Hmmm, total depletion, Aral Sea style? Why isn't that the worst case scenario?
That quote is saying that planners have no way of actually predicting how bad the worst possible drought will be for the area serviced by the reservoir over the lifetime of the reservoir, so they can't actually design the reservoir to handle the worst-case scenario.
If they knew the worst-case drought possible would last N days, they could design the reservoir to hold N+1 days worth of water. Alas, they don't know N.
Edit: Near end of day... Can you tell? Wrote draught instead of drought.
Just before your edit, I spent a while amused at the vision of some guy trying to chug the entire reservoir while being egged-on by inebriated friends.