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: Recall that a unit hydrograph (UH) is built on two basic assumptions: time invariance and linear response (proportionality plus superposition). Any real situation that breaks these assumptions becomes a genuine limitation of the method.
Step 2: Check statement (A). Unit hydrograph theory is derived purely from rainfall excess versus direct runoff. When snow melt mixes into the runoff, the timing and volume relationship no longer comes from rainfall alone, so the linear rainfall-runoff assumption breaks down. This is a genuine, well documented limitation, so (A) is correct.
Step 3: Check statement (B). The linear storage-discharge relationship that the UH depends on gets disturbed if the catchment has large disconnected storages such as tanks, ponds or extensive flood-plain storage, because these store and release water in a non-linear, delayed way. So (B) correctly states a real limitation.
Step 4: Check statement (C). The actual textbook limitation is that a unit hydrograph gives poor results when rainfall is NOT uniformly spread over the catchment (spatially non-uniform storms distort the shape of the resulting hydrograph). Statement (C) reverses this logic by saying uniform precipitation causes poor results, which is factually backwards, so (C) is incorrect as written.
Step 5: Check statement (D). One of the basic working assumptions of the unit hydrograph is that rainfall excess occurs at a constant intensity throughout the unit duration. Since this is exactly the kind of restrictive assumption whose violation limits the method, (D) is a correctly stated limitation.
Step 6: Check statement (E). The accepted guideline in hydrology practice is that the duration of the unit storm should not exceed about one-fifth to one-fourth of the basin lag for the unit hydrograph to remain valid, not a broad 1/6 to 1/2 range. Because the ratio quoted in (E) is wrong, this statement is incorrect.
Step 7: Only (A), (B) and (D) hold up as true and correctly worded limitations, which is exactly option 4.