3.2.2 Monthly Scale
Figure 11 displays monthly averaged turbulent heat fluxes over the whole
study period. Latent heat fluxes always remained positive despite some
condensation events in June (Sect. 3.2.1). LE peaked in
September, when the heat storage reached its maximum (Fig. 13). It
remained above 70 W m−2 from August to October.LE began to rise in July when stratification in the reservoir set
in. It then sharply increased in August all the way to December, during
which 84% of the annual evaporation occurred. This is consistent with
the findings from Blanken et al. (2011) for Lake Superior, where 89% of
the annual evaporation occurred when there was unstable atmospheric
stratification. It is also consistent with the Rouse et al. (2003) study
at Great Slave Lake, where 85% to 90% of evaporation occurred during a
period when the atmosphere was unstable. However, in our study,LE decreased gradually due to the declining vapour pressure
deficit that followed the decline in air temperature. In winter and
spring, LE stayed below 20 W m−2 and reached
its minimum in June, during the ice-free period (Fig. 6).
Sensible heat fluxes were negative from February to July, with values
less than −10 W m−2 and reaching as low as −20 W
m−2 when the reservoir was colder than the air. On
average, values remained positive from August to January. In June, the
reservoir was ice-free while the water column was under vernal
transition, with upper layers that were not warm enough to stratify
(i.e., below 3.98°C, the temperature of maximum water density). The
water surface temperature stayed far below the air temperature,
preventing H from becoming positive and enabling LE to
become negative. Despite the high net radiation term around the summer
solstice, the heat content of the water body only slowly rose because of
the high specific heat and density of water. In summer and fall, the
surface layers were warmer than the air above the water. Hincreased steadily until December, reaching 80 W m−2,
and then decreased abruptly in January as the water surface froze. From
May to August, net radiation (Rn ) was high,
contributing mostly to the storage of heat in the reservoir
(ΔHS ), while turbulent heat fluxes were
relatively low. Nearly all the energy brought in byRn was used to increase the heat storage term.
During the fall and early winter, Rn declined
rapidly while LE and H increased, fuelled by the energy
stored in the reservoir, firmly establishing the heat release period of
the reservoir. We observed a three-month delay between the maximum
summer net radiation that occurred in June and the maximum latent heat
flux. The delay was six months between the peak net radiation and the
maximum sensible heat flux. We also observed different delays between
the maximum surface water temperature and the maximum LE andH , which were delayed by one and four months, respectively.