Exploratory Outdoor Assessment of Low-Coverage Silver-Colored Floating Discs: Daytime Water-Level Decline and Water-Temperature Response

Open-water surfaces, including lakes and reservoirs, absorb solar radiation and can experience substantial evaporative water losses. Partial surface coverage with reflective floating elements may reduce both solar heat loading and evaporation, but coverage fraction, water temperature, evaporation, and boundary conditions are strongly coupled. This exploratory study evaluated water-temperature response and daytime water-level decline associated with partial coverage by silver-colored floating polyethylene discs under three reduced-scale outdoor scenarios in Rende, Italy. Three identical water containers (17 × 15 cm surface area; 10 cm water depth) were tested with 0%, 10%, and 25% nominal projected surface coverage (C0, C10, and C25, respectively). Water temperature, air temperature, and relative humidity were recorded hourly, while water depth was measured at the beginning and end of each trial. Reported hourly solar-radiation data for Rende were used only to characterize the meteorological context. Observed mean C25–C0 temperature differences were +0.5 °C in S1, −0.4 °C in S2, and −1.5 °C in S3, with the largest instantaneous separation of −3.1 °C occurring in S3. The mean differences observed in S1 and S2 were of the same order as the nominal combined PT100 measurement uncertainty, whereas the larger separation in S3 exceeded this uncertainty in magnitude. In all three exploratory trials, the covered vessels exhibited smaller daytime water-level declines than the uncovered control; however, several between-treatment differences were comparable with the ±1 mm water-level measurement resolution. The direction and magnitude of water-temperature differences varied among the three coupled meteorological and support-boundary scenarios. The findings provide preliminary mechanism-screening observations rather than statistically replicated performance validation.