EFFECT OF USING MONOLAYER COVERS ON EVAPORATION RATE IN DAM RESERVOIRS EFFECT OF USING MONOLAYER COVERS ON EVAPORATION RATE IN DAM RESERVOIRS

Each year, millions cubic meters of water are lost from water reservoirs (especially dam reservoirs) due to evaporation. Spreading artificial monolayers on water reservoir is one of the ways to reduce evaporation. Monolayers are one molecule thick films formed at a phase boundary like air/water interface. According to laboratory studies, long chain fatty alcohols such as Cetyl and Stearyl alcohol, are one of the most effective materials for reducing evaporation from water bodies, specially dam reservoirs. Most of the materials used as monolayers are readily broken down by bacteria, moreover these materials could also be evaporated, so redistributing them in specific periods is necessary to keep up their function. Most of Previous studies are experimental (both laboratory and in-field) which resulted a wide range for evaporation reduction (0\% - over 50\%) based on different cases, locations and monolayer materials. The aim of this study is to evaluate the amount of evaporation reduction causes by using this method to reduce evaporation. Energy balance relations were used to calculate the evaporation rate. Minab dam reservoir, was selected as a case study and Cetyl alcohol was chosen as a monolayer material. The evaporation rate was calculated for the 5-year study period (1390-1394) in base condition and in two scenarios, first for full time usage of monolayers and second for only summer usage scenario. Application of monolayers increases the apparent surface boundary layer thickness, thereby, increasing resistance to evaporation. Excess aerodynamic resistance caused by the usage of monolayers was computed according to previous studies as a linear function of wind speed (means the monolayer resistance decreases linearly when wind speed increases) and entered into evaporation rate calculations directly. Results show that average evaporation reductions are 8.1\% and 2.7\% espectively when monolayers were used full-time and when they were used only in summer. This amount of reduction leads to save up to 3.54 million cubic meter annually. A primary cost estimation was also done and resulted about 1 USD per cubic meter of not evaporated water for full-time usage scenario and about 0.8 USD per cubic meter of not evaporated water for the summer usage scenario.