Introduction: Evaporation from open water surfaces is a primary cause of water resource depletion in arid and semi-arid regions, sometimes resulting in the loss of over 50 percent of annual water reserves. Iran, characterized predominantly by an arid to semi-arid climate, is significantly affected by this phenomenon; reports indicate that between 2003 and 2019, the country lost approximately 211 cubic kilometers of its total water reserves—an amount exceeding twice the nation's annual water consumption. Agricultural ponds and reservoirs are particularly vulnerable to such losses due to constant exposure to direct sunlight; indeed, annual evaporation from small agricultural ponds in the Middle East is estimated to exceed 2,400 million cubic meters. Exacerbating this challenge is the fact that nearly 47.8 percent of the country's irrigation pumps still operate on diesel engines, which—in addition to high operating costs—result in significant greenhouse gas emissions. Floating photovoltaic (FPV) systems have emerged as a dual-purpose technology capable of simultaneously reducing surface evaporation and generating clean electricity to power well motors and irrigation pumps. Although previous studies have reported significant evaporation reduction through the use of floating photovoltaic covers—ranging from approximately 40% to over 90%, depending on the coverage extent and layout—the precise impact of panel installation positioning (submerged, flush with the water surface, or elevated above the water) on evaporation reduction in arid climates has not been systematically investigated. In addition to evaporation reduction, the electrical efficiency of the panels under each installation configuration was evaluated as a complementary finding. The present study was designed and conducted to address this knowledge gap.

Materials and Methods This experimental study was conducted during the summer of 2024 (August 6 to September 5) at the Faculty of Agriculture, Ferdowsi University of Mashhad, located in the arid climate of northeastern Iran. Four identical tanks (65 × 79 × 30 cm) were each placed inside a 1 × 1 × 1 m polystyrene enclosure to eliminate the effect of lateral solar radiation on the tank walls. Four treatments were defined: a control tank without a panel; a panel submerged 2 cm below the water surface; a panel flush with the water surface; and a panel positioned 8 cm above the water surface. Four identical 20-watt polycrystalline panels (model Zytech Solar ZT-20P, labeled A through D) were installed horizontally, facing south with a zero-degree tilt angle. To account for panel-specific effects, the positions of the panels were rotated every three days between treatments, resulting in six independent replicates for each treatment. Each panel was connected to an identical resistive circuit—consisting of a 20-watt incandescent bulb—to simulate actual operating temperature conditions. The voltage and current of each panel were measured individually and in immediate succession using a Victor VC97 digital multimeter, with the panel connected to an identical 20-watt incandescent lamp (acting as a resistive load). Voltage was measured across the load, while current was measured in series within the circuit. Besides, electrical output power was calculated . Daily evaporation rates were measured using a digital scale (model FX-3000iWP, precision ±0.01 g) and a 1000 mL graduated beaker, while environmental parameters—including air temperature, relative humidity, and solar radiation intensity—were simultaneously recorded using a Lutron HT-3027SD data logger and the PVGIS database. Assumptions regarding data normality (Shapiro-Wilk test) and sphericity (Mauchly's test) were assessed, and the data were analyzed using repeated measures ANOVA (RM-ANOVA) in SPSS software (version 27), accompanied by pairwise comparisons with Bonferroni correction.
Results and Discussion The average daily evaporation in the control treatment was 4.39 mm, representing the highest value recorded among all treatments. The submerged panel treatment resulted in only a limited reduction (15.7%), bringing the evaporation rate to 3.70 mm; this was likely due to heat transfer from the panel to the water body, which partially offset the shading effect. The treatment with the panel positioned above the water surface showed a reduction of 45.63% (2.38 mm), whereas the treatment with the panel flush with the water surface yielded the greatest reduction (52.37%, or 2.09 mm) as well as the most consistent performance throughout the monitoring period. Repeated measures ANOVA (RM-ANOVA) confirmed statistically significant differences among the treatments (F = 159.17, df = 3, p < 0.001), and Bonferroni post-hoc tests revealed that all panel treatments differed significantly from the control (p < 0.001). The numerical difference between the water-level treatment and the above-water treatment was negligible (0.296 mm) and statistically non-significant (p = 0.065); however, the water-level treatment exhibited greater daily stability throughout the experiment. In contrast to the study by Lopez et al., which achieved a 37% reduction in evaporation using 50% surface coverage on water reservoirs in semi-arid Brazil, the present study recorded a 52.37% reduction with only 42% coverage—a result that exceeds predictions based on a linear relationship between coverage extent and evaporation, highlighting the importance of optimal installation positioning regardless of the surface coverage area. Installation position also significantly affected the panel's electrical efficiency (p < 0.01): the water-surface-level treatment achieved the highest mean efficiency (10.9%), followed by the above-water (10.7%), control/ground-based (10.5%), and submerged (10.2%) treatments; despite recording the highest open-circuit voltage, the submerged treatment exhibited lower efficiency due to optical losses caused by light absorption by the water. The alignment of enhanced evaporation reduction and optimal electrical performance in the water-surface-level treatment confirms the potential of this configuration for dual-use applications.
Conclusion These findings indicate that the installation configuration of floating photovoltaic panels is a decisive factor in their performance—regarding both evaporation reduction and electricity output—under arid climatic conditions. A water-surface-level configuration maximized evaporation reduction while simultaneously yielding the highest electrical efficiency; consequently, this approach is recommended as a practical, dual-purpose solution for agricultural water reservoirs in arid regions of Iran (such as Khorasan Razavi Province), as it enables water resource conservation alongside energy generation for irrigation pumping. Future research should focus on evaluating the structural durability of these systems at an operational scale, investigating their long-term impacts on water quality and reservoir ecosystems, and conducting techno-economic feasibility analyses of the technology.