Question

The limitations to the use of unit hydrographs are:

(A) Snow melt runoff cannot be satisfactorily represented by unit hydrograph.
(B) The catchment should not have unusually large storages in terms of tanks, ponds, large flood bank storages, etc. which affect the linear relationship between storage and discharge.
(C) If the precipitation is decidedly uniform, unit hydrographs can not be expected to give good results.
(D) The rainfall intensity is assumed constant for the duration of the rainfall excess.
(E) The duration of rainfall should be 1/6 to 1/2 of the basin lag.

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (C), (D) and (E) only.
• D. (A), (B) and (D) only.

Hint:

Think about the basic assumptions of unit hydrograph theory and which listed statements actually violate them versus which ones reverse the correct logic.

Answer 1

The Correct Option is D

Step 1: Instead of checking every statement from scratch, start by eliminating the two answer choices that keep option (C) in them, because (C) can be tested quickly. The rule every hydrology text gives is that a unit hydrograph loses accuracy when a storm is NOT spread evenly over the basin. Option (C) claims the opposite, that even, uniform rain is what causes trouble. That is the wrong direction, so (C) must be dropped. This immediately kills choices 1, 2 and 3 in the answer list, since all three include (C).
Step 2: That leaves only option 4, but let's confirm it holds instead of just trusting the elimination. Option 4 keeps (A), (B) and (D) and drops (C) and (E).
Step 3: For (A): snow melt water does not follow the rain-to-runoff timing the UH is built on, so mixing it in ruins the linear model. Keep (A).
Step 4: For (B): a catchment full of ponds, tanks or large flood-bank storage releases water on its own schedule, not in proportion to the rainfall pulse, so the straight-line storage-discharge link the UH needs is broken. Keep (B).
Step 5: For (D): the unit hydrograph model assumes the rain falls at one steady intensity for the whole unit period. That is a strict assumption baked into the derivation, and real storms rarely behave that way, so it is fairly listed as a limitation. Keep (D).
Step 6: For (E), the actual accepted duration rule caps the storm length at around one-fifth to one-fourth of the basin lag, not the 1/6 to 1/2 window given here, so it stays excluded.
Step 7: With (A), (B), (D) confirmed and (C), (E) rejected, the match is exact.\[\boxed{\text{Option 4: (A), (B) and (D) only}}\]