Reducing Evaporation of Standing Waters by Fresh and Non-Living Duckweed Covering

The extremely high rate of evaporation from water surfaces greatly reduces optimal utilization of water reservoirs. One of the biggest problems of water in dams in Iran is the huge amount of water loss through evaporation due to high evaporation rate. In this study, the efficiency of duckweed (Lemna gibba L.) as an environmentally friendly cover on water in reducing water evaporation has been investigated. For this purpose, a pilot system was designed that consisted of two glassy ponds. One pond as duckweed culture medium filled with raw water of the dam without the addition of any duckweed nutrients (plant pond), and another pond filled with raw water of the dam without duckweed (unplanted pond). Duckweed (Lemna gibba L.) was purchased from a typical aquarium market, and it was then cultured and used for batch experiments. The raw water of Aydoghmush dam (Miyaneh, Iran) was used for culture medium. The walls of the ponds were covered to prevent light from entering except at the top and the tow fluorescent lamps (36 W, Pars Co.) as artificial light at the top of each pond were also located (simulating natural climatic conditions). Temperature was maintained at 25 °C, and 14 hours of photoperiod was applied to duckweed. Both ponds were also equipped with an aeration system installed at the bottom of the pond. One of the aquariums as a growth medium and plant covering (planted aquarium) and the other without duckweed as unplanted aquarium were selected (both aquariums were filled with the raw water of the dam without the addition of any duckweed nutrients). For one month, every 24 hours, the amount of water evaporation was measured using the amount of change of water height in the ponds. The effect of plant covering, aeration and comparison of fresh and non-living duckweed covering on reducing surface water evaporation were studied. The effect of plant covering on reducing surface water evaporation in open air (outside) was also investigated.  The results showed that the rate of evaporation in the plant pond is lower than unplanted pond. The content of evaporation in the plant and unplanted ponds is 23.75 and 32.60%, respectively. So, plant growth and creating coverage on the water’s surface can be effective in reducing the surface evaporation of water storages such as dams. The effect of aeration on reduction of evaporation was also investigated. The results indicated that aeration enhanced growth rate of the plant and consequently, reduced the water evaporation. This can be due to the following main reasons: 1) increasing the growth rate of the plant and the formation of more plant covering on the surface of the water; 2) the formation of turbulent flow in the surface layer of water and the replacement of water molecules with those molecules that have the potential to escape from the surface of the water. The results also showed that non-living duckweed can be used as a cover on water to reduce water evaporation, and in the open air, by creating plant covering on the surface of the water, 23.53% of the surface water evaporation can be prevented. By comparing the results obtained in the laboratory and in the open air, it is evident that the evaporation rate in both plant and unplanted aquariums is higher than in the laboratory environment, which can be due to exposure to sunlight and wind.