Abstarct: Freshwater is one of the most precious gifts of nature that human life on earth is going on. Countries have a different share of this vital resource, depending on the climatic and geographical conditions. In recent years, population growth and community development have reduced freshwater supplies in many parts of the world, especially in dry and semi-arid countries. If the necessary measures to manage, suitable use and reduce water losses do not occur, dehydration will become a major and serious threat to human life. These actions, if they are public, can make a significant contribution to preventing water losses. In arid and semi-arid areas, the main reason for water losses can be called a phenomenon called surface evaporation that occurs at temperatures below the boiling point of water. In Surface evaporation, molecules on the surface of the water are exposed to sunlight or wind blowing, and with increasing jerking, they are separated from the water surface and converted to water vapor. Hence, evaporation is an inevitable phenomenon, and plays a significant role in the rate of water losses in a country, given the country's climate and climate conditions. One of the major ways to reduce evaporation from the surface of open reservoirs, which is used in many warm and dry areas, is to use physical coatings. These coatings prevent from direct contacting sunlight with the surface of the water and reduce the rate of evaporation. In most studies that investigate the reduction of evaporation losses by physical coatings, the velocity of water flow was zero and the rate of evaporation reduction in stagnant conditions have been estimated. Considering the fact that many reservoirs have inlet and outlet flows, the study of surface flow effects on evaporation rates and the proper estimation of evaporation rates from these reservoirs can be helpful in managing and controlling evaporation losses. To achieve this, a 500-liter polyethylene reservoir as a water storage tank, six 500-watt projectors to simulate sunlight, plastic balls with two different densities (The first case has a mass of about 14 grams, and the second case, which falls to half the water, has a mass of 60 grams) in two colors, black and white, as a protective coating, a pump with a constant flow to create a surface flow and from A Marriott bottle is used to measure the surface evaporation rate of the reservoir. The results show that, in the uncoated conditions, surface water evaporation in the presence of a surface flow to a specified discharge, which is called an optimal Flow rate decreases as compared to the non-flowing state, With the increase of discharge and the passage of optimal discharge, the evaporation rate begins to increase. Also, in all conditions of existence and absence of surface flow, the results indicate that white coatings due to the high reflection coefficient higher than the black coatings have the highest evaporation reduction efficiency and the evaporation rate in the combined coating conditions is equal to the black balls and White is less than black balls. When floating coatings collapse in water due to the increase in density, the height of the surface roughness decreases. Hence, the heat transfer coefficient increases between the air and the water. Therefore, the rate of evaporation is increased compared to the use of lightweight coatings and reduces the efficiency of evaporation reduction. in order to estimate the evaporation of water in surface flow mode, energy balance modelling was considered at the surface of water and coating. The results show that using the energy balance modeling method with a very good approximation can predict the evaporation rate by having the parameters required for all modes with the coating, without coating, with the presence of surface flow and without surface flow.