1. INTRODUCTION
Reservoirs are subject to substantial water level fluctuations caused by
water release operations and, as such, to shorter water residence times
than lakes. Evaporation (latent heat flux, LE ) is a major
component of the mass and energy budgets of reservoirs, which can
compromise some typical key functions such as freshwater supply,
irrigation, hydropower, navigation, and other related economic
activities (Friedrich et al., 2018). In some arid regions of the world,
structural measures are put in place to limit evaporation, such as
floating balls or lattices (Assouline & Narkis, 2021). Evaporation is
intangible and therefore a difficult hydrological flux to measure,
making it difficult to fully understand its magnitude and controlling
mechanisms.
Studies that have quantified reservoir evaporation are rare. Tanny et
al. (2008) reported an average evaporation rate of 5.5 mm
day–1 from July to September in the Eshkol reservoir
in Israel (33°N), which has a hot and arid climate. Further north, in
the Eastmain-1 reservoir in Canada (52°N), Strachan et al. (2016) found
the evaporation rate to be 3.1 mm day–1 between
August and October. For the Great Slave Lake in Canada (61°N), Rouse et
al. (2003) reported 2 mm day−1 in summer and 5 mm
day−1 in December. These studies indicate that even in
cold regions, reservoir evaporation can be substantial.
In cold climates, reservoirs undergo two turnovers per year (dimictic).
Their thermal regime typically evolves into three successive phases
(Cole & Weihe, 2016). During the ice cover phase, ice acts as a lid
over the water body, preventing direct interactions between the
atmosphere and the water column. The water column then becomes
stratified, with cold water (< 4°C) sitting on top of warmer
water. Latent heat fluxes tend to remain low during this period (Wang et
al., 2016). From ice breakup in spring to the middle or end of summer,
the heat storage phase unfolds. Energy is first stored in the top
several meters of the water column closer to the surface and eventually
reaches deeper layers due to wind-induced mixing and internal
hydrodynamics (Spence et al., 2003; Vincent, 2018). The result is a
surface mixing layer (epilimnion) that is separated from the homogeneous
deep layer (hypolimnion) by a zone with a steep temperature gradient
(metalimnion). Latent heat fluxes remain low during this phase, with
frequent and stable atmospheric stratification. The third and final
phase corresponds to the heat release period. This is characterized by a
decline in water temperature due to a substantial release of energy into
the atmosphere through turbulent heat fluxes that are high and sustained
day and night (Blanken et al., 2011). The epilimnion then slowly deepens
until the fall turnover, during which the temperature of the entire
water column becomes homogenous.
While evaporation varies seasonally in response to the three thermal
phases, it also fluctuates on smaller time scales in response to
meteorological forcing. For instance, incoming shortwave radiation
causes latent heat fluxes to peak during the day, thereby increasing the
rate of evaporation during peak times (Lensky et al., 2018). The
atmospheric demand for water vapour, driven by wind speed and vapour
pressure deficit, is also known to modulate evaporation in water bodies
(Pérez et al., 2020). Evaporative demand can vary within a single day.
For instance, changing wind direction can lead to a reduced or enhanced
sheltering effect, increasing or decreasing evaporation rates (Markfort
et al., 2010; Venäläinen et al., 1998). Evaporation can also vary over
the course of a few days, due to passing synoptic systems that can
generate sustained evaporation (Laird & Kristovich, 2002; Spence et
al., 2013). Blanken et al. (2000) found that 50% of annual evaporation
over the Great Slave Lake occurred over only 25% of the year through
episodic evaporation water losses. Moreover, thermocline depth and
intensity, which depends in part on the reservoir morphometry (Gorham,
1964), influence turbulent heat fluxes by limiting or enhancing the
energy available in the upper water layers. Indeed, Piccolroaz et al.
(2015) identified positive feedback between the lake surface temperature
and the stratification dynamics of Lake Superior, Canada. Therefore, the
timing of evaporation occurs at different scales and remains poorly
documented or correlated to physical drivers (Beck et al., 2018).
Northeastern America is one of the densest regions of lakes and
reservoirs around the world (Downing et al., 2006). These lakes and
reservoirs are considered to be climate sentinels (Adrian et al., 2009;
Williamson, Saros, & Schindler, 2009) as well as integrators and
regulators of climate change (Williamson, Saros, Vincent, et al., 2009).
Wang et al. (2018) showed that modifications in surface energy
allocation under warmer climate conditions will accelerate global lake
evaporation. In-situ evaporation observations are needed to develop and
improve lake models (McJannet et al., 2017) for future climate
estimates, particularly in remote areas.
There is a lack of direct in-situ measurements of turbulent heat fluxes
over reservoirs in remote northern regions. The overarching goal of this
study is to identify the characteristic time scales of evaporation from
a deep subarctic hydropower reservoir. Using four years of
eddy-covariance (EC) measurements, the specific objectives are to
quantify turbulent heat fluxes at daily, monthly and annual time scales,
and to identify the key processes and surface energy budget terms that
govern LE at each time scale. The paper is organized as follows.
We first introduce the study site and measurement methods. Then, we
describe the meteorological conditions over the whole study period and
the driving factors for each time scale. Finally, uncertainties in the
flux data are discussed, given the energy budget of several water layers
in the reservoir.