# Urban Flood Mitigation: Flood volume error?

Hi, I am testing the Urban Flood Mitigation Model using as aoi a single polygon corresponding to the whole city that I am analyzing, and I realized that the flood_vol output might be wrong.
Flood-vol is calculated according to equation (5) in the user manual by multiplying Qi by the design storm depth P. However, Q is already the runoff depth (in mm), which just needs to be multiplied by the area to get the total runoff volume.
In fact, the flood_vol that I obtain is much higher than the value that can be estimated by multiplying rain depth for the whole area and then subtracting the total water retention.
Is it possible that I am correct here? Anyone with the same doubt?

Hi @ChiaraC,

I just wanted to offer a slight clarification, which is that Q_m3 is runoff volume in cubic meters per pixel. The calculations in the model’s source code reflect the equation defined in the user’s guide:

Q_{m3} = Q_{p,i} \cdot P \cdot pixel.area \cdot 0.001

Maybe this adjustment for pixel area accounts for the higher value you’re experiencing?

James

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I am using a raster with 1 meter resolution, so that is not the case.
But if you look at the units in the equation there is something wrong: you have Q in mm, P again in mm, and then pixel area that is in m2.
The problem to my understanding is that Qp,i is not an index, but a physical quantity of water runoff per pixel, so it should not multiplied by P.

Chiara

That is an interesting discrepancy! You’re right, something isn’t quite right with this. Let me check with our science team and get back to you on this.

Thank you @jdouglass!

Hi @ChiaraC, good catch, that P in the runoff volume is nonsensical. I’ve removed it from the user’s guide and I’ve put in a pull request to patch that issue in InVEST https://github.com/natcap/invest/issues/308. That fix will go into the next minor release of InVEST. If you can’t wait for that patch you can hack the output by dividing the current “runoff volume” by P to get the actual runoff volume.

Thanks again for catching this and I apologize for the inconvenience and confusing this caused you.

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Hi @rich, thanks for your reply. Great to know that the issue will be fixed in the next release!
Chiara

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