What does a direct runoff hydrograph result from?

• A. Base flow
• B. Total Rainfall
• C. Rainfall Excess
• D. Interflow

Answer: (C)

A direct runoff hydrograph represents the immediate response of a watershed to precipitation, specifically the portion of rainfall that contributes to surface runoff. It illustrates how much of the rainfall exceeds the infiltration capacity of the soil and becomes direct runoff.

Rainfall excess is the key concept here, as it is the part of the total rainfall that is not absorbed into the soil or held in the form of depression storage. Instead, it is conveyed over the land surface to streams and rivers, resulting in direct runoff. This is what causes the rise in stream flow depicted in the hydrograph.

In contrast, base flow refers to the groundwater contribution to stream flow, which is relatively stable and does not respond quickly to storm events. Total rainfall encompasses both the rainfall that infiltrates and the rainfall excess; hence, it does not solely pertain to the runoff aspect. Interflow involves water movement through the soil layer, contributing to groundwater rather than immediate surface runoff. Thus, rainfall excess most accurately defines the cause of a direct runoff hydrograph.