Runoff generation concept: Surface runoff from a watershed primarily results from which portion of a storm hyetograph?

Introduction / Context:
Surface runoff is the portion of rainfall that appears as overland flow and subsequently as streamflow, after satisfying initial losses such as interception, depression storage, and infiltration capacity. Identifying the effective part of rainfall that produces runoff is crucial for unit hydrograph and rainfall–runoff modeling.

Given Data / Assumptions:

• Watershed initially has losses (interception, depression storage, infiltration to capacity).
• Only rainfall exceeding these losses within the supply interval becomes effective rainfall.
• Baseflow is considered separate from direct runoff.

Concept / Approach:
Effective rainfall (also called excess rainfall) equals gross rainfall minus initial abstraction and continuing infiltration (up to infiltration capacity). This effective portion during the net supply interval drives surface runoff and shapes the direct-runoff hydrograph.

Step-by-Step Solution:
Initial rain often satisfies interception and storage; it seldom contributes immediately to surface runoff.As storm intensity exceeds infiltration capacity and storage, the net (effective) rainfall begins.Thus, surface runoff is linked to rain in this net supply interval, not to initial or residual components that do not exceed losses.

Verification / Alternative check:
Abstraction models (e.g., phi-index, SCS-CN method) explicitly compute excess rainfall, which when convolved with unit hydrographs reproduces observed runoff.

Why Other Options Are Wrong:
Initial rain: commonly consumed by abstractions.Residual rain/baseflow: generally not the effective driver of direct runoff.“All the above”: overly broad; only the net supply interval creates overland flow.

Common Pitfalls:
Assuming any rainfall creates runoff; ignoring infiltration capacity and initial abstractions.

Final Answer:
Rain in the net supply interval (effective rainfall after initial losses).