The global Reservoir Assessment Tool (RAT) Graphical User Interface consists of 1598 dams over South America, Africa, and South-East Asia. This tool provides reservoir information based on topography, satellite imagery and hydrological model in near-real-time scale. User needs to click on any reservoir icon in the main window to see the following options for key reservoir parameters: “AEC” for Area-Elevation relationship, “Surface Area” for surface water extent area, “Storage Change” for storage change, “Inflow” for reservoir inflow, and “outflow” for reservoir outflow. The area-elevation relationship was derived from SRTM Dem. The surface water extent area of reservoirs was calculated in Google Earth Engine (GEE) by applying Normalized Difference Water Index (NDWI) method on Landsat 8 OLI/TIRS imagery datasets. Storage change was calculated using surface water extent area and area-elevation relationship. Inflow into reservoirs was simulated using meteorological observations (i.e. precipitation, maximum and minimum temperature and wind speed) forced into VIC hydrological model. Reservoir outflow calculated using a mass balance approach from inflow and storage change. Typically, every week all the reservoirs are being updated with the latest available hydrological data and satellite imagery. Users should refer to the How to Cite button for the published research this portal is based on.