# Urban flood risk mitigation output

**URL:** <https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070>\
**Category:** Science, Research & Data\
**Tags:** urban-flood\
**Created:** [June 16, 2020, 7:03pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070 "2020-06-16T19:03:28Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Joe](https://avatars.discourse-cdn.com/v4/letter/j/b9e5f3/32.png) [@Joe](https://community.naturalcapitalalliance.org/u/Joe)\
**Post date:** [June 16, 2020, 7:03pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070/1 "2020-06-16T19:03:28Z")

</div>

Hi there,

Would it be possible for a quick explanation of the output “runoff\_retention\_output” from the Urban InVEST (v.3.8.0) flood risk mitigation model?

I guess it is the “R.tif: raster with runoff retention values (no unit, relative to precipitation volume)” mentioned in the user guide? If so, how is it calculated and what do the values specifically mean?

I have values ranging from 0 to 0.62 for example.

Much appreciated,

Joe

---

<div class="post-metadata">

**Author:** ![dcdenu4](https://yyz1.discourse-cdn.com/flex031/user_avatar/community.naturalcapitalalliance.org/dcdenu4/32/544_2.png) [@dcdenu4](https://community.naturalcapitalalliance.org/u/dcdenu4)\
**Post date:** [June 17, 2020, 3:24pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070/2 "2020-06-17T15:24:11Z")

</div>

Hi @Joe,

I do believe you are right in the outputs do not match the User’s Guide. We’ll get that updated.

I can’t help anymore than repeating what the `How it Works` [User’s Guide](https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/urban_flood_mitigation.html#how-it-works) section but can try and track someone down who could give a better indication.

Doug

---

<div class="post-metadata">

**Author:** ![Joe](https://avatars.discourse-cdn.com/v4/letter/j/b9e5f3/32.png) [@Joe](https://community.naturalcapitalalliance.org/u/Joe)\
**Post date:** [June 17, 2020, 11:07pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070/3 "2020-06-17T23:07:02Z")

</div>

Hi Doug,

Thanks for the reply. Yeah, I had a look through the user guide but can’t figure out for sure how this output is calculated and therefore how it can be used. (Hope I’m not overlooking something in there).

I’m wondering if the values within this “no unit” raster simply indicate the proportion of runoff retained.  
For example, my input “depth of rainfall” is 132mm. Perhaps a pixel value of 0.5 in this output simply means that 50% of this rainfall is retained (therefore 66mm retained)?

---

<div class="post-metadata">

**Author:** ![dcdenu4](https://yyz1.discourse-cdn.com/flex031/user_avatar/community.naturalcapitalalliance.org/dcdenu4/32/544_2.png) [@dcdenu4](https://community.naturalcapitalalliance.org/u/dcdenu4)\
**Post date:** [June 18, 2020, 12:06pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070/4 "2020-06-18T12:06:31Z")

</div>

Yea, it looks like `R_m3` is the runoff retention by volume with pixel area in squared meters. I’ll try and have one our science leads say more. Hang tight!

Doug

---

<div class="post-metadata">

**Author:** ![dcdenu4](https://yyz1.discourse-cdn.com/flex031/user_avatar/community.naturalcapitalalliance.org/dcdenu4/32/544_2.png) [@dcdenu4](https://community.naturalcapitalalliance.org/u/dcdenu4)\
**Post date:** [June 29, 2020, 12:47pm UTC](https://community.naturalcapitalalliance.org/t/urban-flood-risk-mitigation-output/1070/5 "2020-06-29T12:47:27Z")

</div>

Hey @Joe,

Just circling back on this. It looks like your interpretation is correct, see Equation 3 in the UG: [https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/urban\_flood\_mitigation.html#the-model](https://storage.googleapis.com/releases.naturalcapitalproject.org/invest-userguide/latest/urban_flood_mitigation.html#the-model)

Our modeler Perrine Hamel says:

> So R.tif is simply the proportion of precipitation that is retained (and does not become runoff).

I hope that helps.