Estimating Flow Rates

Estimating Flow Rates

Flow is the average volume of water that passes a fixed point in a river or stream over time. Flow rates are a key part of understanding how much energy you can produce from your watercourse. Estimating flow rates is quite straight forward, but it is an estimate as accurate measurement of flow rates can take weeks, months, or years to complete.

You can follow the below steps to gain an understanding of the potential at your site. Once you have an estimate of flow you can enter it, along with an estimate of the head, into our Output Calculator.

You need to gain an understanding of your watercourses catchment area.

Using enter your postcode or grid reference (click here for guidance on how to obtain a grid reference) and hit Enter or click ‘Go’.

flow rates

Use the zoom function to show a 1:25000 scale map.

flow rates

Find your proposed site on the map and start visualising the contour lines as actual rises such as hills or mountains. Follow the river/stream and any tributaries upwards until the contours level off. The below example has the catchment area highlighted in red/pink. Looking at the river and waterfalls, and the associated valley around, you can identify the surrounding landscape will be capturing rainwater and which direction the sloping ground will take it.


Identifying hilltops and level ground and drawing a line along the top of surrounding higher ground you can gain a good estimate of your sites catchment area.

There are plenty of geographical and geological variants meaning that estimating flow is open to change. A rough estimate of flow by catchment is 40 litres per second per square kilometre.

Alternatively, for larger flow sites with large catchment areas, local gauging station data may be available at a nearby point. This can be viewed by clicking here.

Calculate Estimating Head