Urban flood risk mitigation model- cn number for waterbodies

hello, I am a new user of Invest and am confused about the cn number for Waterbodies as I checked the sample data the cn number there for water is considered 1 while in the user guide for data sources, it is written “For water bodies and wetlands that are connected to the stream, CN can be set to 99 (i.e. assuming that those pixels rapidly convey quick flow.)”, is this CN number “99” for the new version of Invest?

Hi @Malihe.Moradi -

Thanks for catching that difference in curve number for water. The number itself is not based on how the InVEST model works, it’s based on the curve number method, which is commonly used in hydrology. There is actually a difference in how people assign this value. Most often, it’s assigned a very high number (closer to 100), with the assumption that the water body is saturated, so no water can infiltrate, and all of it runs off.

The key part of that User Guide quote might be “that are connected to the stream”. If a water body is not connected to a stream, then even though it’s saturated, water won’t be running off into a stream, it will just keep filling up the water body and not contribute to flooding (I suppose there’s a limit to that, until it overflows, but the model doesn’t consider that). So if the water body is not connected to a stream, then you can use a very low value (closer to 1).

We should make the sample data and User Guide consistent, so I will do that.

~ Stacie

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Thank you so much Stacie, for the help and clarification!

Malihe