The Global Environmental Flow Calculator (GEFC) is a software package for desktop rapid assessment of Environmental Flows (EFs). The EF estimation technique in GEFC is using monthly time step series reflecting “natural” / unregulated flow conditions and its corresponding Flow Duration Curve (FDC)- a cumulative distribution function of flows. The FDC is represented by 17 percentage points on the probability (X) axis. EFs aim to maintain an ecosystem in, or upgrade it to some prescribed or negotiated condition - “Environmental Management Class (EMC).” The higher the EMC, the more water is needed for ecosystem maintenance and more flow variability needs to be preserved. Six EMCs are used in GEFC, ranging from “Unmodified” to “Critically Modified.” Each EMC is represented by its unique FDC. The FDC for each class is determined by the lateral shift of the original reference FDC to the left along the probability (X) axis by one percentage point. Each EMC is effectively an EF scenario. The EMC best suited for the river in question may be selected-based on expert judgment. A FDC established for each EMC can be converted into an EF time series.