In catchment hydrology, which definition best describes surface runoff?

Introduction / Context:
Surface runoff links precipitation to streamflow and is a critical input to flood routing, reservoir design, and watershed management. It is important to distinguish runoff from other precipitation losses.

Given Data / Assumptions:

• Runoff represents the liquid water contribution to streams and rivers originating from precipitation (and snowmelt) after losses.
• Losses include interception, infiltration, and depression storage plus evapotranspiration.

Concept / Approach:
After rainfall reaches the ground, part is intercepted or infiltrates. Some fills micro-depressions (depression storage). Once these losses are satisfied and rainfall intensity exceeds infiltration plus storage, the excess flows overland to channels; this overland flow, together with direct channel precipitation and interflow contributions, becomes surface runoff reaching and moving through stream channels.

Step-by-Step Solution:

Verification / Alternative check:
Standard hydrology texts define surface runoff (direct runoff) as the portion of water that appears in the channel during or immediately after a storm event, consistent with option (d).

Why Other Options Are Wrong:

• Absorbed by soil / intercepted / evaporated: These are loss components, not runoff.
• Filling surface depressions only: This is storage before runoff initiation.

Common Pitfalls:

• Confusing interflow/throughflow with deep groundwater baseflow.
• Ignoring the threshold behavior when depression storage is satisfied.

Final Answer:
Water that reaches and flows through stream channels