Several inputs are important: (i) maximum and minimum capacity of the reservoir; (ii) projected demand of water usage; (iii) operation rules of the reservoir; (iv) measurement data (or forecast) of precipitation data; (v) evapotranspiration rate; and (vi) any other addition losses from the system such as seepage. Take advantage of all the information above and perform mass balance equation. You should make it right~ 

Thanks Haw Yen.

It is nice answer. How about climatechange? because unpredicted rain can give impact also to flood?

Thanks