An Analysis of the Thermal Regime and Energy Balance of a Subarctic
Hydroelectric Reservoir Using Direct Measurements of Surface and Lateral
Exchanges
Abstract
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.