Instantaneous unit hydrograph is a hydrograph of i) unit duration ii) unit rainfall excess iii) infinitely small duration iv) infinitely small rainfall excess The correct answer is A. (i) and (ii) B. (i) and (iv) C. (ii) and (iii) D. (iii) and (iv)

The correct answer is A. (i) and (ii).

An instantaneous unit hydrograph (IUH) is a hydrograph that represents the response of a watershed to a unit impulse of rainfall excess. It is a dimensionless quantity, and its area is equal to one. The IUH can be used to estimate the runoff hydrograph from a given rainfall hyetograph.

The IUH is a function of the watershed’s characteristics, such as its area, shape, and soil type. It can be determined by field measurements or by using a computer model.

The IUH is a valuable tool for hydrologists and engineers. It can be used to design storm sewers, flood control systems, and water supply reservoirs.

The following are brief explanations of each option:

• Option (i): Unit duration. The IUH is a hydrograph of unit duration, which means that it has a duration of one unit. This is in contrast to a regular hydrograph, which has a finite duration.
• Option (ii): Unit rainfall excess. The IUH is a hydrograph of unit rainfall excess, which means that the total rainfall excess over the duration of the IUH is equal to one. This is in contrast to a regular hydrograph, which may have a total rainfall excess that is greater than or less than one.
• Option (iii): Infinitely small duration. The IUH is not a hydrograph of infinitely
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  small duration. This is because the IUH has a finite duration of one unit.
• Option (iv): Infinitely small rainfall excess. The IUH is not a hydrograph of infinitely small rainfall excess. This is because the IUH has a total rainfall excess that is equal to one.