How is runoff measured with stream gauges and hydrographs?

In hydrologic sciences, runoff is measured by determining stream discharge—the volume of water passing a point per unit of time—and recording it over time to create a hydrograph.

Measuring Discharge

At a gauging station, hydrologists use several methods to calculate discharge:
* Velocity-Area Method: Measuring water velocity using current meters or ultrasonic sounding and multiplying it by the cross-sectional area of the flow.12
* Tracer Dilution: Estimating total discharge by measuring the dilution of a tracer substance.1
* Weirs and Flumes: Using structures like weirs or Venturi flumes that create a unique, predictable relationship between the upstream water level (stage) and discharge.12

Rating Curves and Hydrographs

Because continuous velocity measurements are often impractical, hydrologists establish a rating curve. This graphical formula relates the water surface height to the cross-sectional area and velocity, allowing discharge to be estimated simply by monitoring the water level.2

Continuous water level data, collected via float or probe-based recording gauges, are then used to construct a hydrograph. This record of discharge over time is essential for flood forecasting, water resource planning, and understanding the hydrologic response of a catchment.12