2.4 Water Temperature and Transparency
The vertical water temperature profile was continuously measured with
two chains of temperature probes (HOBO TidBit UTBI-001, Onset, USA) that
were designed with a higher resolution near the surface and a coarser
resolution deeper in the water (Figs 3c and 4). These are referred to as
thermistor chains (TCs). The first measurement site,
TC1, consisted of a 15-m chain deployed in a 30-m deep
section of the reservoir, in the vicinity of the raft. The second
measurement site, TC2, consisted of a 70-m chain located
in a 100-m deep section of the reservoir, 1 km south of the raft. At
both sites, skin temperatures were obtained using floating sensors
sheltered from incoming solar radiation by a white piece of foam. The
chains were designed to withstand variations in water levels by using
additional rope. A pressure sensor (HOBO water level logger u20-001-03,
Onset, USA) was added to each chain to correct for the effects of rope
tilt on the exact vertical position of the probes.
Water transparency was also periodically measured using a Secchi disc.
The mean Secchi depth (SD ) was 4 ± 0.04 m. Assuming a mean
product SD × Kd of 2.28 for water of
moderate transparency following Koenings and Edmundson (1991), the
vertical attenuation coefficient Kd becomes 0.57
m–1. Consequently, 50% of the incident energy flux
density is absorbed in the first 1.2 m of water and 99% over the first
8.1 m of water.