Correct option is B
In hydrology, a unit hydrograph is a tool used to estimate the direct runoff hydrograph resulting from a unit depth of rainfall (typically 1 cm or 1 inch) distributed uniformly over the watershed for a specific duration. The concept is used extensively in flood forecasting and water resource management.
Basin Lag:
Basin lag is the time difference between the centroid of the effective rainfall (excess rainfall) and the peak of the resulting hydrograph. It is a measure of the time it takes for water to move from the rainfall point to the outlet of the basin.
Selection of Rainfall Duration for Unit Hydrograph Derivation:
The duration of the rainfall used to derive a unit hydrograph should be chosen carefully to ensure that the resulting hydrograph accurately represents the basin's response to rainfall.
If the rainfall duration is too short or too long relative to the basin lag, the unit hydrograph may not accurately represent the basin's hydrological response.
For effective unit hydrograph derivation:
The duration of the rainfall used in the derivation of the unit hydrograph should typically be 20% to 30% of the basin lag. This ensures that the unit hydrograph is representative of the basin's response to typical rainfall events, allowing for accurate modeling and prediction of runoff.
