Efficiency of shading materials in reducing evaporation from free water surfaces

The aim of this study was to characterize the influence of various shading materials on the evaporation rate from a Class-A pan and on its driving variables. Hourly evaporation measurements from two pans, one uncovered and the other covered with different types of shading meshes, were carried out under summer conditions, along with measurements of air temperature and humidity, water temperature, solar global radiation and wind speed. Single and double-layered polyethylene meshes of different colours and a single layered aluminized mesh were tested. The aerodynamic and shading effects responsible for the evaporation reduction, as well as the additional processes of condensation and water recovery, were analyzed and discussed. In all cases, shading the pan induced a significant decrease of the daily evaporation rate, ranging from 50% for the aluminized screen to near 80% for the coloured-polyethylene meshes. The latter materials combined an efficient aerodynamic effect (decrease of about 70% of the mass transfer coefficient) with a strong diminution of surface-to-air water vapour gradient. The condensation over the meshes was far for being negligible for the black polyethylene meshes, reaching up to 15-20% of the daily evaporation losses. It was concluded that the choice of the shading material should be based on criteria related to porosity and optical properties, but also to the capacity of the material in enhancing the condensation process during the night and collecting the condensed water. The extrapolation of the results to the case of agricultural water reservoirs was discussed and an economic analysis was carried out to assess the viability of shading covers under the conditions of water use and pricing prevailing in Southern Spain. (c) 2006 Elsevier B.V. All rights reserved.