Stream gauging location in upper Gordon Gulch

Stage height of a stream is automatically recorded by a pressure transducer in the stream. Discharge is initially measured by field personnel, and a plot of stage height vs. discharge produces a rating curve. Discharge can then be estimated by stage height.

Hydrograph comparing discharge over 3 years in Como Creek, a tributary to Boulder Creek (courtesy Ty Atkins, INSTAAR/Niwot LTER).

Discharge, or the volumetric flow rate of water, is commonly estimated in rivers and streams from the measured water-surface elevation (stage) using an empirical fit to measurements of stage and concurrent discharge. This is referred to as the stage discharge relationship or the stream rating curve.

To determine stage height, a Solinst level logger is installed at the stream gauging location and records the stage based on pressure within a sensor.  A pressure transducer sensor measures the combined pressure exerted on it by the atmosphere and the head of water above it. The sensor is fixed below the minimum expected water level, and a cable containing the sensor signals and vent tube runs from the sensor to the data logger. The pressure sensor is the most common type of stream gauge sensor because it is relatively cost-effective to install and maintain.

Measuring the changes in water level over time is only one component in developing a stage discharge relationship or stream rating. In order to obtain a good estimate of water flow (total discharge), point discharge measurements should be taken periodically to establish and maintain a good rating curve. Each discharge measurement is correlated to the water level at the time the measurement was taken. With this information, discharge data can be interpolated and applied to the full range of water level measurements.

Discharge, or the volume of water flowing in a stream over a set interval of time, can be determined with the equation:

Q = AV,

where Q is discharge (volume/unit time: e.g. m³/second), A is the cross-sectional area of the stream (e.g. m²), and V is the average velocity (e.g. m/s).

A hydrograph is a chart that displays the change of a hydrologic variable such as discharge over time.  An annual discharge hydrograph will provide information such as the timing and intensity of snowmelt runoff, storm events and baseflow levels.  Understanding a stream's hydrograph is a necessary step to evaluating other hydrologic properties of a watershed such as biogeochemical cycling, surface/groundwater interactions, and flowpaths within the critical zone.

Written by Rory Cowie