ± 66 mm (~70% of the annual precipitation), with 84% of the evaporation occurring at a high rate from August to freeze-up in late December through episodic pulses. It was difficult to close the energy balance because of the magnitude and the large time lag of the heat storage term. In order to circumvent this problem, we opted to perform calculations for a year that started from the first of March, as the heat storage in the water column was at its lowest at that point and could thus be ignored. From June to December, monthly Bowen ratios increased from near-zero negative values to about 1.5. After September, due to smaller vapor pressure deficits, latent heat fluxes steadily declined until an ice cover sealed up the reservoir. Two opposite diurnal cycles of sensible and latent heat fluxes were revealed during the open water period, with sensible heat fluxes peaking at night and latent heat fluxes peaking in the afternoon.