Barend :
The storage (level pool) reservoir routing method tends to more conservative since it assumes that all of the water is available to leave the reservoir at an instant. This method does not account for the travel time necessary in a large reservoir for the water to work its way from end of the reservoir to the other end. For large reservoirs (more than a couple miles long), this can be overly conservative in the analysis.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com