The influence of floating spheres on evaporation suppression under different climatic conditions

Global warming and population growth increase the need for better management of freshwater resources, in particular in arid and semi-arid regions. Due to increased rainfall variability,  reservoirs became a vital management tool that stores the water during rainfall, thus decrease flood risks and supply water during drought periods. However, large amounts of water are lost by evaporation, which markedly affects reservoirs’ function of ensuring water availability. In Egypt, about 20.0% of the country's Nile share (12.1 to 15.4 billion m³) are lost annually by evaporation from Lake Nasser. The floating covers, i.e. spheres, shade the water surface and act as a physical barrier that decreases energy flux into the water, thus decrease evaporation. Studies that compare the evaporation suppression efficiency of the floating covers, over different climatic conditions, while considering its impact on the water ecology are limited.

A field experiment in an outdoor setting (class A-pan) was conducted for nine months (March to November) in two locations that vary in their climatic conditions, i.e. Aswan and Damanhur, representing northern and southern Egypt, respectively. The water surface was covered by white, black, or multicolor spheres, in addition to the control. Daily evaporation rate (ER), water temperature (WT), evaporation suppression efficiency (ESE), were determined. Moreover, the microalgae growth was measured as an indicator of water ecology.  

Obtained results revealed massive evaporation losses from the uncovered water surface (control) in Aswan location, in which the nine-month average was 2.25 times higher than in Damanhour location. The floating spheres reduced ER in both locations, in particular the white spheres. The ESE in Aswan was less than in Damanhour location. The ESE in Damanhour was 63.38, 58.13, and 54.8%, while in Aswan was 48., 42.5, and 41.6% for white, multicolor and black spheres, respectively. Floating spheres decreased WT in the morning and mid-day, while in the evening the control treatment was the coldest, indicating partial isolation of covered water surface. Irrespective of the spheres’ color, the spheres had no detrimental effect on microalgae growth, indicating enough light penetration and gas exchange through the gaps between spheres.

In conclusion, the floating spheres is an effective mean for evaporation suppression and its efficiency is dependent on the climate and spheres’ color. The ESE of spheres is lower in environments with lower relative humidity. The white spheres are recommended for evaporation suppression without negative impacts on microalgae growth which could be a viable indicator for the ecology of the water ecosystem. Further studies on larger water reservoirs are needed while considering several aquatic organisms.