One of the challenges in the field of dam construction and surface storage resources is the evaporation losses. Evaporation, as one of the main components of the hydrological cycle, results in the loss of significant amounts of volume of stored water in the reservoirs. Since wind is considered as one of the factors affecting the evaporation from free water levels, on the other hand, The implementation of some methods of evaporation control requires the identification of the wind swept situation, Therefore, in the this study, by calculating the wind shelter index in different locations and the cluster analysis of the obtained values, different wind shelter and wind swept conditions in different points of the free water surface are determined. For this purpose, lake of Dez reservoir dam, located between latitude 32º 35' 45" and 32º 43' 38" northern and longitude 48º 18' 51" to 48º 30' 20" east, with an area of over 26 square kilometers, selected and wind shelter index values were calculated in a 30 × 30(m) network, including 28321 points. It should be noted that this parameter was studied for each of the warm months of the year, including June, July, August and August, where evaporation rates are higher. Then, using cluster analysis methods k-means and ward, the lake surface in terms of wind shelter, was divided into homogeneous areas with different wind shelters. The results showed that the homogeneous areas formed in each of the months studied, were in different wind shelter conditions. So, the percentage of the area of the lake which was less windy than other places was 75.6%, 70.8%, 62.3% and 55.5% respectively in each of June, July, August and September. Therefore, it was possible to evaluate free water levels in terms of wind shelter conditions in open reservoir. Also, by examining the values of wind shelter index during the months studied, it was found that the parameter is sensitive to changes in water level. So that, a decrease of 14.5 meters in the level of water level during the months of June to September showed an average increase of 47.6 percent in the wind shelter index, on average. Therefore, it is predicted that determining the water level corresponding to the maximum wind shelter can play an effective role in the management of reservoirs in terms of evaporation control.