Hydrology basics: Runoff is best described as the quantity of water which flows where, as part of the catchment response to precipitation?

Introduction / Context:
Runoff is a core hydrologic term used to estimate yields, design stormwater systems, and assess flood risks. It links rainfall to streamflow after subtracting abstractions like infiltration and interception.

Given Data / Assumptions:

• Precipitation occurs over a catchment.
• Some water infiltrates, some is stored, some evaporates or is transpired.
• The remainder becomes overland flow contributing to channels.

Concept / Approach:
Runoff generally refers to the portion of precipitation that appears as flow in natural streams and rivers (surface runoff plus quick interflow contributions). It is distinct from sewer flows (which may include wastewater) and from leakage losses in pressurized pipes.

Step-by-Step Solution:

Verification / Alternative check:
Hydrograph separation methods distinguish baseflow from direct runoff; both contribute to river discharge records used in design.

Why Other Options Are Wrong:
Sewer and leakage flows are not hydrologic runoff; subsoil drains are engineered systems; evaporation is a loss, not runoff.

Common Pitfalls:
Confusing catchment runoff with urban sanitary flows; ignoring infiltration capacity variations that alter runoff magnitude.

Final Answer:
In rivers and natural streams (surface flow)