River flow monitoring in summer

Water levels in rivers drop during the summer time, and the rivers can sometimes dry up altogether. This natural phenomenon called low-flow level is often accentuated by water needs such as drinking water supply, irrigation or watering parks & gardens, at a time when water resources are scarce. How and when do these low-flow levels occur during the summer ? Why should this be monitored and how ? What variation can be observed over time ? Field observations between 2012 and 2016 show that August 2012 and September 2016 were the months where the most rivers dried up, with significant variation across France.