Hydrograph fundamentals: A hydrograph is a plot of which primary variable against time at a given stream section?

Introduction / Context:
A hydrograph is one of the most important tools in surface-water hydrology. It provides a time history of stream discharge at a gauging station, capturing responses to rainfall, snowmelt, and upstream regulation.

Given Data / Assumptions:

• Single stream cross-section (gauging station).
• Discharge Q(t) measured or estimated over time.
• Hydrograph features: rising limb, crest, recession limb, baseflow separation.

Concept / Approach:
By plotting Q on the vertical axis and time on the horizontal axis, engineers analyze flood peaks, lag times, runoff volumes, and recession constants for design and forecasting.

Step-by-Step Solution:
Identify variable of interest: discharge (flow rate).Associate with time at a fixed location.Hence, hydrograph = Q(t) at a station.

Verification / Alternative check:
Hyetograph is rainfall intensity vs time; mass curve is cumulative volume vs time. The hydrograph specifically represents discharge vs time.

Why Other Options Are Wrong:
Surface runoff (qualitative) and groundwater flow depth are not the standard hydrograph axes; rainfall intensity belongs to hyetographs.

Common Pitfalls:
Confusing hydrograph (Q–t) with hyetograph (i–t) and mass curves.

Final Answer:
Stream discharge (flow rate) versus time.