Impact of floating photovoltaic power plant on reservoir evaporation: insights from eddy-covariance measurements

Aim and Approach

Increasing pressure on water resources, driven by the growing demand for drinking water, irrigation, and industrial uses, calls for improved water management strategies (Unesco, 2024). In this context, floating photovoltaic (FPV) systems have emerged as a promising solution. Initially developed to address land-use constraints, FPV installations also present a substantial potential for reducing evaporation losses from the reservoirs on which they are deployed (Sahu, 2016). By partially covering the water surface, these systems modify air–water interactions, reducing incoming solar radiation and altering convective heat and mass exchanges, thereby potentially limiting evaporative losses (Taboada, 2017).

However, despite this widely assumed benefit (Taboada, 2017; Gonzalez, 2025; Bontempo, 2021), evaporation reduction induced by FPV systems has not yet been robustly demonstrated or quantified at the scale of industrial installations. This lack of large-scale assessment primarily stems from the complexity of the physical processes involved, including the coupled effects of surface shading, altered turbulence, and modified atmospheric boundary-layer dynamics, which cannot be reliably captured by indirect or simplified approaches and require direct, high-resolution measurements (Tanny, 2008).

To address this gap, two eddy-covariance (EC) systems were deployed on a reservoir partially covered by an industrial-scale FPV plant (see Figure 1). This experimental setup enables a direct and simultaneous monitoring of evaporative fluxes over both covered and uncovered water surfaces, providing new insights into the impact of FPV installations on reservoir-scale evaporation dynamics.

Figure 1 - Location of EC measurements on the reservoir partially covered by an FPV power plant.

Results and Perspectives

Figure 2 presents the daily evaporation rates measured for several days in July over the covered area (E_C) and the adjacent uncovered area (E_UC), and compares them with evaporation from the reservoir assuming free-water conditions (E_{free, PM}). The results clearly indicate a substantial reduction in evaporation over the partially covered reservoir compared to the free-water reference.

Over the full observation period (2025-05 to 2025-10), an average evaporation reduction of 44% was observed above the FPV-covered area. More unexpectedly, this reduction extends beyond the direct footprint of the FPV installation. Evaporation over the uncovered area is also significantly reduced, with a mean decrease of 35%. This finding is particularly significant, as it challenges the common assumption in the literature that covered and uncovered areas behave as weakly coupled systems. Instead, our results reveal a strong coupling between these zones, indicating that FPV installations induce non-local modifications of the surface–atmosphere exchanges that affect evaporation at the reservoir scale.

Building on these observations, the next objective is to identify the key physical drivers controlling evaporation under FPV deployment and to explain the observed differences. Ultimately, this work aims to develop a simplified, physically based model capable of estimating evaporation losses from reservoirs partially covered by FPV systems.

Figure 2 - Daily mean evaporation from several days of 2025-07 over covered (orange) and uncovered (blue) areas. Gray bars correspond to the estimated free-lake evaporation of the reservoir.