River Discharge

Here’s another task for a rainy day.  If you have already measured river velocity and calculated the area of the cross-section at each of your sites, you already have all the information that you need to work out river discharge.

Discharge is the volume of water flowing along your river channel.

Discharge = cross-section x velocity.

For each site calculate the average of your velocity measurements.

If you haven’t already calculated the cross-section, look back to the previous post to see how to come up with a figure.

Discharge is usually quoted in cumecs, that’s cubic metres per second.  The longest river in the British Isles, the River Shannon, Ireland, has the highest discharge with more than 200 cumecs. 

River Severn

But the second longest river, the River Severn, only has just over 60 cumecs.  The river with the second largest discharge is the River Tay, in Scotland, with around 180 cumecs. 

If you want to be able to compare your river’s discharge then you need to also have a figure in cumecs.  To get that your velocity needs to be in metres per second and your cross-section needs to be in square metres.  If your river is really small you might have been working in square centimetres but google can sort that out for you.  Use this link.

So having got your cross-section in square metres and your velocity in metres per second, multiply them together to get the discharge in cumecs.

As tributaries join the stream you would expect the discharge to increase, but if you are in a limestone area, maybe some of the water is sinking into the bed of the channel, giving you different results.