1. The problem statement, all variables and given/known data Lag time in hydrograph means teh time of center of rainfall to center of hydrograph , according to my book . 2. Relevant equations 3. The attempt at a solution Since lag time can be understood by the time taken for the water to reach river , contributing to max discharge ... So , i think in photo 755 , it's wrongly labelled , it sjould be labelled in the beginning of the graph , which is the green line , correct me if i am wrong.