3.2.3 Annual Scale
Calculating annual reservoir evaporation values is challenging. It is
well understood that eddy covariance data underestimate turbulent heat
fluxes (Foken, 2008), and in addition, the calculation of the heat
storage term is especially difficult for a deep water column. For this
reason, an alternative approach was used: the corrected turbulent heat
fluxes were obtained by redistributing the missing energy according to
the annual EBR over a so-called ”energy year,” from March 1 to February
28 of the next year. Since energy storage is typically at its minimum
value at this time of year, this allowed us to discard that variable and
make Rn = H +LE . SinceRn > H + LE , the
missing energy can then be redistributed by preserving the observed
Bowen ratio, as discussed in Mauder et al. (2018). By doing so, the EBR
was 80%, 69%, and 76% for the years 2019−20, 2020−21, and 2021−22,
respectively.
Figure 14 illustrates the yearly cumulative evaporation for three energy
years, from 2019 to 2022. The mean annual non-corrected evaporation was
439 ± 23 mm and did not vary much from year to year. When correcting for
the energy imbalance using the annual EBR values, the annual evaporation
values reached 555 mm, 656 mm and 559 mm for 2019−20, 2020−21 and
2021−22, respectively (refer to Table 1). Note that the inter-annual
variabilities of cumulative evaporation were 7.2% and 18% for
non-corrected and corrected values, respectively. In fact, 50% of the
total measured evaporation occurred over 24%, 27% and 26% of the days
for 2019−20, 2020−21 and 2021−22, respectively. This is consistent with
Blanken et al. (2000), who found a mean value of 22.5% for the Great
Slave Lake.