Abstarct: Abstract Climate change and air pollution are two major crises in the present era, but water scarcity is even more important compared to these two, as it intensifies them. Therefore, accurately estimating parameters of the hydrological cycle is essential. In the hydrological cycle, evapotranspiration is considered one of the most important and complex parameters. Accurate estimation of this parameter greatly aids in managing water resources. Nowadays, with remote sensing technology, it is possible to determine this parameter easily and without the usual challenges across various areas. For this reason, this study focused on estimating evapotranspiration. The study area is Shahid Rajaee Dam Lake and part of its upstream basin in Mazandaran Province. In this study, the SEBAL algorithm was used to calculate evapotranspiration in the region through the Google Earth Engine platform, and its performance was evaluated against the Penman-Monteith method and empirical methods including Mayer, Marciano, Shahtin, Hefner, and the evaporation pan method. For the water body, 8 images from Landsat 8 satellite were used, and for the upstream basin, 8 images from the MODIS sensor for the 2022–2023 hydrological year were used. The results of this study indicate a high correlation between the SEBAL algorithm and the Penman-Monteith method (with a correlation coefficient of 0.86 for the Aqua Area and 0.94 for the upstream basin). The Landsat 8 satellite provides evapotranspiration zoning with higher spatial resolution, suitable for rugged and small areas, while the MODIS is more suitable for large, flat areas.These days, with the help of remote sensing and the SEBAL algorithm, evapotranspiration can be estimated over large areas by using appropriate images and satellites for each type of land use, which helps better and more targeted water resource management.