Abstract
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.