Lake Urmia is a thalassohaline ecosystem. It is an extremely simple ecological pyramid. This makes it a very sensitive ecosystem. This ecosystem has been facing various threats regarding the amount of water released from the associated basin. To calculate the lake water requirement with an ecological approach, we identified three variables: ecology, water quality, and water quantity indices as environmental indicators. The ecological index represented by Artemia urmiana is considered as an independent variable; while, the water quality index represented by concentration of NaCl, and the water quantity index represented by water elevation are regarded as dependent variables. The salinity threshold of 240 ppt (parts per thousand) is selected as the water quality requirement for Artemia urmiana. Finally, the aggregation of these variables shows that Lake Urmia requires annually 3086 MCM of inflow water to have a sustainable ecosystem.