Inferring reservoir operating patterns across the Mekong Basin using only space observations

This study explores the operating pattern of artificial reservoirs by examining their impact on streamflow through two parameters, residence time and flow alteration, using a purely satellite-based technique for the Mekong Basin. Overall residence times of individual reservoirs ranged from 0.09 to 4.04 years, while streamflow was altered between 11 and 130% of its natural variability. The current set of reservoirs appears to have increased the residence time of the entire Mekong basin by about 1 month. However, if subbasin variability is considered, the satellite-based method depicts a different picture. Residence time increases to 4 months when only regulated flows are considered. If low residence time reservoirs on major rivers are excluded and reservoirs on higher stream-order rivers considered, residence time increases to 1.3 years. Predictable strong seasonal patterns emerged in residence time, where reservoirs experience higher residence time in the dry season and lower residence time in the wet season and residence time varies inversely with precipitation. High variability in reservoir effects on streamflow between reservoirs could not be explained by any reservoir properties (e.g., size, use, location, etc.), highlighting the variability in the human decisions operating these reservoirs. The take-home message of this study is that satellite observations, in combination with physical models forced with satellite data, can elucidate the spatiotemporal variability of reservoir behavior in ungauged basins of the developing world. We demonstrate in this study that the requirement for ground data to monitor current or historical behavior of dams is not necessary. Plain Language Summary The key take home message of our study is that satellites can now see the diverse variability of surface water residence time due to reservoir construction in ungauged and international river basins that is otherwise intractable. As satellite observations become increasingly more widespread in the near future, the scientific community will be able to rely on space observations to understand the potential impact of extensive reservoir development planned by each riparian nation of major river basins in the developing world. Such an ability will improve water management, inform planning decisions, and better reservoir operations. Such an understanding can also counter the secrecy or the lack of capacity that is common among nations, and result in a more cooperative environment for the benefit of all the stakeholders of the basin.