4. CONCLUSION
This study quantified the temporal dynamics of evaporation over a deep,
boreal, dimictic hydroelectric reservoir using two eddy-covariance
setups, one mounted on a raft and one onshore. Data were collected for
four years, from June 2018 to June 2022, in order to examine the daily,
monthly and annual patterns of the turbulent heat fluxes.
Turbulent heat fluxes revealed opposite diurnal cycles of H andLE during heat storage and heat release periods, and the absence
of a diurnal pattern during the rest of the year. LE reached its
maximum at 15:00 when H reached its minimum, and LE was
minimal at night at 00:00 while H peaked at 05:00 in the morning.
Our monthly analysis showed that most of the latent and sensible heat
fluxes occurred from August to December. The cumulative latent heat flux
amounted to 84% of the annual evaporation. Three- and six-month delays
occurred between maximum summer net radiation and maximum values ofLE and H , respectively, suggesting the impact of the heat
storage release. Moreover, one- and four-month delays were observed
between the maximum surface water temperature and maximum LE andH , respectively.
Results showed an annual evaporation of 590 ± 66 mm
yr–1 that was quite constant from year to year, with
frequent 1-day to 2-day sustained events. Latent heat flux increased
earlier than the sensible heat flux but also decreased before the
sensible heat flux, resulting in a Bowen ratio that varied from a
near-zero negative value in July to 1.5 in December. Vapour
pressure-controlled evaporation induced a steady decline from September
to December due to decreasing air temperature.
The large time lag and the magnitude of the energy storage within the
water column made it difficult to close the energy balance. Therefore,
in this study, we have taken that issue into account by correcting the
annual cumulative evaporation while preserving of the measured Bowen
ratio.
Moreover, monthly and seasonal patterns of evaporation can be related to
the energy state of the reservoir. Indeed, depending on the time of
year, the reservoir was either under ice cover or in heat storage or
heat release conditions, which drives the magnitude of evaporation.
Results clearly illustrate that water loss through evaporation has a low
impact on hydroelectricity production and water availability downstream.
However, because evaporation is likely to increase in the region due to
climate change, the assessment of this energy and the associated
hydraulic components remains topical and essential to understanding
future trends.