Hi Mehdi,  while others have ...

Hi Mehdi,  while others have given very good advice, I think you are asking for the base flow calculation and not the total or storm flows.  This is usually determined from the rainfall/runoff for the driest months of the year, and then you may also wish to determine this for the average year, or alternatively for e.g. a 10 year drought year.  Now in many cases the rainfall in the driest months of the year may be zero or close to zero, so using the rainfall-runoff calculations does not give you an acceptable result.  Instead you will need to estimate the contribution to the stream flow from underground sources (springs and seeps) which are geologically controlled as well as linked to the recharge of the groundwater.  A simpler way is to measure the streamflow in the critical period (if it has not already been abstracted for human endeavours upstream).

I am pleased though that you are following this path - it is often neglected and we ignorantly disturb our river eco-systems thrrough ignoring this factor.  Best wishes