In an S-curve derived by a unit hydrograph U of D-h durationa)the equi...
Explanation:
S-curve derived by a unit hydrograph U of D-h duration represents the cumulative runoff at the outlet of a catchment in response to a unit depth of excess rainfall that is uniformly distributed over the catchment for a duration of D hours.
The correct option is B, i.e., the time base of U represents the time at which the S-curve attains its maximum value. This can be explained as follows:
- Definition of S-curve: An S-curve represents the cumulative runoff at the outlet of a catchment over time, starting from the time of onset of excess rainfall until the runoff ceases.
- Relationship between unit hydrograph and S-curve: The S-curve is derived by convolving the unit hydrograph with excess rainfall of varying duration and intensity. The unit hydrograph represents the response of the catchment to a unit pulse of excess rainfall, and its time base represents the time taken for the peak of the hydrograph to occur.
- Effect of duration on S-curve: As the duration of excess rainfall increases, the volume of runoff also increases, and hence the S-curve attains higher values at later times.
- Effect of time base of U on S-curve: The time base of U determines the time at which the peak of the S-curve occurs. This is because the peak of the S-curve corresponds to the time when the excess rainfall is fully converted to runoff and is equal to the peak flow rate of the unit hydrograph. Hence, the time base of U represents the time at which the S-curve attains its maximum value.
- Other options: Option A is incorrect because the equilibrium discharge depends on the catchment area, and hence on the volume of excess rainfall. Option C is incorrect because the equilibrium discharge depends on the catchment area. Option D is incorrect because the ordinate of the S-curve is in units of volume per unit area, not volume.