Hi, if you mean by ...

Hi,

if you mean by environmental flow is the minimum flow that should exist within one section of the river, it involves many ecological parameters. An example is by using sustainable index of living habitats of certain sensitive organism (fish, riparian animals, etc.) which needs minimum amount of river flow in order to sustain.

However, if what you mean is baseflow of a river, there are various studies on baseflow separation method which you can search over the internet.

Hope this helps.

Best regards,

Muhammad Alfalah Fauzi