Figure
13: Cumulative heat storage in the
upper 70 m of the water column for years 2018, 2020 and 2021. There are
no data shown for 2019 due to a major thermistor chain malfunction.
Calculations start from 24 June as no data were recorded before this
date for 2018 and 2020. For the purposes of the figure, to create a
reference point based on our data availability, we set the annual
cumulative heat storage at the end of the year to zero
(ΔHS = 0 MJ m−2).
Note that Romaine-2 experienced a sharp drop in heat storage in
September 2020. This is linked to the approximately 10 m decline in
water level in August and September. Indeed, the epilimnion moved closer
to the water intake and therefore the accumulated heat within the top
layers was drained out as turbine flow. The year 2020 was therefore
unusual in terms of heat storage.
When looking at the monthly layer energy balance of the reservoir using
equation 3, we noted that there was a significant non-closure. This is
due to the time scale difference between the heat storage in the water
column and the remaining terms (turbulent heat fluxes and net
radiation). Indeed, due to the high thermal inertia and the high
specific heat of water, the net radiation that was received in summer
accumulated in the water column and was only released in autumn. This
feature makes it difficult to calculate the EBR on a monthly scale.
Within this context, correcting turbulent heat fluxes to account for the
energy imbalance (Foken, 2008) using the monthly EBR is problematic,
unless accurate measurements of lateral energy inputs are available
(sect. 3.3).