No problem, it is a very tricky subject and I can never remember how it works off the top of my head (well, now I can just look back at this post!)
Regarding the unit hydrograph duration, imagine if the problem was structured this way:
Given:
Average Effective Rainfall from Hour 0-3: 1.5 in/hr (4.5 in total)
Average Effective Rainfall from Hour 3-6: 0.7 in/hr (2.1 in total)
and given the 1-hour unit hydrograph in your problem statement.
Solution:
You could not solve the same way I did in the above post, because originally the rainfall was given in 1-hour increments. Now, in this particular problem, you have two pulses of 3-hour rainfall increments (aka two mini storms lasting 3 hours long each). Therefore, you would have to convert your 1-hour UH to a 3-hour UH. Once you did that, you could solve the same way I posted above, except you would "lag" the second pulse by 3 hours instead of 1.
And why would the exam give you 3-hour pulses in the first place? Well maybe the "effective rainfall gauge" (quotes because while that would be awesome, I'm not sure this exists) only collects data every 3-hours. So the gauge says it collected 4.5 inches of water from 7AM to 10AM, then clears the data and starts a new reading for the next 3-hours.
Luckily, I really don't think the depth exam will be this tricky..the problems will probably only be as complicated as your post above, or converting an x-duration UH to a y-duration UH. It would be a little cruel to combine both in one problem, but you never know. Good luck!
