Time of Concentration:
The time of concentration is an important parameter in hydrology that measures the time taken for rainfall water to travel from the most distant point of a watershed to a specific location, such as the inlet of a sewer or a point of interest in a drainage system. It is a key factor in determining the design of drainage systems and evaluating the runoff characteristics of a watershed.

Definition:
The time of concentration is defined as the sum of two components:
1. The time taken by rainfall water to run from the most distant point of the watershed to the inlet of the sewer.
2. The time required for the flow of water in the sewer from the inlet to the point under consideration.

Explanation of the Correct Answer (Option C):
The correct answer is option C, which states that the time of concentration is the sum of the time taken for runoff from the most distant point of the watershed to the inlet of the sewer and the time required for the flow of water in the sewer to the point under consideration.

Components of Time of Concentration:
1. Time taken for runoff from the most distant point of the watershed to the inlet of the sewer:
- This component represents the time required for the rainfall water to travel over the land surface, through the channels, and reach the inlet of the sewer.
- It depends on factors such as the slope of the land, surface roughness, and channel characteristics.
- This component is often referred to as the "overland flow time" or "surface flow time."

2. Time required for the flow of water in the sewer to the point under consideration:
- Once the runoff water reaches the inlet of the sewer, it starts flowing through the sewer system.
- This component represents the time taken for the water to travel through the sewer pipes from the inlet to the point under consideration.
- It depends on factors such as the length and diameter of the sewer pipes, the hydraulic gradient, and the flow velocity.

Importance of Time of Concentration:
The time of concentration is a crucial parameter in hydrologic calculations and design of drainage systems. It helps in:
- Estimating the peak flow rates and volumes of runoff from a watershed.
- Designing the capacity of stormwater management facilities such as detention ponds, culverts, and sewers.
- Determining the time of concentration is vital for accurate hydrologic modeling and predicting the behavior of a drainage system during rainfall events.

Conclusion:
The time of concentration is the sum of the time taken for runoff from the most distant point of the watershed to the inlet of the sewer and the time required for the flow of water in the sewer to the point under consideration. It is an important parameter in hydrology and is used in the design and analysis of drainage systems.