Dear Dr. Santosh S Mali,

I am not sure if this exactly you askin for, but maybe it helps:

In Slovak Republic, we have these calculations for "Ecological Flow" - aka Environmental flow" - MQ:

- for streams under reservoirs:

MQ = Q355d

- for other streams:

MQ = (Qmin m + Q100 min d)/2

where:

Qmin m - a value taken from probabilistic field of the average month flows for a high level of safeguard, usually 98 %;

Q100 min d - a value of minimal day-flow with an average occurence once in 100 years

It is necessary to re-evaluate these results for every catchment due to local specific conditions.

Minimal flow in Czech Republic:

-streams with flow Q355d < 0,05 m3.s-1: a long-term value of uninfluenced Q330d recommended

-streams with flow Q355d = 0,05–0,5 m3.s-1: a long-term value of uninfluenced (Q330d+Q355d) * 0,5

-streams with flow Q355d = Q355d = 0,51–5,0 m3.s-1: a long-term value of uninfluenced Q355d

-streams with flow Q355d > 5.0 m3.s-1: a long-term value of uninfluenced (Q355d+Q364d) * 0,5

Some useful links:

http://ieeexplore.ieee.org/document/5893103/?reload=true&tp=&arnumber=5893103&url=http:%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5893103

http://www.sciencedirect.com/science/article/pii/S1877705812007576

http://ocw.upm.es/ingenieria-agroforestal/management-of-fluvial-ecosystems/contenido/lecture-notes/the-spanish-experience-in-determining-minimum-flow-regimes-in-regulated-streams

https://www.hindawi.com/journals/jam/2014/127067/

Best regards,

Tomas

About 30 years ago, I was detailed to the Caribbean National Forest to estimate amounts needed for water rights to protect resources.  The amount of aquatic biology data relative to streamflow was limited to non existent.  I did the best I could with the available stream records, and use the flow duration curves to identify flows by rio (river) or quebrata (creek) for a variety of administrative and environmental needs.  At that time, there was no advice on instream flows needed for channel maintenance, but since then the general advice is to allow for a bankfull event at enough frequency to maintain a clear channel that is not encroached upon by vegetation so it can avoid the circumstance of aggradation and channel instability due to lack of being able to move its sediment load.  I uploaded my short publication ~1995 into researchgate.  The flow duration curves were normalized by area, so you could develop a value up or down a stream system based on watershed size.  Since I did not have much biological or other data, some of the values one might consider arbitrary when estimating minimum or environmental flow needs for recreation, scenery, aquatic biology function, etc.  

There are a variety of instream or environmental flow methodologies that exist, and most require extensive study.  Since I was under pressure to determine an approach, and estimate all needs in a month or two, and all calculations and plots had to be done on hand calculator and manual plots on graph paper, the approach I came up with probably does not meet the rigors of the more detailed and data intensive approaches of others.  

However, by identifying and setting some minimum flow needs for specific uses, and using the area normalized flow duration curves by watershed, it is an approach that relied heavily on the flow data from the existing stream gauging stations.  It would have helped to have more aquatic biology data and a fisheries or aquatic biologist involved, but by maintaining flows above the periodic boundary of low flow was felt to be a reasonable starting point that probably would need monitoring to ensure it was adequate.  

Since many of the stream gauges were on moderate to large drainages, the area normalized flows applies to headwaters and also ungauged small streams would probably protect these smaller streams and headwaters, as they typically do not have as much baseflow as the lower stations.  There are numerous papers on how important headwaters are in supplying nutrients and refugia for specific species, contributing to a river function, so giving them some elevated protection may be worthwhile.  This approach shows how much water might be available for use, but the downside for dry areas is that some effort to store water is probably needed, and that was not factored into the analysis.  Using dams to store water can have their own environmental impacts, cost, maintenance and failure concerns.    

There are several to many approaches that have been used for instream or environmental flow needs, and I do not have the detail on them.  There are several companies that have developed expertise and experience on these methods that you might search out, but I believe much available on internet.  Decisions of which approach to use are often based on the available data and resources (funds, personnel, scientists) available to put into an analysis.  You might chose a quick method as preliminary approach to be developed as monitoring and data is obtained. Typically though if there are endangered or sensitive species involved, the analysis will become more complicated.  There are many streams that are dry because of excessive withdrawals, all water used for irrigation, drinking, etc.  It would be hard to reverse these uses when people's lives and food sources become so dependent on the water.  

Conservation measures may help as well as stopping some water uses in the driest low flow periods and saving the water for the instream needs or critical human and animal needs associated needs such as drinking water supply.

If I am not wrong IWMI have E-Flow calculator, which regarded as one of the best tools for estimation of E-Flow as far as Indian conditions is concern. 

There are numerous methods some of which are detailed in Chapter 3 ofthis Handbook see: http://www.samsamwater.com/library/handbook_catchment_water.pdf

It is very good question and is dire need of the current societal demands keeping in view the climatic changes. Thanks all the experts for their valuable opinions which all are no doubt true recommendations. 

I think there are many factors to decide the minimum flow to maintain the ecological conditions in the river and in the aquifer underlying the river. To maintain the biodiversity, channel sections, recharge to aquifers under stress, leaching requirements, quantity and quality of drainage effluents being thrown in the rivers, mitigation of the sea water intrusion in coastal areas, maintenance of wetlands, pond areas at barrages and many similar other factors are to be kept into consideration while estimating the environmental flows in rivers or streams. The min flow required may vary from site to site. 

Sorry, not my area of expertise.