Hydrology – What Is an Isochrone on a Basin Map? Choose the correct definition of an isochrone plotted on a catchment (basin) map used for runoff and hydrograph studies.

Introduction / Context:
Isochrones are key to transforming spatial rainfall into temporal runoff at the basin outlet. By delineating lines of equal travel time, engineers visualize how different parts of the basin contribute to the hydrograph at successive times after a rainfall event.

Given Data / Assumptions:

• Catchment with defined outlet and drainage paths.
• Overland and channel flow velocities estimated or mapped.
• Focus on surface runoff travel time, not rainfall amount or duration.

Concept / Approach:

An isochrone connects points that deliver runoff to the outlet after the same elapsed time since effective rainfall begins. Stacking the areas between successive isochrones and multiplying by an effective rainfall rate yields the unit hydrograph by time-area method.

Step-by-Step Solution:

Verification / Alternative check:

Results can be checked against observed hydrographs; calibration adjusts velocities and losses to align modeled peaks and timing.

Why Other Options Are Wrong:

• Equal rainfall duration/depth: describe isohyetal analysis (isohyets), not isochrones.
• Equal distance: ignores hydraulic velocity differences; distance is not time.

Common Pitfalls:

Assuming constant velocity everywhere; neglecting channelized flow that shortens travel time relative to overland flow.

Final Answer:

Joining points having equal time of travel of surface runoff to the catchment outlet