Runoff versus surface runoff and stream discharge: pick the single best correct statement.

Introduction / Context:
Hydrologic terminology distinguishes between pathway components and the total discharge at the outlet. Clarity is essential when interpreting models, data, and design criteria.

Given Data / Assumptions:

• Surface runoff (overland flow) is one component of total runoff.
• Baseflow (groundwater contribution) is another component of total discharge.
• Some literature uses “runoff” as shorthand for stream discharge; precision matters.

Concept / Approach:
Runoff at the outlet equals the sum of various contributions, including surface runoff. Therefore, it certainly includes overland flow. However, equating “runoff” strictly with surface runoff is inaccurate, and calling surface runoff “streamflow” (which includes baseflow) is also imprecise.

Step-by-Step Solution:
Option (a) is incorrect: runoff is broader than surface runoff.Option (b) is correct: runoff includes surface runoff among other components.Option (c) can be contextually true, but it is ambiguous and not universally precise; usage varies.Option (d) is misleading: surface runoff is only the direct component, not the entire streamflow.

Verification / Alternative check:
Hydrograph separation confirms that streamflow = direct runoff + baseflow; thus surface runoff is a subset of total discharge.

Why Other Options Are Wrong:
(a) collapses categories; (c) is semantically variable; (d) incorrectly elevates a component to the total.

Common Pitfalls:
Using terms interchangeably in reports without definition; misinterpreting model outputs where “runoff” may mean event direct runoff only.

Final Answer:
Runoff includes the water flowing over the land surface.