Urban Cooling and the Eto layer

Hi - please help me to finally understand the role of ETo in the UC model!

after referencing these other questions (the 2nd of which is mine)

  • Doubts about urban cooling model
  • Urban cooling: advice on scenario generation
  • Reference evapotranspiration raster data

I read in the userguide :

  • Reference evapotranspiration varies with elevation, latitude, humidity, and slope aspect

and i read at fao

  • …reference evapotranspiration, denoted as ETo. The reference surface is a hypothetical grass reference crop etc etc

So whatever value I apply, whether its from a local rural site or within my urban location, it is likely to be a single value across my small (15km2) AOI. is that correct?

In that case when the normalized version is used for ETi, ETo/Etmax will always be 1 and Eti = kc

is that correct? But this makes it seem like Eto has no purpose!

So what am i misundestanding?!!? please help!


Well, I think that’s up to you to decide based on your study area. The Urban Cooling model requires that you provide your ET0 values as a spatial raster, so if you know your reference evapotranspiration to vary spatially, you could have that be reflected in this input.

But I don’t have much experience with this. Maybe @swolny, @jesseG or someone else could weigh in here?

Hi @southplainfield -

It’s entirely possible that ET0 is consistent across your small AOI. It may also differ, if there are significant differences in elevation or other topographic things that might affect ET. I’m not sure what data you’re considering using, but climate data like ET is usually on the coarse side, ~1km resolution or so. So it could show differences across your AOI, although how real they actually are depends on the source data and how it was processed.

~ Stacie


thanks for replying.

just to be super sure…if my Eto is consistant across the AOI then ETo/Etmax will always be 1 and Eti = kc?

According to the model equations in the User Guide, that’s correct.

~ Stacie