Tisa River Basin Hydrologic Data Collection Program

How the State Water Management Committee, in Cooperation with the USGS and ULRMC, Collects the Hydrologic Data

Continued from Previous Page

3.  Streamflow and Velocity are Measured

The volume of water passing a specific point in a given interval of time is called streamflow or discharge.  Streamflow generally is measured in cubic meters per second and most often is determined by measuring the velocity of the water at a particular cross section of a river.  Although streamflow data are more difficult to collect than stage data, the streamflow data are needed for flood forecasting and also are needed by many users of water-resource information.

The velocity-area method is the most common method used to measure streamflow.  The method, which is used by the USGS and by Ukrainian hydrometeorological staff, requires the physical measurement of the cross section and of the velocity of the water.  The velocity is measured using a current meter that consists of a propeller that is rotated by the action of flowing water.  Each rotation completes an electrical circuit and is recorded.  The propeller rotation speed depends on the velocity of the water passing by the propeller.

4.  A Relation Between Water Level and Streamflow is Developed

Streamflow data for a specific stream location are compared to stage data for that location and used to develop a rating curve.  After the rating curve has been developed, the streamflow can be estimated from the curve.

The rating curve for a specific stream location is developed by making successive streamflow measurements to define and maintain a stage-streamflow relation.  The streamflow measurements and their corresponding stages (or gage heights) are plotted on a graph for the stream location.  Continuous streamflow throughout the year then can be determined from the rating curve and the stage record.  The data are checked periodically to ensure accuracy because factors such as ice cover, debris, and vegetation growth can affect the stage-streamflow relation.

As part of a cooperative agreement among the SWMC, USAID, ULRMC, and USGS, both stage and streamflow data will be available to all potential users through organization websites and through published media.