Why do we measure the flow rate?
Each hydrologically closed area (catchment) is drained by a network of streams that gradually merge into one main stream. This stream leaves the catchment at its lowest point (the closure profile) and drains all surface water from rainfall. Rainwater joins the stream at the surface or infiltrates into the soil and emerges as groundwater. To assess the hydrological balance of a catchment, it is necessary to know the amount of water entering the catchment as atmospheric precipitation and the amount of water leaving the catchment as surface runoff.
How do we measure the flow rate?
Flow rate is expressed as the volume of water flowing through the closure profile per unit of time, most commonly in cubic metres per second (m3/s). Long-term flow measurements are made with apparatuses that record the water level at points where the bottom profile has been modified so that the flow is uniform, and the flow cross-section has a regular shape. This ensures accuracy and makes it easier to calculate the volumetric flow rate based on the measured water level, which is most often measured by floats, pressure sensors or ultrasound. For each specific profile, we construct a specific (consumption) curve describing the dependence of the water level on the volume of water flowing through. We use this curve to derive the flow rate from the measured level.
Flow profile on Brejlský potok
The profile you are standing at measures the amount of water flowing out of the Brejlský potok catchment. The values help with assessing the effectiveness of water retention measures applied in the landscape. An ultrasonic sensor measures the height of the stream level, which is directed in a hardened section in the channel bottom. This modification refines the measurements, especially during low flows, and at the same time does not hinder the natural migration of animals, especially the present stone crayfish.
