3.2 Heat Fluxes
3.2.1 Daily Scale
Figure 7 illustrates the average diurnal cycles of H andLE during ice cover (January to mid-May), the heat storage phase
(mid-May to August), and heat release (September to December). The last
two phases are defined based on the heat storage curve presented in
Figure 13. The 70-m heat storage peaked in early September, marking a
transition from the storage period to the release period. In fact, it is
important to note that the storage and release phases of the reservoir
overlap during August, as heat release starts in early August while heat
storage continues to grow in the water column until early September.
During the ice cover periods, LE remained low (< 20 W
m−2) and positive, which is indicative of sublimation,
while H was negative (< –10 W m−2).
Both turbulent heat fluxes had diurnal cycles with an amplitude lower
than 10 W m−2. The corresponding Bowen ratios were
small and negative when LE was between 5 W m−2and 10 W m−2. These results were expected, as ice and
snow covered the water surface from early January until mid-May,
suggesting low albedo and preventing the surface from heating up enough
to produce stronger turbulent heat fluxes.
From mid-May to August, LE values remained low, with an average
of nearly 30 W m−2 and a maximum of up to 60 W
m−2 at 14:00. H then showed a clear diurnal cycle, at
−30 W m−2 at 15:00 and close to 0 W
m−2 at night. A value of −60 W m−2was reached in early afternoon. As a result, the Bowen ratio also
exhibited a diurnal cycle.
During the heat release phase (September to December), LEremained larger than H at all times, meaning that energy was
mainly released through phase change (evaporation). One striking feature
is that LE and H had opposite daily patterns: (i)LE reached a maximum at around 15:00 while H reached a
minimum, and (ii) LE reached a minimum at 00:00 while Hpeaked at 5:00. LE peaked in the afternoon when the incident
solar radiation reached its maximum value, warming up the water surface
during the day. Maximum H values occurred at night, when the
maximum water−air temperature difference was observed. At a similar
temporal scale, Nordbo et al. (2011) observed fluxes over the small,
boreal Lake Valkea‐Kotinen in Finland and showed that the highest values
of LE and H occurred in June, peaking at 15:00 and at
06:00 respectively. Therefore, in this study, the Bowen ratio exhibited
an explicit diurnal cycle. The highest values were around 0.95 at night
because of high H and low LE values. The lowest values
were around 0.40 in the day because of low H and high LEvalues. As a result, heat was released by two different mechanisms
depending on the time of the day: through both turbulent heat fluxes at
night and mainly through evaporation during the day. Overall, the
sensible heat fluxes H contributed to the heat stored by the
water body early in summer, that will later be released as latent heat
flux.