In surface hydrology, the time taken by runoff from the most remote point of a watershed to reach the outlet is called the __________.

Introduction / Context:
Design of storm drains, culverts, and small basins frequently relies on the concept of time of concentration, which defines when the entire watershed contributes simultaneously to flow at the outlet. Correct terminology is essential for selecting rainfall intensities from IDF curves and applying rational or unit hydrograph methods.

Given Data / Assumptions:

• Watershed with a definable hydraulic path from the hydraulically most remote point (in time) to the outlet.
• Uniform rainfall intensity considered for a design duration equal to or exceeding this time.

Concept / Approach:
The time of concentration, Tc, is the travel time of runoff from the critical remote point to the outlet. When storm duration equals or exceeds Tc, peak discharge is expected because the whole area contributes. Terms like “overland flow time” are components of Tc only for the sheet flow segment.

Step-by-Step Clarification:
Identify the hydraulically longest (in time) path including sheet flow, shallow concentrated flow, and channel flow segments.Compute travel times for each segment using appropriate empirical or kinematic formulas.Sum to obtain Tc, which dictates the critical design rainfall duration.

Verification / Alternative check:
Using Tc to select rainfall intensity from IDF curves is standard. Peak flow from the Rational method Q = C * I * A is taken with I corresponding to duration ≈ Tc.

Why Other Options Are Wrong:

• Time of overland flow / Concentration time of overland flow: Only part of the total concentration time; excludes channel segments.
• Duration of rainfall: Independent storm characteristic, not a watershed property.
• Baseflow recession time: Describes groundwater discharge decay, not travel time of surface runoff.

Common Pitfalls:

• Equating Tc to just channel travel time or just overland flow time.
• Using Tc from one method outside its valid watershed size/land use range.

Final Answer:
Time of concentration