The use of a hybrid method for reconstructing water levels and volumes of water mass in a reservoir is considered on the example of the Arakhley Lake of the Trans-Baikal Territory. The method makes it possible to obtain high spatial resolution cuts of water levels on the relief based on satellite images of the Landsat system of different time intervals and images from unmanned aerial vehicles (UAVs) as a source of a highly detailed digital elevation model. As a result of processing the Landsat satellite data, the values of the Arakhley Lake surface areas for the period 1987–2018 were obtained. Based on the results of the UAV survey, the water levels in the lake were extracted according to the survey dates corresponding to the areas. The root mean square error of water level determination (RMSE) was 0.23 m, which is lower than the horizontal resolution of the elevation model (0.3 m) obtained from the UAV data. Also, the characteristics of the water mass volume were obtained for the variable part of the lake volume for the period 1987–2018. The use of the hybrid method considered in the article will solve the problem of insufficient or complete absence of data on the long-term water regime of unexplored lakes and reservoirs. Evaluation of the possibilities of using this technology by comparing with the instrumental characteristics of water levels at the regime point of hydrological observations, shows the boundaries of its use, advantages and disadvantages. At the same time, the main advantage can be recognized as the possibility of obtaining time series of changes in levels and volumes over the past years in those lakes and reservoirs where there have never been ground observations and are unlikely to be. In the case of establishing the dependences of the water mass volume on the areas of the water surface, it becomes possible to perform operational hydrological monitoring of water bodies using only Landsat satellite images.
